Stack
A stack is a container of elements with last in, first out access policy. Sometimes it also called LIFO.
You are encouraged to solve this task according to the task description, using any language you may know.
Data Structure
This illustrates a data structure, a means of storing data within a program.
The stack is accessed through its top.
The basic stack operations are:
- push stores a new element onto the stack top;
- pop returns the last pushed stack element, while removing it from the stack;
- empty tests if the stack contains no elements.
Sometimes the last pushed stack element is made accessible for immutable access (for read) or mutable access (for write):
- top (sometimes called peek to keep with the p theme) returns the topmost element without modifying the stack.
Stacks allow a very simple hardware implementation.
They are common in almost all processors.
In programming, stacks are also very popular for their way (LIFO) of resource management, usually memory.
Nested scopes of language objects are naturally implemented by a stack (sometimes by multiple stacks).
This is a classical way to implement local variables of a re-entrant or recursive subprogram. Stacks are also used to describe a formal computational framework.
See stack machine.
Many algorithms in pattern matching, compiler construction (e.g. recursive descent parsers), and machine learning (e.g. based on tree traversal) have a natural representation in terms of stacks.
- Task
Create a stack supporting the basic operations: push, pop, empty.
- See also
- Array
- Associative array: Creation, Iteration
- Collections
- Compound data type
- Doubly-linked list: Definition, Element definition, Element insertion, List Traversal, Element Removal
- Linked list
- Queue: Definition, Usage
- Set
- Singly-linked list: Element definition, Element insertion, List Traversal, Element Removal
- Stack
11l
<lang 11l>[Int] stack
L(i) 1..10
stack.append(i)
L 10
print(stack.pop())</lang>
- Output:
10 9 8 7 6 5 4 3 2 1
6502 Assembly
The 6502 has a built-in stack, which is located at memory addresses $0100-$01FF. The first thing most boot ROMs will do is set the stack to equal $FF. Only the X register can interact with the stack pointer's value directly, and it does so using TSX (transfer stack to X) and TXS (transfer X to stack.) Each push will decrement S by 1 and write that byte to the stack memory. On the original 6502, only the accumulator could be pushed to the stack, so programs running on those CPUs would often use sequences such as TXA PHA and TYA PHA to save the X and Y registers. This had the nasty habit of destroying the accumulator, which made saving these registers difficult. Fortunately, the 65c02 and its later revisions can push/pop X and Y directly without having to go through the accumulator first.
Push: <lang 6502asm>PHA</lang> Pop: <lang 6502asm>PLA</lang> Empty: <lang 6502asm>TSX CPX $FF BEQ stackEmpty</lang> Peek: <lang 6502asm>TSX LDA $0101,x</lang>
68000 Assembly
The 68000 is well-suited to stack data structures. Register A7 contains the stack pointer, however any address register can be used for a similar purpose. Any register from A0-A6 can be pointed to work RAM and used as a stack.
Push
You can push the contents of one or more variables. <lang 68000devpac>LEA userStack,A0 ;initialize the user stack, points to a memory address in user RAM. Only do this once! MOVEM.L D0-D3,-(A0) ;moves the full 32 bits of registers D0,D1,D2,D3 into the address pointed by A0, with pre-decrement</lang>
Unlike the "true" stack (A7), you can push a single byte onto the user stack and it won't get automatically padded with a trailing null byte.
Pop
The pop is just a reverse push. <lang 68000devpac>MOVEM.L (A0)+,D0-D3 ;returns the four longs stored in the stack back to where they came from.</lang>
Empty
The stack is empty if and only if the stack pointer equals its initialized value. This is only true provided you have never adjusted the stack pointer except by pushing and popping. <lang 68000devpac>CMPA.L #userStack,A0 BEQ StackIsEmpty</lang>
Manually adjusting the stack
You can offset the user stack (and the real stack) as follows: <lang 68000devpac>LEA (4,SP),SP ;does the same thing to the stack as popping 4 bytes, except those bytes are not retrieved.</lang>
Peek
If you know the intended length of the last item on the stack (1, 2, or 4 bytes), you can load it into memory without popping it. This applies to both the real stack and a user stack you may have created. Since this operation doesn't alter the value of the stack pointer, you don't have to worry about misaligning the stack, but the value you peek at should be of the correct size or you'll be "peeking" at more than one item at the same time.
<lang 68000devpac>MOVE.W (SP),D0 ;load the top two bytes of the stack into D0 MOVE.W (A0),D0 ;load the top two bytes of A0 into D0</lang>
8086 Assembly
The 8086's hardware stack is very similar to that of Z80 Assembly. This is no coincidence, as the Z80 was based on the predecessor to the 8086.
<lang asm>push ax ;push ax onto the stack pop ax ; pop the top two bytes of the stack into ax</lang>
The "high" byte is pushed first, then the low byte. Popping does the opposite.
Depending on your assembler, the stack's initial value may be set using the .stack directive.
Like the Z80, the 8086 can only push or pop 2 bytes at a time. It's not possible to push AH without pushing AL alongside it. The stack can be used to exchange values of registers that even the XCHG command can't work with. This is done by deliberately pushing two registers and popping them in the "wrong" order.
The easiest way to "peek" is to pop then push that same register again. <lang asm>;get the top item of the stack pop ax push ax</lang>
The stack need not be accessed using these push and pop commands, it can also be read like any other area of memory. This is actually how C programs store and recall local variables and function arguments.
ABAP
This works for ABAP Version 7.40 and above
<lang ABAP> report z_stack.
interface stack.
methods:
push
importing
new_element type any
returning
value(new_stack) type ref to stack,
pop
exporting
top_element type any
returning
value(new_stack) type ref to stack,
empty
returning
value(is_empty) type abap_bool,
peek
exporting
top_element type any,
get_size
returning
value(size) type int4,
stringify
returning
value(stringified_stack) type string.
endinterface.
class character_stack definition.
public section.
interfaces:
stack.
methods:
constructor
importing
characters type string optional.
private section.
data:
characters type string.
endclass.
class character_stack implementation.
method stack~push.
characters = |{ new_element }{ characters }|.
new_stack = me. endmethod.
method stack~pop.
if not me->stack~empty( ).
top_element = me->characters(1).
me->characters = me->characters+1. endif.
new_stack = me. endmethod.
method stack~empty. is_empty = xsdbool( strlen( me->characters ) eq 0 ). endmethod.
method stack~peek. check not me->stack~empty( ).
top_element = me->characters(1). endmethod.
method stack~get_size. size = strlen( me->characters ). endmethod.
method stack~stringify.
stringified_stack = cond string(
when me->stack~empty( )
then `empty`
else me->characters ).
endmethod.
method constructor. check characters is not initial.
me->characters = characters. endmethod.
endclass.
class integer_stack definition.
public section.
interfaces:
stack.
methods:
constructor
importing
integers type int4_table optional.
private section.
data:
integers type int4_table.
endclass.
class integer_stack implementation.
method stack~push. append new_element to me->integers.
new_stack = me. endmethod.
method stack~pop.
if not me->stack~empty( ).
top_element = me->integers[ me->stack~get_size( ) ].
delete me->integers index me->stack~get_size( ). endif.
new_stack = me. endmethod.
method stack~empty. is_empty = xsdbool( lines( me->integers ) eq 0 ). endmethod.
method stack~peek. check not me->stack~empty( ).
top_element = me->integers[ lines( me->integers ) ]. endmethod.
method stack~get_size. size = lines( me->integers ). endmethod.
method stack~stringify.
stringified_stack = cond string(
when me->stack~empty( )
then `empty`
else reduce string(
init stack = ``
for integer in me->integers
next stack = |{ integer }{ stack }| ) ).
endmethod.
method constructor. check integers is not initial.
me->integers = integers. endmethod.
endclass.
start-of-selection.
data: stack1 type ref to stack, stack2 type ref to stack, stack3 type ref to stack,
top_character type char1, top_integer type int4.
stack1 = new character_stack( ). stack2 = new integer_stack( ). stack3 = new integer_stack( ).
write: |Stack1 = { stack1->stringify( ) }|, /.
stack1->push( 'a' )->push( 'b' )->push( 'c' )->push( 'd' ).
write: |push a, push b, push c, push d -> Stack1 = { stack1->stringify( ) }|, /.
stack1->pop( )->pop( importing top_element = top_character ).
write: |pop, pop and return element -> { top_character }, Stack1 = { stack1->stringify( ) }|, /, /.
write: |Stack2 = { stack2->stringify( ) }|, /.
stack2->push( 1 )->push( 2 )->push( 3 )->push( 4 ).
write: |push 1, push 2, push 3, push 4 -> Stack2 = { stack2->stringify( ) }|, /.
stack2->pop( )->pop( importing top_element = top_integer ).
write: |pop, pop and return element -> { top_integer }, Stack2 = { stack2->stringify( ) }|, /, /.
write: |Stack3 = { stack3->stringify( ) }|, /.
stack3->pop( ).
write: |pop -> Stack3 = { stack3->stringify( ) }|, /, /.
</lang>
- Output:
Stack1 = empty push a, push b, push c, push d -> Stack1 = dcba pop, pop and return element -> c, Stack1 = ba Stack2 = empty push 1, push 2, push 3, push 4 -> Stack2 = 4321 pop, pop and return element -> 3, Stack2 = 21 Stack3 = empty pop -> Stack3 = empty
Action!
Static memory
<lang Action!>DEFINE MAXSIZE="200" BYTE ARRAY stack(MAXSIZE) BYTE stacksize=[0]
BYTE FUNC IsEmpty()
IF stacksize=0 THEN RETURN (1) FI
RETURN (0)
PROC Push(BYTE v)
IF stacksize=maxsize THEN
PrintE("Error: stack is full!")
Break()
FI
stack(stacksize)=v
stacksize==+1
RETURN
BYTE FUNC Pop()
IF IsEmpty() THEN
PrintE("Error: stack is empty!")
Break()
FI
stacksize==-1
RETURN (stack(stacksize))
PROC TestIsEmpty()
IF IsEmpty() THEN
PrintE("Stack is empty")
ELSE
PrintE("Stack is not empty")
FI
RETURN
PROC TestPush(BYTE v)
PrintF("Push: %B%E",v)
Push(v)
RETURN
PROC TestPop()
BYTE v
Print("Pop: ")
v=Pop()
PrintBE(v)
RETURN
PROC Main()
TestIsEmpty() TestPush(10) TestIsEmpty() TestPush(31) TestPop() TestIsEmpty() TestPush(5) TestPop() TestPop() TestPop()
RETURN</lang>
Dynamic memory
The user must type in the monitor the following command after compilation and before running the program!
SET EndProg=*
<lang Action!>CARD EndProg ;required for ALLOCATE.ACT
INCLUDE "D2:ALLOCATE.ACT" ;from the Action! Tool Kit. You must type 'SET EndProg=*' from the monitor after compiling, but before running this program!
DEFINE PTR="CARD" DEFINE NODE_SIZE="3" TYPE StackNode=[BYTE data PTR nxt]
StackNode POINTER stack
BYTE FUNC IsEmpty()
IF stack=0 THEN RETURN (1) FI
RETURN (0)
PROC Push(BYTE v)
StackNode POINTER node
node=Alloc(NODE_SIZE) node.data=v node.nxt=stack stack=node
RETURN
BYTE FUNC Pop()
StackNode POINTER node
BYTE v
IF IsEmpty() THEN
PrintE("Error stack is empty!")
Break()
FI
node=stack v=node.data stack=node.nxt Free(node,NODE_SIZE)
RETURN (v)
PROC TestIsEmpty()
IF IsEmpty() THEN
PrintE("Stack is empty")
ELSE
PrintE("Stack is not empty")
FI
RETURN
PROC TestPush(BYTE v)
PrintF("Push: %B%E",v)
Push(v)
RETURN
PROC TestPop()
BYTE v
Print("Pop: ")
v=Pop()
PrintBE(v)
RETURN
PROC Main()
AllocInit(0) stack=0
Put(125) PutE() ;clear screen
TestIsEmpty() TestPush(10) TestIsEmpty() TestPush(31) TestPop() TestIsEmpty() TestPush(5) TestPop() TestPop() TestPop()
RETURN</lang>
- Output:
Error at the end of program is intentional.
Screenshot from Atari 8-bit computer
Stack is empty Push: 10 Stack is not empty Push: 31 Pop: 31 Stack is not empty Push: 5 Pop: 5 Pop: 10 Pop: Error: stack is empty! RETURN Error: 128
ActionScript
In ActionScript an Array object provides stack functionality. <lang actionscript>var stack:Array = new Array(); stack.push(1); stack.push(2); trace(stack.pop()); // outputs "2" trace(stack.pop()); // outputs "1"</lang>
Ada
This is a generic stack implementation. <lang ada>generic
type Element_Type is private;
package Generic_Stack is
type Stack is private; procedure Push (Item : Element_Type; Onto : in out Stack); procedure Pop (Item : out Element_Type; From : in out Stack); function Create return Stack; Stack_Empty_Error : exception;
private
type Node;
type Stack is access Node;
type Node is record
Element : Element_Type;
Next : Stack := null;
end record;
end Generic_Stack;</lang> <lang ada>with Ada.Unchecked_Deallocation;
package body Generic_Stack is
------------
-- Create --
------------
function Create return Stack is
begin
return (null);
end Create;
---------- -- Push -- ----------
procedure Push(Item : Element_Type; Onto : in out Stack) is
Temp : Stack := new Node;
begin
Temp.Element := Item;
Temp.Next := Onto;
Onto := Temp;
end Push;
--------- -- Pop -- ---------
procedure Pop(Item : out Element_Type; From : in out Stack) is
procedure Free is new Ada.Unchecked_Deallocation(Node, Stack);
Temp : Stack := From;
begin
if Temp = null then
raise Stack_Empty_Error;
end if;
Item := Temp.Element;
From := Temp.Next;
Free(Temp);
end Pop;
end Generic_Stack;</lang>
ALGOL 68
ALGOL 68: Using linked list
ALGOL 68 uses "HEAP" variables for new LINKs in a linked list. Generally ALGOL 68's garbage collector should recover the LINK memory some time after a value is popped.
File: prelude/next_link.a68<lang algol68># -*- coding: utf-8 -*- # CO REQUIRES:
MODE OBJVALUE = ~ # Mode/type of actual obj to be stacked #
END CO
MODE OBJNEXTLINK = STRUCT(
REF OBJNEXTLINK next, OBJVALUE value # ... etc. required #
);
PROC obj nextlink new = REF OBJNEXTLINK:
HEAP OBJNEXTLINK;
PROC obj nextlink free = (REF OBJNEXTLINK free)VOID:
next OF free := obj stack empty # give the garbage collector a BIG hint #</lang>File: prelude/stack_base.a68<lang algol68># -*- coding: utf-8 -*- #
CO REQUIRES:
MODE OBJNEXTLINK = STRUCT( REF OBJNEXTLINK next, OBJVALUE value ); PROC obj nextlink new = REF OBJNEXTLINK: ~, PROC obj nextlink free = (REF OBJNEXTLINK free)VOID: ~
END CO
- actually a pointer to the last LINK, there ITEMs are ADDED, pushed & popped #
MODE OBJSTACK = REF OBJNEXTLINK;
OBJSTACK obj stack empty = NIL;
BOOL obj stack par = FALSE; # make code thread safe # SEMA obj stack sema = LEVEL ABS obj stack par;
- Warning: 1 SEMA for all stacks of type obj, i.e. not 1 SEMA per stack #
PROC obj stack init = (REF OBJSTACK self)REF OBJSTACK:
self := obj stack empty;
- see if the program/coder wants the OBJ problem mended... #
PROC (REF OBJSTACK #self#)BOOL obj stack index error mended
:= (REF OBJSTACK self)BOOL: (abend("obj stack index error"); stop);
PROC on obj stack index error = (REF OBJSTACK self, PROC(REF OBJSTACK #self#)BOOL mended)VOID:
obj stack index error mended := mended;
PROC obj stack push = (REF OBJSTACK self, OBJVALUE obj)REF OBJSTACK:(
IF obj stack par THEN DOWN obj stack sema FI; self := obj nextlink new := (self, obj); IF obj stack par THEN UP obj stack sema FI; self
);
- aliases: define a useful put (+=:) operator... #
OP +=: = (OBJVALUE obj, REF OBJSTACK self)REF OBJSTACK: obj stack push(self, obj);
PROC obj stack pop = (REF OBJSTACK self)OBJVALUE: (
- DOWN obj stack sema; #
IF self IS obj stack empty THEN
IF NOT obj stack index error mended(self) THEN abend("obj stack index error") FI FI;
OBJNEXTLINK old head := self; OBJSTACK new head := next OF self; OBJVALUE out := value OF old head; obj nextlink free(old head); # freeing nextlink, NOT queue! # self := new head;
- UP obj stack sema; #
out
);
PROC obj stack is empty = (REF OBJSTACK self)BOOL:
self IS obj stack empty;
SKIP</lang>File: test/data_stigler_diet.a68<lang algol68># -*- coding: utf-8 -*- # MODE DIETITEM = STRUCT(
STRING food, annual quantity, units, REAL cost
);
- Stigler's 1939 Diet ... #
FORMAT diet item fmt = $g": "g" "g" = $"zd.dd$; []DIETITEM stigler diet = (
("Cabbage", "111","lb.", 4.11),
("Dried Navy Beans", "285","lb.", 16.80),
("Evaporated Milk", "57","cans", 3.84),
("Spinach", "23","lb.", 1.85),
("Wheat Flour", "370","lb.", 13.33),
("Total Annual Cost", "","", 39.93)
)</lang>File: test/stack.a68<lang algol68>#!/usr/bin/a68g --script #
- -*- coding: utf-8 -*- #
MODE OBJVALUE = DIETITEM; PR read "prelude/next_link.a68" PR; PR read "prelude/stack_base.a68" PR;
PR read "test/data_stigler_diet.a68" PR; OBJSTACK example stack; obj stack init(example stack);
FOR i TO UPB stigler diet DO
- obj stack push(example stack, stigler diet[i]) #
stigler diet[i] +=: example stack
OD;
printf($"Items popped in reverse:"l$); WHILE NOT obj stack is empty(example stack) DO
- OR example stack ISNT obj stack empty #
printf((diet item fmt, obj stack pop(example stack), $l$))
OD</lang>
- Output:
Items popped in reverse: Total Annual Cost: = $39.93 Wheat Flour: 370 lb. = $13.33 Spinach: 23 lb. = $ 1.85 Evaporated Milk: 57 cans = $ 3.84 Dried Navy Beans: 285 lb. = $16.80 Cabbage: 111 lb. = $ 4.11
See also: Queue
ALGOL 68: Using FLEX array
An alternative to using a linked list is to use a FLEX array. <lang algol68> MODE DIETITEM = STRUCT (
STRING food, annual quantity, units, REAL cost
);
MODE OBJVALUE = DIETITEM;
- PUSH element to stack #
OP +:= = (REF FLEX[]OBJVALUE stack, OBJVALUE item) VOID:
BEGIN
FLEX[UPB stack + 1]OBJVALUE newstack;
newstack[2:UPB newstack] := stack;
newstack[1] := item;
stack := newstack
END;
OP POP = (REF FLEX[]OBJVALUE stack) OBJVALUE:
IF UPB stack > 0 THEN
OBJVALUE result = stack[1];
stack := stack[2:UPB stack];
result
ELSE
# raise index error; # SKIP
FI;
- Stigler's 1939 Diet ... #
FORMAT diet item fmt = $g": "g" "g" = $"zd.dd$; []DIETITEM stigler diet = (
("Cabbage", "111","lb.", 4.11),
("Dried Navy Beans", "285","lb.", 16.80),
("Evaporated Milk", "57","cans", 3.84),
("Spinach", "23","lb.", 1.85),
("Wheat Flour", "370","lb.", 13.33),
("Total Annual Cost", "","", 39.93)
);
FLEX[0]DIETITEM example stack;
FOR i TO UPB stigler diet DO
example stack +:= stigler diet[i]
OD;
printf($"Items popped in reverse:"l$); WHILE UPB example stack > 0 DO
printf((diet item fmt, POP example stack, $l$))
OD </lang>
- Output:
Items popped in reverse: Total Annual Cost: = $39.93 Wheat Flour: 370 lb. = $13.33 Spinach: 23 lb. = $ 1.85 Evaporated Milk: 57 cans = $ 3.84 Dried Navy Beans: 285 lb. = $16.80 Cabbage: 111 lb. = $ 4.11
ALGOL W
<lang algolw>begin
% define a Stack type that will hold StringStackElements %
% and the StringStackElement type %
% we would need separate types for other element types %
record StringStack ( reference(StringStackElement) top );
record StringStackElement ( string(8) element
; reference(StringStackElement) next
);
% adds e to the end of the StringStack s %
procedure pushString ( reference(StringStack) value s
; string(8) value e
) ;
top(s) := StringStackElement( e, top(s) );
% removes and returns the top element from the StringStack s %
% asserts the Stack is not empty, which will stop the %
% program if it is %
string(8) procedure popString ( reference(StringStack) value s ) ;
begin
string(8) v;
assert( not isEmptyStringStack( s ) );
v := element(top(s));
top(s):= next(top(s));
v
end popStringStack ;
% returns the top element of the StringStack s %
% asserts the Stack is not empty, which will stop the %
% program if it is %
string(8) procedure peekStringStack ( reference(StringStack) value s ) ;
begin
assert( not isEmptyStringStack( s ) );
element(top(s))
end popStringStack ;
% returns true if the StringStack s is empty, false otherwise %
logical procedure isEmptyStringStack ( reference(StringStack) value s ) ; top(s) = null;
begin % test the StringStack operations %
reference(StringStack) s;
s := StringStack( null );
pushString( s, "up" );
pushString( s, "down" );
pushString( s, "strange" );
pushString( s, "charm" );
while not isEmptyStringStack( s ) do write( popString( s )
, if isEmptyStringStack( s ) then "(empty)"
else peekStringStack( s )
)
end
end.</lang>
- Output:
charm strange strange down down up up (empty)
Applesoft BASIC
<lang basic>100 DIM STACK$(1000) 110 DATA "(2*A)","PI","","TO BE OR","NOT TO BE" 120 FOR I = 1 TO 5 130 READ ELEMENT$ 140 GOSUB 500_PUSH 150 NEXT 200 GOSUB 400 POP AND PRINT 210 GOSUB 300_EMPTY AND PRINT 220 FOR I = 1 TO 4 230 GOSUB 400 POP AND PRINT 240 NEXT 250 GOSUB 300_EMPTY AND PRINT 260 END 300 GOSUB 700_EMPTY 310 PRINT "STACK IS "; 320 IF NOT EMPTY THEN PRINT "NOT "; 330 PRINT "EMPTY" 340 RETURN 400 GOSUB 600 POP 410 PRINT ELEMENT$ 420 RETURN 500 REM 510 REM PUSH 520 REM 530 LET STACK$(SP) = ELEMENT$ 540 LET SP = SP + 1 550 RETURN 600 REM 610 REM POP 620 REM 630 IF SP THEN SP = SP - 1 640 LET ELEMENT$ = STACK$(SP) 650 LET STACK$(SP) = "" 660 RETURN 700 REM 710 REM EMPTY 720 REM 730 LET EMPTY = SP = 0 740 RETURN </lang>
- Output:
NOT TO BE STACK IS NOT EMPTY TO BE OR PI (2*A) STACK IS EMPTY
ARM Assembly
The stack is held in register 13, or r13 but more commonly referred to as SP for clarity.
Pushing and popping multiple values is very similar to 68000 Assembly. <lang ARM Assembly>STMFD sp!,{r0-r12,lr} ;push r0 thru r12 and the link register LDMFD sp!,{r0-r12,pc} ;pop r0 thru r12, and the value that was in the link register is put into the program counter.
;This acts as a pop and return command all-in-one. (Most programs use bx lr to return.)</lang>
Like in 68000 Assembly, you are not limited to using SP as the source/destination for these commands; any register can fulfill that role. If you wish to have multiple stacks, then so be it.
The stack pointer will work with any operation the other registers can. As such, a peek can be done by using an LDR with the stack pointer as the address register:
<lang ARM Assembly>LDR r0,[sp] ;load the top of the stack into r0</lang>
The order in which registers are pushed/popped is always the same, no matter which order you list the registers in your source code. If you want to push some registers and purposefully pop them into different registers, you'll need to push/pop them separately.
A check if the stack is empty is also very simple, provided the initial value of the stack pointer was saved at the start of the program, or (more likely) was loaded from a nearby memory location.
<lang ARM Assembly>;this example uses VASM syntax which considers a "word" to be 16-bit regardless of the architecture InitStackPointer: .long 0x3FFFFFFF ;other assemblers would call this a "word"
MOV R1,#InitStackPointer LDR SP,[R1] ;set up the stack pointer LDR R2,[R1] ;also load it into R2
- There's no point in checking since we haven't pushed/popped anything but just for demonstration purposes we'll check now
CMP SP,R2 BEQ StackIsEmpty</lang>
In THUMB mode, the PUSH and POP commands replace STMFD and LDMFD. They work in a similar fashion, but are limited to just the stack unlike the real STMFD and LDMFD commands which can use any register as the "stack pointer."
Arturo
<lang rebol>Stack: $[]-> []
pushTo: function [st val]-> 'st ++ val popStack: function [st] [
result: last st remove 'st .index (size st)-1 return result
] emptyStack: function [st]-> empty 'st printStack: function [st]-> print st
st: new Stack
pushTo st "one" pushTo st "two" pushTo st "three" printStack st
print popStack st printStack st
emptyStack st print ["finally:" st]</lang>
- Output:
one two three three one two finally: []
ATS
<lang ATS>(* Stacks implemented as linked lists. *)
(* A nonlinear stack type of size n, which is good for when you are
using a garbage collector or can let the memory leak. *)
typedef stack_t (t : t@ype+, n : int) = list (t, n) typedef stack_t (t : t@ype+) = [n : int] stack_t (t, n)
(* A linear stack type of size n, which requires (and will enforce)
explicit freeing. (Note that a "peek" function for a linear stack is a complicated topic. But the task avoids this issue.) *)
viewtypedef stack_vt (vt : vt@ype+, n : int) = list_vt (vt, n) viewtypedef stack_vt (vt : vt@ype+) = [n : int] stack_vt (vt, n)
(* Proof that a given nonlinear stack does not have a nonnegative
size. *)
prfn lemma_stack_t_param {n : int} {t : t@ype}
(stack : stack_t (t, n)) :<prf> [0 <= n] void = lemma_list_param stack
(* Proof that a given linear stack does not have a nonnegative
size. *)
prfn lemma_stack_vt_param {n : int} {vt : vt@ype}
(stack : !stack_vt (vt, n)) :<prf> [0 <= n] void = lemma_list_vt_param stack
(* Create an empty nonlinear stack. *) fn {} stack_t_nil {t : t@ype} () :<> stack_t (t, 0) =
list_nil ()
(* Create an empty linear stack. *) fn {} stack_vt_nil {vt : vt@ype} () :<> stack_vt (vt, 0) =
list_vt_nil ()
(* Is a nonlinear stack empty? *) fn {} stack_t_is_empty {n : int} {t : t@ype}
(stack : stack_t (t, n)) :<> [empty : bool | empty == (n == 0)] bool empty = case+ stack of | list_nil _ => true | list_cons _ => false
(* Is a linear stack empty? *) fn {} stack_vt_is_empty {n : int} {vt : vt@ype}
(* ! = pass by value; stack is preserved. *)
(stack : !stack_vt (vt, n)) :<>
[empty : bool | empty == (n == 0)]
bool empty =
case+ stack of
| list_vt_nil _ => true
| list_vt_cons _ => false
(* Push to a nonlinear stack that is stored in a variable. *) fn {t : t@ype} stack_t_push {n : int}
(stack : &stack_t (t, n) >> stack_t (t, m),
x : t) :<!wrt>
(* It is proved that the stack is raised one higher. *)
#[m : int | 1 <= m; m == n + 1]
void =
let
prval _ = lemma_stack_t_param stack
prval _ = prop_verify {0 <= n} ()
in
stack := list_cons (x, stack)
end
(* Push to a linear stack that is stored in a variable. Beware: if x
is linear, it is consumed. *)
fn {vt : vt@ype} stack_vt_push {n : int}
(stack : &stack_vt (vt, n) >> stack_vt (vt, m),
x : vt) :<!wrt>
(* It is proved that the stack is raised one higher. *)
#[m : int | 1 <= m; m == n + 1]
void =
let
prval _ = lemma_stack_vt_param stack
prval _ = prop_verify {0 <= n} ()
in
stack := list_vt_cons (x, stack)
end
(* Pop from a nonlinear stack that is stored in a variable. It is
impossible (unless you cheat the typechecker) to pop from an empty stack. *)
fn {t : t@ype} stack_t_pop {n : int | 1 <= n}
(stack : &stack_t (t, n) >> stack_t (t, m)) :<!wrt>
(* It is proved that the stack is lowered by one. *)
#[m : int | m == n - 1]
t =
case+ stack of
| list_cons (x, tail) =>
begin
stack := tail;
x
end
(* Pop from a linear stack that is stored in a variable. It is
impossible (unless you cheat the typechecker) to pop from an empty stack. *)
fn {vt : vt@ype} stack_vt_pop {n : int | 1 <= n}
(stack : &stack_vt (vt, n) >> stack_vt (vt, m)) :<!wrt>
(* It is proved that the stack is lowered by one. *)
#[m : int | m == n - 1]
vt =
case+ stack of
| ~ list_vt_cons (x, tail) => (* ~ = the top node is consumed. *)
begin
stack := tail;
x
end
(* A linear stack has to be consumed. *) extern fun {vt : vt@ype} stack_vt_free$element_free (x : vt) :<> void fn {vt : vt@ype} stack_vt_free {n : int}
(stack : stack_vt (vt, n)) :<> void =
let
fun
loop {m : int | 0 <= m}
.<m>. (* <-- proof of loop termination *)
(stk : stack_vt (vt, m)) :<> void =
case+ stk of
| ~ list_vt_nil () => begin end
| ~ list_vt_cons (x, tail) =>
begin
stack_vt_free$element_free x;
loop tail
end
prval _ = lemma_stack_vt_param stack in loop stack end
implement main0 () =
let var nonlinear_stack : stack_t (int) = stack_t_nil () var linear_stack : stack_vt (int) = stack_vt_nil () implement stack_vt_free$element_free<int> x = begin end
overload is_empty with stack_t_is_empty overload is_empty with stack_vt_is_empty
overload push with stack_t_push overload push with stack_vt_push
overload pop with stack_t_pop
overload pop with stack_vt_pop
in
println! ("nonlinear_stack is empty? ", is_empty nonlinear_stack);
println! ("linear_stack is empty? ", is_empty linear_stack);
println! ("pushing 3, 2, 1...");
push (nonlinear_stack, 3);
push (nonlinear_stack, 2);
push (nonlinear_stack, 1);
push (linear_stack, 3);
push (linear_stack, 2);
push (linear_stack, 1);
println! ("nonlinear_stack is empty? ", is_empty nonlinear_stack);
println! ("linear_stack is empty? ", is_empty linear_stack);
println! ("popping nonlinear_stack: ", (pop nonlinear_stack) : int);
println! ("popping nonlinear_stack: ", (pop nonlinear_stack) : int);
println! ("popping nonlinear_stack: ", (pop nonlinear_stack) : int);
println! ("popping linear_stack: ", (pop linear_stack) : int);
println! ("popping linear_stack: ", (pop linear_stack) : int);
println! ("popping linear_stack: ", (pop linear_stack) : int);
println! ("nonlinear_stack is empty? ", is_empty nonlinear_stack);
println! ("linear_stack is empty? ", is_empty linear_stack);
stack_vt_free<int> linear_stack end</lang>
- Output:
$ patscc -O2 -DATS_MEMALLOC_LIBC stack-postiats.dats && ./a.out nonlinear_stack is empty? true linear_stack is empty? true pushing 3, 2, 1... nonlinear_stack is empty? false linear_stack is empty? false popping nonlinear_stack: 1 popping nonlinear_stack: 2 popping nonlinear_stack: 3 popping linear_stack: 1 popping linear_stack: 2 popping linear_stack: 3 nonlinear_stack is empty? true linear_stack is empty? true
AutoHotkey
<lang AutoHotkey>msgbox % stack("push", 4) msgbox % stack("push", 5) msgbox % stack("peek") msgbox % stack("pop") msgbox % stack("peek") msgbox % stack("empty") msgbox % stack("pop") msgbox % stack("empty") return
stack(command, value = 0) {
static
if !pointer pointer = 10000
if (command = "push")
{
_p%pointer% := value
pointer -= 1
return value
}
if (command = "pop")
{
pointer += 1
return _p%pointer%
}
if (command = "peek")
{ next := pointer + 1 return _p%next% }
if (command = "empty")
{
if (pointer == 10000)
return "empty"
else return 0
}
}</lang>
AWK
<lang awk>function deque(arr) {
arr["start"] = 0 arr["end"] = 0
}
function dequelen(arr) {
return arr["end"] - arr["start"]
}
function empty(arr) {
return dequelen(arr) == 0
}
function push(arr, elem) {
arr[++arr["end"]] = elem
}
function pop(arr) {
if (empty(arr)) {
return
}
return arr[arr["end"]--]
}
function unshift(arr, elem) {
arr[arr["start"]--] = elem
}
function shift(arr) {
if (empty(arr)) {
return
}
return arr[++arr["start"]]
}
function peek(arr) {
if (empty(arr)) {
return
}
return arr[arr["end"]]
}
function printdeque(arr, i, sep) {
printf("[")
for (i = arr["start"] + 1; i <= arr["end"]; i++) {
printf("%s%s", sep, arr[i])
sep = ", "
}
printf("]\n")
}
BEGIN {
deque(q)
for (i = 1; i <= 10; i++) {
push(q, i)
}
printdeque(q)
for (i = 1; i <= 10; i++) {
print pop(q)
}
printdeque(q)
}</lang>
Axe
<lang axe>0→S Lbl PUSH r₁→{L₁+S}ʳ S+2→S Return
Lbl POP S-2→S {L₁+S}ʳ Return
Lbl EMPTY S≤≤0 Return</lang>
Babel
<lang babel>main :
{ (1 2 3) foo set -- foo = (1 2 3)
4 foo push -- foo = (1 2 3 4)
0 foo unshift -- foo = (0 1 2 3 4)
foo pop -- foo = (0 1 2 3)
foo shift -- foo = (1 2 3)
check_foo
{ foo pop } 4 times -- Pops too many times, but this is OK and Babel won't complain
check_foo }
empty? : nil? -- just aliases 'empty?' to the built-in operator 'nil?'
check_foo! :
{ "foo is "
{foo empty?) {nil} {"not " .} ifte
"empty" .
cr << }
</lang>
- Output:
foo is not empty foo is empty
Batch File
This implementation uses an environment variable naming convention to implement a stack as a pseudo object containing a pseudo dynamic array and top attribute, as well as an empty "method" that is a sort of macro. The implementation depends on delayed expansion being enabled at the time of each call to a stack function. More complex variations can be written that remove this limitation. <lang dos>@echo off setlocal enableDelayedExpansion
- LIFO stack usage
- Define the stack
call :newStack myStack
- Push some values onto the stack
for %%A in (value1 value2 value3) do call :pushStack myStack %%A
- Test if stack is empty by examining the top "attribute"
if myStack.top==0 (echo myStack is empty) else (echo myStack is NOT empty)
- Peek at the top stack value
call:peekStack myStack val && echo a peek at the top of myStack shows !val!
- Pop the top stack value
call :popStack myStack val && echo popped myStack value=!val!
- Push some more values onto the stack
for %%A in (value4 value5 value6) do call :pushStack myStack %%A
- Process the remainder of the stack
- processStack
call :popStack myStack val || goto :stackEmpty echo popped myStack value=!val! goto :processStack
- stackEmpty
- Test if stack is empty using the empty "method"/"macro". Use of the
- second IF statement serves to demonstrate the negation of the empty
- "method". A single IF could have been used with an ELSE clause instead.
if %myStack.empty% echo myStack is empty if not %myStack.empty% echo myStack is NOT empty exit /b
- LIFO stack definition
- newStack stackName
set /a %~1.top=0
- Define an empty "method" for this stack as a sort of macro
set "%~1.empty=^!%~1.top^! == 0" exit /b
- pushStack stackName value
set /a %~1.top+=1 set %~1.!%~1.top!=%2 exit /b
- popStack stackName returnVar
- Sets errorlevel to 0 if success
- Sets errorlevel to 1 if failure because stack was empty
if !%~1.top! equ 0 exit /b 1 for %%N in (!%~1.top!) do (
set %~2=!%~1.%%N! set %~1.%%N=
) set /a %~1.top-=1 exit /b 0
- peekStack stackName returnVar
- Sets errorlevel to 0 if success
- Sets errorlevel to 1 if failure because stack was empty
if !%~1.top! equ 0 exit /b 1 for %%N in (!%~1.top!) do set %~2=!%~1.%%N! exit /b 0</lang>
BBC BASIC
<lang bbcbasic> STACKSIZE = 1000
FOR n = 3 TO 5
PRINT "Push ";n : PROCpush(n)
NEXT
PRINT "Pop " ; FNpop
PRINT "Push 6" : PROCpush(6)
REPEAT
PRINT "Pop " ; FNpop
UNTIL FNisempty
PRINT "Pop " ; FNpop
END
DEF PROCpush(n) : LOCAL f%
DEF FNpop : LOCAL f% : f% = 1
DEF FNisempty : LOCAL f% : f% = 2
PRIVATE stack(), sptr%
DIM stack(STACKSIZE-1)
CASE f% OF
WHEN 0:
IF sptr% = DIM(stack(),1) ERROR 100, "Error: stack overflowed"
stack(sptr%) = n
sptr% += 1
WHEN 1:
IF sptr% = 0 ERROR 101, "Error: stack empty"
sptr% -= 1
= stack(sptr%)
WHEN 2:
= (sptr% = 0)
ENDCASE
ENDPROC</lang>
- Output:
Push 3 Push 4 Push 5 Pop 5 Push 6 Pop 6 Pop 4 Pop 3 Pop Error: stack empty
beeswax
Beeswax is a stack-based language. The instruction pointers (bees) carry small local stacks (lstacks) of fixed length 3 that can interact with the global stack (gstack) of unrestricted length. The local stacks do not behave exactly like the stack specified in this task, but the global stack does.
Push (1): f pushes the topmost value of lstack on gstack.
instruction: _f
gstack: UInt64[0]• (at the beginning of a program lstack is initialized to [0 0 0]
Push (2): e pushes all three lstack values on gstack, in reversed order.
instruction: _e
gstack: UInt64[0 0 0]• (at the beginning of a program lstack is initialized to [0 0 0]
Push (3): i pushes an integer from STDIN as UInt64 value on gstack.
instruction: _i
input: i123
gstack: UInt64[123]•
Push (4): c pushes the Unicode codepoint value of a character from STDIN as UInt64 value on gstack.
instruction: _c
input: cH
gstack: UInt64[72]•
Push (5): V pushes the Unicode codepoint values of the characters of a string given at STDIN as UInt64 values on gstack, last character, followed by newline on top.
instruction: _V
input: sHello, α∀
gstack: UInt64[72 101 108 108 111 44 32 945 8704 10]•
Pop: g{? reads the top value of gstack and stores it on top of lstack. Then outputs top value of lstack to STDOUT and finally pops gstack.
Empty: Ag?';`gstack is empty` pushes length of gstack on gstack, reads top value of gstack, stores it as top value of lstack and prints gstack is empty if lstack top=0.
Top: g{ reads the top value of gstack, stores it on top of lstack. Then outputs top value of lstack to STDOUT. If gstack is empty, this instruction does not do anything but return the topmost value of lstack.
To make sure that there is any value on gstack, you would need to check for gstack length first, using the method shown in the “Empty” example above:
*Ag'{`gstack empty, no value to return`
This method returns the top value of gstack only if gstack is not empty, otherwise it outputs gstack empty, no value to return to STDOUT.
BQN
Representing the stack as an array, pushing is appending, popping is removing the last element, and checking emptiness is checking the length.
<lang bqn> Push ← ∾ ∾
Pop ← ¯1⊸↓
¯1⊸↓
Empty ← 0=≠
0=≠
1‿2‿3 Push 4
⟨ 1 2 3 4 ⟩
Pop 1‿2‿3
⟨ 1 2 ⟩
Empty 1‿2‿3
0
Empty ⟨⟩
1</lang>
Bracmat
A stack is easiest implemented as a dotted list top.top-1.top-2.[...].. In the example below we also introduce a 'class' stack, instantiated in the 'object' Stack. The class has a member variable S and methods push,pop, top and empty. As a side note, . is to .. as C's . is to ->. In a method's body, its refers to the object itself. (Analogous to (*this) in C++.)
<lang bracmat>( ( stack
= (S=)
(push=.(!arg.!(its.S)):?(its.S))
( pop
= top.!(its.S):(%?top.?(its.S))&!top
)
(top=top.!(its.S):(%?top.?)&!top)
(empty=.!(its.S):)
)
& new$stack:?Stack & (Stack..push)$(2*a) & (Stack..push)$pi & (Stack..push)$ & (Stack..push)$"to be or" & (Stack..push)$"not to be" & out$((Stack..pop)$|"Cannot pop (a)") & out$((Stack..top)$|"Cannot pop (b)") & out$((Stack..pop)$|"Cannot pop (c)") & out$((Stack..pop)$|"Cannot pop (d)") & out$((Stack..pop)$|"Cannot pop (e)") & out$((Stack..pop)$|"Cannot pop (f)") & out$((Stack..pop)$|"Cannot pop (g)") & out$((Stack..pop)$|"Cannot pop (h)") & out
$ ( str
$ ( "Stack is "
((Stack..empty)$&|not)
" empty"
)
)
& );</lang>
- Output:
not to be to be or to be or pi 2*a Cannot pop (g) Cannot pop (h) Stack is empty
Brat
Built in arrays have push, pop, and empty? methods:
<lang Brat>stack = [] stack.push 1 stack.push 2 stack.push 3
until { stack.empty? } { p stack.pop }</lang>
- Output:
3 2 1
C
Macro expanding to type flexible stack routines. <lang c>#include <stdio.h>
- include <stdlib.h>
/* to read expanded code, run through cpp | indent -st */
- define DECL_STACK_TYPE(type, name) \
typedef struct stk_##name##_t{type *buf; size_t alloc,len;}*stk_##name; \ stk_##name stk_##name##_create(size_t init_size) { \ stk_##name s; if (!init_size) init_size = 4; \ s = malloc(sizeof(struct stk_##name##_t)); \ if (!s) return 0; \ s->buf = malloc(sizeof(type) * init_size); \ if (!s->buf) { free(s); return 0; } \ s->len = 0, s->alloc = init_size; \ return s; } \ int stk_##name##_push(stk_##name s, type item) { \ type *tmp; \ if (s->len >= s->alloc) { \ tmp = realloc(s->buf, s->alloc*2*sizeof(type)); \ if (!tmp) return -1; s->buf = tmp; \ s->alloc *= 2; } \ s->buf[s->len++] = item; \ return s->len; } \ type stk_##name##_pop(stk_##name s) { \ type tmp; \ if (!s->len) abort(); \ tmp = s->buf[--s->len]; \ if (s->len * 2 <= s->alloc && s->alloc >= 8) { \ s->alloc /= 2; \ s->buf = realloc(s->buf, s->alloc * sizeof(type));} \ return tmp; } \ void stk_##name##_delete(stk_##name s) { \ free(s->buf); free(s); }
- define stk_empty(s) (!(s)->len)
- define stk_size(s) ((s)->len)
DECL_STACK_TYPE(int, int)
int main(void) { int i; stk_int stk = stk_int_create(0);
printf("pushing: "); for (i = 'a'; i <= 'z'; i++) { printf(" %c", i); stk_int_push(stk, i); }
printf("\nsize now: %d", stk_size(stk)); printf("\nstack is%s empty\n", stk_empty(stk) ? "" : " not");
printf("\npoppoing:"); while (stk_size(stk)) printf(" %c", stk_int_pop(stk)); printf("\nsize now: %d", stk_size(stk)); printf("\nstack is%s empty\n", stk_empty(stk) ? "" : " not");
/* stk_int_pop(stk); <-- will abort() */ stk_int_delete(stk); return 0; }</lang>
Or
<lang c>#include <stdio.h>
- include <stdlib.h>
- include <stddef.h>
- include <stdbool.h>
- define check_pointer(p) if (!p) {puts("Out of memory."); exit(EXIT_FAILURE);}
- define MINIMUM_SIZE 1
/* Minimal stack size (expressed in number of elements) for which space is allocated. It should be at least 1. */
- define GROWTH_FACTOR 2
/* How much more memory is allocated each time a stack grows out of its allocated segment. */
typedef int T;
// The type of the stack elements.
typedef struct
{T *bottom;
T *top;
T *allocated_top;} stack;
stack * new(void) /* Creates a new stack. */
{stack *s = malloc(sizeof(stack));
check_pointer(s);
s->bottom = malloc(MINIMUM_SIZE * sizeof(T));
check_pointer(s->bottom);
s->top = s->bottom - 1;
s->allocated_top = s->bottom + MINIMUM_SIZE - 1;
return s;}
void destroy(stack *s) /* Frees all the memory used for a stack. */
{free(s->bottom);
free(s);}
bool empty(stack *s) /* Returns true iff there are no elements on the stack. This is different from the stack not having enough memory reserved for even one element, which case is never allowed to arise. */
{return s->top < s->bottom ? true : false;}
void push(stack *s, T x) /* Puts a new element on the stack, enlarging the latter's memory allowance if necessary. */
{if (s->top == s->allocated_top)
{ptrdiff_t qtty = s->top - s->bottom + 1;
ptrdiff_t new_qtty = GROWTH_FACTOR * qtty;
s->bottom = realloc(s->bottom, new_qtty * sizeof(T));
check_pointer(s->bottom);
s->top = s->bottom + qtty - 1;
s->allocated_top = s->bottom + new_qtty - 1;}
*(++s->top) = x;}
T pop(stack *s) /* Removes and returns the topmost element. The result of popping an empty stack is undefined. */
{return *(s->top--);}
void compress(stack *s) /* Frees any memory the stack isn't actually using. The allocated portion still isn't allowed to shrink smaller than MINIMUM_SIZE. If all the stack's memory is in use, nothing happens. */
{if (s->top == s->allocated_top) return;
ptrdiff_t qtty = s->top - s->bottom + 1;
if (qtty < MINIMUM_SIZE) qtty = MINIMUM_SIZE;
size_t new_size = qtty * sizeof(T);
s->bottom = realloc(s->bottom, new_size);
check_pointer(s->bottom);
s->allocated_top = s->bottom + qtty - 1;}</lang>
C#
<lang csharp>// Non-Generic Stack System.Collections.Stack stack = new System.Collections.Stack(); stack.Push( obj ); bool isEmpty = stack.Count == 0; object top = stack.Peek(); // Peek without Popping. top = stack.Pop();
// Generic Stack System.Collections.Generic.Stack<Foo> stack = new System.Collections.Generic.Stack<Foo>(); stack.Push(new Foo()); bool isEmpty = stack.Count == 0; Foo top = stack.Peek(); // Peek without Popping. top = stack.Pop();</lang>
C++
The C++ standard library already provides a ready-made stack class. You get it by writing <lang cpp>#include <stack></lang> and then using the std::stack class.
An example of an explicit implementation of a stack class (which actually implements the standard stack class, except that the standard one is in namespace std): <lang cpp>#include <deque> template <class T, class Sequence = std::deque<T> > class stack {
friend bool operator== (const stack&, const stack&); friend bool operator< (const stack&, const stack&);
public:
typedef typename Sequence::value_type value_type; typedef typename Sequence::size_type size_type; typedef Sequence container_type; typedef typename Sequence::reference reference; typedef typename Sequence::const_reference const_reference;
protected:
Sequence seq;
public:
stack() : seq() {}
explicit stack(const Sequence& s0) : seq(s0) {}
bool empty() const { return seq.empty(); }
size_type size() const { return seq.size(); }
reference top() { return seq.back(); }
const_reference top() const { return seq.back(); }
void push(const value_type& x) { seq.push_back(x); }
void pop() { seq.pop_back(); }
};
template <class T, class Sequence> bool operator==(const stack<T,Sequence>& x, const stack<T,Sequence>& y) {
return x.seq == y.seq;
} template <class T, class Sequence> bool operator<(const stack<T,Sequence>& x, const stack<T,Sequence>& y) {
return x.seq < y.seq;
}
template <class T, class Sequence> bool operator!=(const stack<T,Sequence>& x, const stack<T,Sequence>& y) {
return !(x == y);
} template <class T, class Sequence> bool operator>(const stack<T,Sequence>& x, const stack<T,Sequence>& y) {
return y < x;
} template <class T, class Sequence> bool operator<=(const stack<T,Sequence>& x, const stack<T,Sequence>& y) {
return !(y < x);
} template <class T, class Sequence> bool operator>=(const stack<T,Sequence>& x, const stack<T,Sequence>& y) {
return !(x < y);
}</lang>
Clojure
As is mentioned in the Common Lisp example below, built in cons-based lists work just fine. In this implementation, the list is wrapped in a datatype, providing a stateful solution. <lang lisp>(deftype Stack [elements])
(def stack (Stack (ref ())))
(defn push-stack
"Pushes an item to the top of the stack." [x] (dosync (alter (:elements stack) conj x)))
(defn pop-stack
"Pops an item from the top of the stack."
[] (let [fst (first (deref (:elements stack)))]
(dosync (alter (:elements stack) rest)) fst))
(defn top-stack
"Shows what's on the top of the stack." [] (first (deref (:elements stack))))
(defn empty-stack?
"Tests whether or not the stack is empty." [] (= () (deref (:elements stack))))</lang>
We can make this a bit smaller and general by using defprotocol along with deftype. Here is a revised version using defprotocol.
<lang lisp>(defprotocol StackOps
(push-stack [this x] "Pushes an item to the top of the stack.") (pop-stack [this] "Pops an item from the top of the stack.") (top-stack [this] "Shows what's on the top of the stack.") (empty-stack? [this] "Tests whether or not the stack is empty."))
(deftype Stack [elements]
StackOps (push-stack [x] (dosync (alter elements conj x))) (pop-stack [] (let [fst (first (deref elements))]
(dosync (alter elements rest)) fst))
(top-stack [] (first (deref elements))) (empty-stack? [] (= () (deref elements))))
(def stack (Stack (ref ())))</lang>
CLU
<lang clu>% Stack stack = cluster [T: type] is new, push, pop, peek, empty
rep = array[T]
new = proc () returns (cvt)
return (rep$new())
end new
empty = proc (s: cvt) returns (bool)
return (rep$size(s) = 0)
end empty;
push = proc (s: cvt, val: T)
rep$addh(s, val)
end push;
pop = proc (s: cvt) returns (T) signals (empty)
if rep$empty(s)
then signal empty
else return(rep$remh(s))
end
end pop
peek = proc (s: cvt) returns (T) signals (empty)
if rep$empty(s)
then signal empty
else return(s[rep$high(s)])
end
end peek
end stack
start_up = proc ()
po: stream := stream$primary_output()
% Make a stack
s: stack[int] := stack[int]$new()
% Push 1..10 onto the stack
for i: int in int$from_to(1, 10) do
stack[int]$push(s, i)
end
% Pop items off the stack until the stack is empty
while ~stack[int]$empty(s) do
stream$putl(po, int$unparse(stack[int]$pop(s)))
end
% Trying to pop off the stack now should raise 'empty'
begin
i: int := stack[int]$pop(s)
stream$putl(po, "Still here! And I got: " || int$unparse(i))
end except when empty:
stream$putl(po, "The stack is empty.")
end
end start_up</lang>
- Output:
10 9 8 7 6 5 4 3 2 1 The stack is empty.
COBOL
Based loosely on the C stack implementation in Evangel Quiwa's Data Structures.
This example (ab)uses the COPY procedure to ensure that there is a consistently-defined stack type, node type, node information type, p(redicate) type, and set of stack-utilities.
stack.cbl <lang COBOL> 01 stack.
05 head USAGE IS POINTER VALUE NULL.
</lang>
node.cbl <lang COBOL> 01 node BASED.
COPY node-info REPLACING
01 BY 05
node-info BY info.
05 link USAGE IS POINTER VALUE NULL.
</lang>
node-info.cbl <lang COBOL> 01 node-info PICTURE X(10) VALUE SPACES. </lang>
p.cbl <lang COBOL> 01 p PICTURE 9.
88 nil VALUE ZERO WHEN SET TO FALSE IS 1.
88 t VALUE 1 WHEN SET TO FALSE IS ZERO.
</lang>
stack-utilities.cbl <lang COBOL> IDENTIFICATION DIVISION.
PROGRAM-ID. push.
DATA DIVISION.
LOCAL-STORAGE SECTION.
COPY p.
COPY node.
LINKAGE SECTION.
COPY stack.
01 node-info-any PICTURE X ANY LENGTH.
PROCEDURE DIVISION USING stack node-info-any.
ALLOCATE node
CALL "pointerp" USING
BY REFERENCE ADDRESS OF node
BY REFERENCE p
END-CALL
IF nil
CALL "stack-overflow-error" END-CALL
ELSE
MOVE node-info-any TO info OF node
SET link OF node TO head OF stack
SET head OF stack TO ADDRESS OF node
END-IF
GOBACK.
END PROGRAM push.
IDENTIFICATION DIVISION.
PROGRAM-ID. pop.
DATA DIVISION.
LOCAL-STORAGE SECTION.
COPY p.
COPY node.
LINKAGE SECTION.
COPY stack.
COPY node-info.
PROCEDURE DIVISION USING stack node-info.
CALL "empty" USING
BY REFERENCE stack
BY REFERENCE p
END-CALL
IF t
CALL "stack-underflow-error" END-CALL
ELSE
SET ADDRESS OF node TO head OF stack
SET head OF stack TO link OF node
MOVE info OF node TO node-info
END-IF
FREE ADDRESS OF node
GOBACK.
END PROGRAM pop.
IDENTIFICATION DIVISION.
PROGRAM-ID. empty.
DATA DIVISION.
LOCAL-STORAGE SECTION.
LINKAGE SECTION.
COPY stack.
COPY p.
PROCEDURE DIVISION USING stack p.
CALL "pointerp" USING
BY CONTENT head OF stack
BY REFERENCE p
END-CALL
IF t
SET t TO FALSE
ELSE
SET t TO TRUE
END-IF
GOBACK.
END PROGRAM empty.
IDENTIFICATION DIVISION.
PROGRAM-ID. head.
DATA DIVISION.
LOCAL-STORAGE SECTION.
COPY p.
COPY node.
LINKAGE SECTION.
COPY stack.
COPY node-info.
PROCEDURE DIVISION USING stack node-info.
CALL "empty" USING
BY REFERENCE stack
BY REFERENCE p
END-CALL
IF t
CALL "stack-underflow-error" END-CALL
ELSE
SET ADDRESS OF node TO head OF stack
MOVE info OF node TO node-info
END-IF
GOBACK.
END PROGRAM head.
IDENTIFICATION DIVISION.
PROGRAM-ID. peek.
DATA DIVISION.
LOCAL-STORAGE SECTION.
LINKAGE SECTION.
COPY stack.
COPY node-info.
PROCEDURE DIVISION USING stack node-info.
CALL "head" USING
BY CONTENT stack
BY REFERENCE node-info
END-CALL
GOBACK.
END PROGRAM peek.
IDENTIFICATION DIVISION.
PROGRAM-ID. pointerp.
DATA DIVISION.
LINKAGE SECTION.
01 test-pointer USAGE IS POINTER.
COPY p.
PROCEDURE DIVISION USING test-pointer p.
IF test-pointer EQUAL NULL
SET nil TO TRUE
ELSE
SET t TO TRUE
END-IF
GOBACK.
END PROGRAM pointerp.
IDENTIFICATION DIVISION.
PROGRAM-ID. stack-overflow-error.
PROCEDURE DIVISION.
DISPLAY "stack-overflow-error" END-DISPLAY
STOP RUN.
END PROGRAM stack-overflow-error.
IDENTIFICATION DIVISION.
PROGRAM-ID. stack-underflow-error.
PROCEDURE DIVISION.
DISPLAY "stack-underflow-error" END-DISPLAY
STOP RUN.
END PROGRAM stack-underflow-error.
IDENTIFICATION DIVISION.
PROGRAM-ID. copy-stack.
DATA DIVISION.
LOCAL-STORAGE SECTION.
COPY p.
COPY node-info.
LINKAGE SECTION.
COPY stack.
COPY stack REPLACING stack BY new-stack.
PROCEDURE DIVISION USING stack new-stack.
CALL "empty" USING
BY REFERENCE stack
BY REFERENCE p
END-CALL
IF nil
CALL "pop" USING
BY REFERENCE stack
BY REFERENCE node-info
END-CALL
CALL "copy-stack" USING
BY REFERENCE stack
BY REFERENCE new-stack
END-CALL
CALL "push" USING
BY REFERENCE stack
BY REFERENCE node-info
END-CALL
CALL "push" USING
BY REFERENCE new-stack
BY REFERENCE node-info
END-CALL
END-IF
GOBACK.
END PROGRAM copy-stack.
IDENTIFICATION DIVISION.
PROGRAM-ID. reverse-stack.
DATA DIVISION.
LOCAL-STORAGE SECTION.
COPY p.
COPY node-info.
LINKAGE SECTION.
COPY stack.
COPY stack REPLACING stack BY new-stack.
PROCEDURE DIVISION USING stack new-stack.
CALL "empty" USING
BY REFERENCE stack
BY REFERENCE p
END-CALL
IF nil
CALL "pop" USING
BY REFERENCE stack
BY REFERENCE node-info
END-CALL
CALL "push" USING
BY REFERENCE new-stack
BY REFERENCE node-info
END-CALL
CALL "reverse-stack" USING
BY REFERENCE stack
BY REFERENCE new-stack
END-CALL
CALL "push" USING
BY REFERENCE stack
BY REFERENCE node-info
END-CALL
END-IF
GOBACK.
END PROGRAM reverse-stack.
IDENTIFICATION DIVISION.
PROGRAM-ID. traverse-stack.
DATA DIVISION.
LOCAL-STORAGE SECTION.
COPY p.
COPY node-info.
COPY stack REPLACING stack BY new-stack.
LINKAGE SECTION.
COPY stack.
PROCEDURE DIVISION USING stack.
CALL "copy-stack" USING
BY REFERENCE stack
BY REFERENCE new-stack
END-CALL
CALL "empty" USING
BY REFERENCE new-stack
BY REFERENCE p
END-CALL
IF nil
CALL "head" USING
BY CONTENT new-stack
BY REFERENCE node-info
END-CALL
DISPLAY node-info END-DISPLAY
CALL "peek" USING
BY CONTENT new-stack
BY REFERENCE node-info
END-CALL
DISPLAY node-info END-DISPLAY
CALL "pop" USING
BY REFERENCE new-stack
BY REFERENCE node-info
END-CALL
DISPLAY node-info END-DISPLAY
CALL "traverse-stack" USING
BY REFERENCE new-stack
END-CALL
END-IF
GOBACK.
END PROGRAM traverse-stack.
</lang>
stack-test.cbl <lang COBOL> IDENTIFICATION DIVISION.
PROGRAM-ID. stack-test.
DATA DIVISION.
LOCAL-STORAGE SECTION.
COPY stack.
COPY stack REPLACING stack BY new-stack.
PROCEDURE DIVISION.
CALL "push" USING
BY REFERENCE stack
BY CONTENT "daleth"
END-CALL
CALL "push" USING
BY REFERENCE stack
BY CONTENT "gimel"
END-CALL
CALL "push" USING
BY REFERENCE stack
BY CONTENT "beth"
END-CALL
CALL "push" USING
BY REFERENCE stack
BY CONTENT "aleph"
END-CALL
CALL "traverse-stack" USING
BY REFERENCE stack
END-CALL
CALL "reverse-stack" USING
BY REFERENCE stack
BY REFERENCE new-stack
END-CALL
CALL "traverse-stack" USING
BY REFERENCE new-stack
END-CALL
STOP RUN.
END PROGRAM stack-test.
COPY stack-utilities.
</lang>
- Output:
aleph aleph beth beth beth gimel gimel gimel daleth daleth daleth daleth daleth daleth gimel gimel gimel beth beth beth aleph aleph aleph
CoffeeScript
<lang CoffeeScript>stack = [] stack.push 1 stack.push 2 console.log stack console.log stack.pop() console.log stack</lang>
Common Lisp
It's a bit unusual to write a wrapper for a stack in Common Lisp; built-in cons-based lists work just fine. Nonetheless, here's an implementation where the list is wrapped in a structure, providing a stateful solution. <lang lisp>(defstruct stack
elements)
(defun stack-push (element stack)
(push element (stack-elements stack)))
(defun stack-pop (stack)(deftype Stack [elements])
(defun stack-empty (stack)
(endp (stack-elements stack)))
(defun stack-top (stack)
(first (stack-elements stack)))
(defun stack-peek (stack)
(stack-top stack))</lang>
Component Pascal
Works with BlackBox Component Builder <lang oberon2> MODULE Stacks; IMPORT StdLog;
TYPE (* some pointers to records *) Object* = POINTER TO ABSTRACT RECORD END;
Integer = POINTER TO RECORD (Object) i: INTEGER END;
Point = POINTER TO RECORD (Object) x,y: REAL END;
Node = POINTER TO LIMITED RECORD next- : Node; data-: ANYPTR; END;
(* Stack *) Stack* = POINTER TO RECORD top- : Node; END;
PROCEDURE (dn: Object) Show*, NEW, ABSTRACT;
PROCEDURE (i: Integer) Show*; BEGIN StdLog.String("Integer(");StdLog.Int(i.i);StdLog.String(");");StdLog.Ln END Show;
PROCEDURE (p: Point) Show*; BEGIN StdLog.String("Point(");StdLog.Real(p.x);StdLog.Char(','); StdLog.Real(p.y);StdLog.String(");");StdLog.Ln END Show;
PROCEDURE (s: Stack) Init, NEW; BEGIN s.top := NIL; END Init;
PROCEDURE (s: Stack) Push*(data: ANYPTR), NEW; VAR n: Node; BEGIN NEW(n);n.next := NIL;n.data := data; IF s.top = NIL THEN s.top := n ELSE n.next := s.top; s.top := n END END Push;
PROCEDURE (s: Stack) Pop*(): ANYPTR, NEW; VAR x: ANYPTR; BEGIN IF s.top # NIL THEN x := s.top.data; s.top := s.top.next ELSE x := NIL END; RETURN x END Pop;
PROCEDURE (s: Stack) Empty*(): BOOLEAN, NEW; BEGIN RETURN s.top = NIL END Empty;
PROCEDURE NewStack*(): Stack; VAR s: Stack; BEGIN NEW(s);s.Init; RETURN s END NewStack;
PROCEDURE NewInteger*(data: INTEGER): Integer; VAR i: Integer; BEGIN NEW(i);i.i := data; RETURN i END NewInteger;
PROCEDURE NewPoint*(x,y: REAL): Point; VAR p: Point; BEGIN NEW(p);p.x := x;p.y := y; RETURN p END NewPoint;
PROCEDURE TestStack*; VAR s: Stack; BEGIN s := NewStack(); s.Push(NewInteger(1)); s.Push(NewPoint(2.0,3.4)); s.Pop()(Object).Show(); s.Pop()(Object).Show(); END TestStack;
END Stacks. </lang>
Execute: ^Q Stacks.TestStack
- Output:
Point( 2.0, 3.4); Integer( 1);
Crystal
<lang ruby>stack = [] of Int32 (1..10).each do |x|
stack.push x
end
10.times do
puts stack.pop
end</lang>
Output:
10 9 8 7 6 5 4 3 2 1
D
Generic stack class implemented with a dynamic array. <lang d>import std.array;
class Stack(T) {
private T[] items;
@property bool empty() { return items.empty(); }
void push(T top) { items ~= top; }
T pop() {
if (this.empty)
throw new Exception("Empty Stack.");
auto top = items.back;
items.popBack();
return top;
}
}
void main() {
auto s = new Stack!int(); s.push(10); s.push(20); assert(s.pop() == 20); assert(s.pop() == 10); assert(s.empty());
}</lang>
Delphi
<lang Delphi>program Stack;
{$APPTYPE CONSOLE}
uses Generics.Collections;
var
lStack: TStack<Integer>;
begin
lStack := TStack<Integer>.Create; try lStack.Push(1); lStack.Push(2); lStack.Push(3); Assert(lStack.Peek = 3); // 3 should be at the top of the stack
Writeln(lStack.Pop); // 3 Writeln(lStack.Pop); // 2 Writeln(lStack.Pop); // 1 Assert(lStack.Count = 0); // should be empty finally lStack.Free; end;
end.</lang>
DWScript
Dynamic arrays have pseudo-methods that allow to treat them as a stack. <lang Delphi> var stack: array of Integer;
stack.Push(1); stack.Push(2); stack.Push(3);
PrintLn(stack.Pop); // 3 PrintLn(stack.Pop); // 2 PrintLn(stack.Pop); // 1
Assert(stack.Length = 0); // assert empty </lang>
Dyalect
<lang dyalect>type Stack() {
var xs = []
}
func Stack.IsEmpty() => this!xs.Length() == 0
func Stack.Peek() => this!xs[this!xs.Length() - 1]
func Stack.Pop() {
var e = this!xs[this!xs.Length() - 1] this!xs.RemoveAt(this!xs.Length() - 1) return e
}
func Stack.Push(item) => this!xs.Add(item)
var stack = Stack() stack.Push(1) stack.Push(2) print(stack.Pop()) print(stack.Peek()) stack.Pop() print(stack.IsEmpty())</lang>
- Output:
2 1 true
Déjà Vu
<lang dejavu>local :stack [] #lists used to be stacks in DV
push-to stack 1 push-to stack 2 push-to stack 3
!. pop-from stack #prints 3 !. pop-from stack #prints 2 !. pop-from stack #prints 1
if stack: #empty lists are falsy
error #this stack should be empty now!</lang>
Diego
Diego has a stack object and posit:
<lang diego>set_ns(rosettacode)_me();
add_stack({int},a)_values(1..4); // 1,2,3,4 (1 is first/bottom, 4 is last/top)
with_stack(a)_pop(); // 1,2,3
with_stack(a)_push()_v(5,6); // 1,2,3,5,6
add_var({int},b)_value(7);
with_stack(a)_push[b]; // 1,2,3,5,6,7
with_stack(a)_pluck()_at(2); // callee will return `with_stack(a)_err(pluck invalid with stack);`
me_msg()_stack(a)_top(); // "7" me_msg()_stack(a)_last(); // "7" me_msg()_stack(a)_peek(); // "7"
me_msg()_stack(a)_bottom(); // "1" me_msg()_stack(a)_first(); // "1" me_msg()_stack(a)_peer(); // "1"
me_msg()_stack(a)_isempty(); // "false" with_stack(a)_empty(); with_stack(a)_msg()_isempty()_me(); // "true" (alternative syntax)
me_msg()_stack(a)_history()_all(); // returns th entire history of stack 'a' since its creation
reset_ns[];</lang>
stack is a derivative of array, so arrays can also be used as stacks.
E
The standard FlexList data structure provides operations for use as a stack. <lang e>? def l := [].diverge()
- value: [].diverge()
? l.push(1) ? l.push(2) ? l
- value: [1, 2].diverge()
? l.pop()
- value: 2
? l.size().aboveZero()
- value: true
? l.last()
- value: 1
? l.pop()
- value: 1
? l.size().aboveZero()
- value: false</lang>
Here's a stack implemented out of a reference to a linked list: <lang e>def makeStack() {
var store := null
def stack {
to push(x) { store := [x, store] }
to pop() { def [x, next] := store; store := next; return x }
to last() { return store[0] }
to empty() { return (store == null) }
}
return stack
}
? def s := makeStack()
- value: <stack>
? s.push(1) ? s.push(2) ? s.last()
- value: 2
? s.pop()
- value: 2
? s.empty()
- value: false
? s.pop()
- value: 1
? s.empty()
- value: true</lang>
EchoLisp
Named stacks are native objects. The following demonstrates the available operations : <lang lisp>
- build stack [0 1 ... 9 (top)] from a list
(list->stack (iota 10) 'my-stack) (stack-top 'my-stack) → 9 (pop 'my-stack) → 9 (stack-top 'my-stack) → 8 (push 'my-stack '🐸) ; any kind of lisp object in the stack (stack-empty? 'my-stack) → #f (stack->list 'my-stack) ; convert stack to list
→ (0 1 2 3 4 5 6 7 8 🐸)
(stack-swap 'my-stack) ; swaps two last items
→ 8 ; new top
(stack->list 'my-stack)
→ (0 1 2 3 4 5 6 7 🐸 8) ; swapped
(while (not (stack-empty? 'my-stack)) (pop 'my-stack)) ; pop until empty (stack-empty? 'my-stack) → #t ; true
(push 'my-stack 7) my-stack ; a stack is not a variable, nor a symbol - cannot be evaluated
⛔ error: #|user| : unbound variable : my-stack
(stack-top 'my-stack) → 7 </lang>
Eiffel
<lang Eiffel> class STACK_ON_ARRAY
create make
feature -- Implementation
empty: BOOLEAN do Result := stack.is_empty ensure empty: Result = (stack.count = 0) end
push (item: ANY) do stack.force (item, stack.count) ensure pushed: stack [stack.upper] = item growth: stack.count = old stack.count + 1 end
pop: ANY require not_empty: not empty do Result := stack.at (stack.upper) stack.remove_tail (1) ensure reduction: stack.count = old stack.count - 1 end
feature {NONE} -- Initialization
stack: ARRAY [ANY]
make do create stack.make_empty end
end </lang>
Elena
<lang elena>public program() {
var stack := new system'collections'Stack(); stack.push:2; var isEmpty := stack.Length == 0; var item := stack.peek(); // Peek without Popping. item := stack.pop()
}</lang>
Elisa
This is a generic Stack component based on arrays. See how in Elisa generic components are defined. <lang Elisa> component GenericStack ( Stack, Element );
type Stack;
Stack (MaxSize = integer) -> Stack;
Empty ( Stack ) -> boolean;
Full ( Stack ) -> boolean;
Push ( Stack, Element) -> nothing;
Pull ( Stack ) -> Element;
begin
Stack(MaxSize) =
Stack:[ MaxSize; index:=0; area=array (Element, MaxSize) ];
Empty( stack ) = (stack.index <= 0);
Full ( stack ) = (stack.index >= stack.MaxSize);
Push ( stack, element ) =
[ exception (Full (stack), "Stack Overflow");
stack.index:=stack.index + 1;
stack.area[stack.index]:=element ];
Pull ( stack ) =
[ exception (Empty (stack), "Stack Underflow");
stack.index:=stack.index - 1;
stack.area[stack.index + 1] ];
end component GenericStack;</lang> Another example of a generic Stack component is based on an unlimited sequence. A sequence is a uni-directional list. See how Elisa defines sequences. The component has the same interface as the array based version. <lang Elisa>component GenericStack ( Stack, ElementType );
type Stack;
Stack(MaxSize = integer) -> Stack;
Empty( Stack ) -> boolean;
Full ( Stack ) -> boolean;
Push ( Stack, ElementType)-> nothing;
Pull ( Stack ) -> ElementType;
begin
type sequence = term;
ElementType & sequence => sequence;
nil = null (sequence);
head (sequence) -> ElementType;
head (X & Y) = ElementType:X;
tail (sequence) -> sequence;
tail (X & Y) = Y;
Stack (Size) = Stack:[ list = nil ];
Empty ( stack ) = (stack.list == nil);
Full ( stack ) = false;
Push ( stack, ElementType ) = [ stack.list:= ElementType & stack.list ];
Pull ( stack ) = [ exception (Empty (stack), "Stack Underflow");
Head = head(stack.list); stack.list:=tail(stack.list); Head];
end component GenericStack;</lang> Both versions give the same answers to the following tests: <lang Elisa>use GenericStack (StackofBooks, Book); type Book = text; BookStack = StackofBooks(50);
Push (BookStack, "Peter Pan"); Push (BookStack, "Alice in Wonderland");
Pull (BookStack)? "Alice in Wonderland"
Pull (BookStack)? "Peter Pan"
Pull (BookStack)?
- Exception: Stack Underflow</lang>
Elixir
<lang elixir>defmodule Stack do
def new, do: []
def empty?([]), do: true
def empty?(_), do: false
def pop([h|t]), do: {h,t}
def push(h,t), do: [h|t]
def top([h|_]), do: h
end</lang>
Example:
iex(2)> stack = Stack.new
[]
iex(3)> Stack.empty?(stack)
true
iex(4)> newstack = List.foldl([1,2,3,4,5], stack, fn x,acc -> Stack.push(x,acc) end)
[5, 4, 3, 2, 1]
iex(5)> Stack.top(newstack)
5
iex(6)> {popped, poppedstack} = Stack.pop(newstack)
{5, [4, 3, 2, 1]}
iex(7)> Stack.empty?(newstack)
false
Erlang
Erlang has no built-in stack, but its lists behave basically the same way. A stack module can be implemented as a simple wrapper around lists: <lang erlang>-module(stack). -export([empty/1, new/0, pop/1, push/2, top/1]).
new() -> [].
empty([]) -> true; empty(_) -> false.
pop([H|T]) -> {H,T}.
push(H,T) -> [H|T].
top([H|_]) -> H.</lang> Note that as Erlang doesn't have mutable data structure (destructive updates), pop returns the popped element and the new stack as a tuple.
The module is tested this way: <lang erlang>1> c(stack). {ok,stack} 2> Stack = stack:new(). [] 3> NewStack = lists:foldl(fun stack:push/2, Stack, [1,2,3,4,5]). [5,4,3,2,1] 4> stack:top(NewStack). 5 5> {Popped, PoppedStack} = stack:pop(NewStack). {5,[4,3,2,1]} 6> stack:empty(NewStack). false 7> stack:empty(stack:new()). true</lang>
F#
.NET provides a mutable stack type in System.Collections.Generic.Stack.
A list-based immutable stack type could be implemented like this: <lang fsharp>type Stack<'a> //'//(workaround for syntax highlighting problem)
(?items) = let items = defaultArg items []
member x.Push(A) = Stack(A::items)
member x.Pop() =
match items with
| x::xr -> (x, Stack(xr))
| [] -> failwith "Stack is empty."
member x.IsEmpty() = items = []
// example usage let anEmptyStack = Stack<int>() let stack2 = anEmptyStack.Push(42) printfn "%A" (stack2.IsEmpty()) let (x, stack3) = stack2.Pop() printfn "%d" x printfn "%A" (stack3.IsEmpty())</lang>
Factor
Factor is a stack based language, but also provides stack "objects", because all resizable sequences can be treated as stacks (see docs). Typically, a vector is used: <lang factor> V{ 1 2 3 } {
[ 6 swap push ] [ "hi" swap push ] [ "Vector is now: " write . ] [ "Let's pop it: " write pop . ] [ "Vector is now: " write . ] [ "Top is: " write last . ] } cleave
Vector is now: V{ 1 2 3 6 "hi" }
Let's pop it: "hi"
Vector is now: V{ 1 2 3 6 }
Top is: 6</lang>
Forth
<lang forth>: stack ( size -- )
create here cell+ , cells allot ;
- push ( n st -- ) tuck @ ! cell swap +! ;
- pop ( st -- n ) -cell over +! @ @ ;
- empty? ( st -- ? ) dup @ - cell+ 0= ;
10 stack st
1 st push 2 st push 3 st push st empty? . \ 0 (false) st pop . st pop . st pop . \ 3 2 1 st empty? . \ -1 (true)</lang>
Fortran
This solution can easily be adapted to data types other than floating point numbers. <lang fortran>module mod_stack
implicit none type node ! data entry in each node real*8, private :: data ! pointer to the next node of the linked list type(node), pointer, private :: next end type node private node
type stack ! pointer to first element of stack. type(node), pointer, private :: first ! size of stack integer, private :: len=0 contains procedure :: pop procedure :: push procedure :: peek procedure :: getSize procedure :: clearStack procedure :: isEmpty end type stack
contains
function pop(this) result(x)
class(stack) :: this
real*8 :: x
type(node), pointer :: tmp
if ( this%len == 0 ) then
print*, "popping from empty stack"
!stop
end if
tmp => this%first
x = this%first%data
this%first => this%first%next
deallocate(tmp)
this%len = this%len -1
end function pop
subroutine push(this, x)
real*8 :: x
class(stack), target :: this
type(node), pointer :: new, tmp
allocate(new)
new%data = x
if (.not. associated(this%first)) then
this%first => new
else
tmp => this%first
this%first => new
this%first%next => tmp
end if
this%len = this%len + 1
end subroutine push
function peek(this) result(x) class(stack) :: this real*8 :: x x = this%first%data end function peek
function getSize(this) result(n) class(stack) :: this integer :: n n = this%len end function getSize
function isEmpty(this) result(empty)
class(stack) :: this
logical :: empty
if ( this%len > 0 ) then
empty = .FALSE.
else
empty = .TRUE.
end if
end function isEmpty
subroutine clearStack(this)
class(stack) :: this
type(node), pointer :: tmp
integer :: i
if ( this%len == 0 ) then
return
end if
do i = 1, this%len
tmp => this%first
if ( .not. associated(tmp)) exit
this%first => this%first%next
deallocate(tmp)
end do
this%len = 0
end subroutine clearStack
end module mod_stack
program main
use mod_stack type(stack) :: my_stack integer :: i real*8 :: dat do i = 1, 5, 1 dat = 1.0 * i call my_stack%push(dat) end do do while ( .not. my_stack%isEmpty() ) print*, my_stack%pop() end do call my_stack%clearStack()
end program main</lang>
Free Pascal
Delphi adaptation
Example taken and adapted from the Delphi entry. <lang pascal>program Stack;
{$IFDEF FPC}{$MODE DELPHI}{$IFDEF WINDOWS}{$APPTYPE CONSOLE}{$ENDIF}{$ENDIF}
{$ASSERTIONS ON}
uses Generics.Collections;
var
lStack: TStack<Integer>;
begin
lStack := TStack<Integer>.Create; try lStack.Push(1); lStack.Push(2); lStack.Push(3); Assert(lStack.Peek = 3); // 3 should be at the top of the stack Write(lStack.Pop:2); // 3 Write(lStack.Pop:2); // 2 Writeln(lStack.Pop:2); // 1 Assert(lStack.Count = 0, 'Stack is not empty'); // should be empty finally lStack.Free; end;
end.</lang>
Output: 3 2 1
Object version from scratch
<lang Pascal> PROGRAM StackObject.pas; {$IFDEF FPC}
{$mode objfpc}{$H+}{$J-}{$m+}{$R+}
{$ELSE}
{$APPTYPE CONSOLE}
{$ENDIF} (*)
Free Pascal Compiler version 3.2.0 [2020/06/14] for x86_64
TheStack free and readable alternative at C/C++ Sidxeeds
compiles natively to almost any platform, including raSidxberry PI *
Can run independently from DELPHI / Lazarus
For debian Linux: apt -y install fpc
It contains a text IDE called fp
This is an experiment for a stack that can handle almost any
simple type of variable.
What happens after retrieving the variable is TBD by you.
(*)
USES
Classes ,
Crt ,
Variants ;
{$WARN 6058 off : Call to subroutine "$1" marked as inline is not inlined} // Use for variants
TYPE
Stack = OBJECT
CONST
CrLf = #13#10 ;
TYPE
VariantArr = array of variant ;
PRIVATE
Ar : VariantArr ;
{$MACRO ON}
{$DEFINE STACKSIZE := Length ( Ar ) * Ord ( Length ( Ar ) > 0 ) }
{$DEFINE TOP := STACKSIZE - 1 * Ord ( STACKSIZE > 0 ) }
{$DEFINE SLEN := length ( Ar [ TOP ] ) * Ord ( Length ( Ar [ TOP ] ) > 0 ) }
FUNCTION IsEmpty : boolean ;
PROCEDURE Print ;
FUNCTION Pop : variant ;
FUNCTION Peep : variant ;
PROCEDURE Push ( item : variant ) ;
FUNCTION SecPop : variant ;
PUBLIC
CONSTRUCTOR Create ;
END;
CONSTRUCTOR Stack.Create ;
BEGIN
SetLength ( Ar, STACKSIZE ) ;
END;
FUNCTION Stack.IsEmpty : boolean ;
BEGIN
IsEmpty := ( STACKSIZE < 1 ) ;
END;
PROCEDURE Stack.Print ;
VAR
i : shortint ;
BEGIN
IF ( TOP < 1 ) or ( IsEmpty ) THEN
BEGIN
WriteLn ( CrLf + '<empty stack>' ) ;
EXIT ;
END;
WriteLn ( CrLf , '<top>') ;
FOR i := ( TOP ) DOWNTO 0 DO WriteLn ( Ar [ i ] ) ;
WriteLn ( '<bottom>' ) ;
END;
FUNCTION Stack.Pop : variant ;
BEGIN
IF IsEmpty THEN EXIT ;
Pop := Ar [ TOP ] ;
SetLength ( Ar, TOP ) ;
END;
FUNCTION Stack.Peep : variant ;
BEGIN
IF IsEmpty THEN EXIT ;
Peep := Ar [ TOP ] ;
END;
PROCEDURE Stack.Push ( item : variant ) ;
BEGIN
SetLength ( Ar, STACKSIZE + 1 ) ;
Ar [ TOP ] := item ;
END;
FUNCTION Stack.SecPop : variant ;
(*) Pop and Wipe (*)
BEGIN
IF IsEmpty THEN EXIT ;
SecPop := Ar [ TOP ] ;
Ar [ TOP ] := StringOfChar ( #255 , SLEN ) ;
Ar [ TOP ] := StringOfChar ( #0 , SLEN ) ;
SetLength ( Ar, TOP ) ;
END;
VAR
n : integer ;
r : real ;
S : string ;
So : Stack ;
BEGIN
So.Create ;
So.Print ;
n := 23 ;
So.Push ( n ) ;
S := '3 guesses ' ;
So.Push ( S ) ;
r := 1.23 ;
So.Push ( r ) ;
WriteLn ( 'Peep : ', So.Peep ) ;
So.Push ( 'Nice Try' ) ;
So.Print ;
WriteLn ;
WriteLn ( 'SecPop : ',So.SecPop ) ;
WriteLn ( 'SecPop : ',So.SecPop ) ;
WriteLn ( 'SecPop : ',So.SecPop ) ;
WriteLn ( 'SecPop : ',So.SecPop ) ;
So.Print ;
END.
</lang>JPD 2021/07/03
Output:
<empty stack>
Peep : 1.23
<top>
Nice Try
1.23
3 guesses
23
<bottom>
SecPop : Nice Try
SecPop : 1.23
SecPop : 3 guesses
SecPop : 23
<empty stack>
FreeBASIC
We first use a macro to define a generic Stack type : <lang freebasic>' FB 1.05.0 Win64
' stack_rosetta.bi ' simple generic Stack type
- Define Stack(T) Stack_##T
- Macro Declare_Stack(T)
Type Stack(T)
Public: Declare Constructor() Declare Destructor() Declare Property capacity As Integer Declare Property count As Integer Declare Property empty As Boolean Declare Property top As T Declare Function pop() As T Declare Sub push(item As T) Private: a(any) As T count_ As Integer = 0 Declare Function resize(size As Integer) As Integer
End Type
Constructor Stack(T)()
Redim a(0 To 0) create a default T instance for various purposes
End Constructor
Destructor Stack(T)()
Erase a
End Destructor
Property Stack(T).capacity As Integer
Return UBound(a)
End Property
Property Stack(T).count As Integer
Return count_
End Property
Property Stack(T).empty As Boolean
Return count_ = 0
End Property
Property Stack(T).top As T
If count_ > 0 Then Return a(count_) End If Print "Error: Attempted to access 'top' element of an empty stack" Return a(0) return default element
End Property
Function Stack(T).pop() As T
If count_ > 0 Then Dim value As T = a(count_) a(count_) = a(0) zero element to be removed count_ -= 1 Return value End If Print "Error: Attempted to remove 'top' element of an empty stack" Return a(0) return default element
End Function
Sub Stack(T).push(item As T)
Dim size As Integer = UBound(a) count_ += 1 If count_ > size Then size = resize(size) Redim Preserve a(0 to size) End If a(count_) = item
End Sub
Function Stack(T).resize(size As Integer) As Integer
If size = 0 Then size = 4 ElseIf size <= 32 Then size = 2 * size Else size += 32 End If Return size
End Function
- EndMacro</lang>
We now use this type to create a Stack of Dog instances : <lang freebasic>' FB 1.05.0 Win64
- Include "stack_rosetta.bi"
Type Dog
name As String age As Integer Declare Constructor Declare Constructor(name_ As string, age_ As integer) Declare Operator Cast() As String
end type
Constructor Dog default constructor End Constructor
Constructor Dog(name_ As String, age_ As Integer)
name = name_ age = age_
End Constructor
Operator Dog.Cast() As String
Return "[" + name + ", " + Str(age) + "]"
End Operator
Declare_Stack(Dog) expand Stack type for Dog instances
Dim dogStack As Stack(Dog)
Var cerberus = Dog("Cerberus", 10) Var rover = Dog("Rover", 3) Var puppy = Dog("Puppy", 0) With dogStack push these Dog instances onto the stack
.push(cerberus) .push(rover) .push(puppy)
End With Print "Number of dogs on the stack :" ; dogStack.count Print "Capacity of dog stack :" ; dogStack.capacity Print "Top dog : "; dogStack.top dogStack.pop() Print "Top dog now : "; dogStack.top Print "Number of dogs on the stack :" ; dogStack.count dogStack.pop() Print "Top dog now : "; dogStack.top Print "Number of dogs on the stack :" ; dogStack.count Print "Is stack empty now : "; dogStack.empty Print Print "Press any key to quit" Sleep</lang>
- Output:
Number of dogs on the stack : 3 Capacity of dog stack : 4 Top dog : [Puppy, 0] Top dog now : [Rover, 3] Number of dogs on the stack : 2 Top dog now : [Cerberus, 10] Number of dogs on the stack : 1 Is stack empty now : false
Frink
Frink's array class has all of the methods to make it usable as a stack or a deque. The methods are called array.push[x], array.pop[], and array.isEmpty[]
<lang frink>a = new array
a.push[1]
a.push[2]
a.peek[]
while ! a.isEmpty[]
println[a.pop[]]</lang>
Genie
<lang genie>[indent=4] /*
Stack, in Genie, with GLib double ended Queues valac stack.gs
- /
init
var stack = new Queue of int()
// push stack.push_tail(2) stack.push_tail(1)
// pop (and peek at top) print stack.pop_tail().to_string() print stack.peek_tail().to_string()
// empty print "stack size before clear: " + stack.get_length().to_string() stack.clear() print "After clear, stack.is_empty(): " + stack.is_empty().to_string()</lang>
- Output:
prompt$ valac stack.gs prompt$ ./stack 1 2 stack size before clear: 1 After clear, stack.is_empty(): true
Go
Go slices make excellent stacks without defining any extra types, functions, or methods. For example, to keep a stack of integers, simply declare one as, <lang go>var intStack []int</lang> Use the built in append function to push numbers on the stack: <lang go>intStack = append(intStack, 7)</lang> Use a slice expression with the built in len function to pop from the stack: <lang go>popped, intStack = intStack[len(intStack)-1], intStack[:len(intStack)-1]</lang> The test for an empty stack: <lang go>len(intStack) == 0</lang> And to peek at the top of the stack: <lang go>intStack[len(intStack)-1]</lang> It is idiomatic Go to use primitive language features where they are sufficient, and define helper functions or types and methods only as they make sense for a particular situation. Below is an example using a type with methods and idiomatic "ok" return values to avoid panics. It is only an example of something that might make sense in some situation. <lang go>package main
import "fmt"
type stack []interface{}
func (k *stack) push(s interface{}) {
*k = append(*k, s)
}
func (k *stack) pop() (s interface{}, ok bool) {
if k.empty() {
return
}
last := len(*k) - 1
s = (*k)[last]
*k = (*k)[:last]
return s, true
}
func (k *stack) peek() (s interface{}, ok bool) {
if k.empty() {
return
}
last := len(*k) - 1
s = (*k)[last]
return s, true
}
func (k *stack) empty() bool {
return len(*k) == 0
}
func main() {
var s stack
fmt.Println("new stack:", s)
fmt.Println("empty?", s.empty())
s.push(3)
fmt.Println("push 3. stack:", s)
fmt.Println("empty?", s.empty())
s.push("four")
fmt.Println(`push "four" stack:`, s)
if top, ok := s.peek(); ok {
fmt.Println("top value:", top)
} else {
fmt.Println("nothing on stack")
}
if popped, ok := s.pop(); ok {
fmt.Println(popped, "popped. stack:", s)
} else {
fmt.Println("nothing to pop")
}
}</lang>
- Output:
new stack: [] empty? true push 3. stack: [3] empty? false push "four" stack: [3 four] top value: four four popped. stack: [3]
Groovy
In Groovy, all lists have stack semantics, including "push()" and "pop()" methods, an "empty" property, and a "last()" method as a stand-in for "top/peek" semantics. Calling "pop()" on an empty list throws an exception.
Of course, these stack semantics are not exclusive. Elements of the list can still be accessed and manipulated in myriads of other ways. <lang groovy>def stack = [] assert stack.empty
stack.push(55) stack.push(21) stack.push('kittens') assert stack.last() == 'kittens' assert stack.size() == 3 assert ! stack.empty
println stack
assert stack.pop() == "kittens" assert stack.size() == 2
println stack
stack.push(-20)
println stack
stack.push( stack.pop() * stack.pop() ) assert stack.last() == -420 assert stack.size() == 2
println stack
stack.push(stack.pop() / stack.pop()) assert stack.size() == 1
println stack
println stack.pop() assert stack.size() == 0 assert stack.empty
try { stack.pop() } catch (NoSuchElementException e) { println e.message }</lang>
- Output:
[55, 21, kittens] [55, 21] [55, 21, -20] [55, -420] [-7.6363636364] -7.6363636364 Cannot pop() an empty List
Haskell
The Haskell solution is trivial, using a list. Note that pop returns both the element and the changed stack, to remain purely functional.
<lang haskell>type Stack a = [a]
create :: Stack a create = []
push :: a -> Stack a -> Stack a push = (:)
pop :: Stack a -> (a, Stack a) pop [] = error "Stack empty" pop (x:xs) = (x,xs)
empty :: Stack a -> Bool empty = null
peek :: Stack a -> a
peek [] = error "Stack empty"
peek (x:_) = x</lang>
We can make a stack that can be destructively popped by hiding the list inside a State monad.
<lang haskell>import Control.Monad.State
type Stack a b = State [a] b
push :: a -> Stack a () push = modify . (:)
pop :: Stack a a pop = do
nonEmpty x <- peek modify tail return x
empty :: Stack a Bool empty = gets null
peek :: Stack a a peek = nonEmpty >> gets head
nonEmpty :: Stack a () nonEmpty = empty >>= flip when (fail "Stack empty")</lang>
Icon and Unicon
Stacks (and double ended queues) are built into Icon and Unicon as part of normal list access. In addition to 'push' and 'pop', there are the functions 'put', 'get' (alias for pop), 'pull', list element addressing, and list sectioning (like sub-strings). Unicon extended 'insert' and 'delete' to work with lists. The programmer is free to use any or all of the list processing functions on any problem. The following illustrates typical stack usage: <lang Icon>procedure main() stack := [] # new empty stack push(stack,1) # add item push(stack,"hello",table(),set(),[],5) # add more items of mixed types in order left to right y := top(stack) # peek x := pop(stack) # remove item write("The stack is ",if isempty(stack) then "empty" else "not empty") end
procedure isempty(x) #: test if a datum is empty, return the datum or fail (task requirement) if *x = 0 then return x # in practice just write *x = 0 or *x ~= 0 for is/isn't empty end
procedure top(x) #: return top element w/o changing stack return x[1] # in practice, just use x[1] end</lang>
Io
aside from using built-in lists, a stack can be created using nodes like so: <lang io>Node := Object clone do(
next := nil obj := nil
)
Stack := Object clone do(
node := nil
pop := method(
obj := node obj
node = node next
obj
)
push := method(obj,
nn := Node clone
nn obj = obj
nn next = self node
self node = nn
)
)</lang>
Ioke
<lang ioke>Stack = Origin mimic do(
initialize = method(@elements = []) pop = method(@elements pop!) empty = method(@elements empty?) push = method(element, @elements push!(element))
)</lang>
IS-BASIC
<lang IS-BASIC>100 LET N=255 ! Size of stack 110 NUMERIC STACK(1 TO N) 120 LET PTR=1 130 DEF PUSH(X) 140 IF PTR>N THEN 150 PRINT "Stack is full.":STOP 160 ELSE 170 LET STACK(PTR)=X:LET PTR=PTR+1 180 END IF 190 END DEF 200 DEF POP 210 IF PTR=1 THEN 220 PRINT "Stack is empty.":STOP 230 ELSE 240 LET PTR=PTR-1:LET POP=STACK(PTR) 250 END IF 260 END DEF 270 DEF EMPTY 280 LET PTR=1 290 END DEF 300 DEF TOP=STACK(PTR-1) 310 CALL PUSH(3):CALL PUSH(5) 320 PRINT POP+POP</lang>
J
<lang J>stack=: push=: monad def '0$stack=:stack,y' pop=: monad def 'r[ stack=:}:stack[ r=.{:stack' empty=: monad def '0=#stack'</lang> Example use: <lang J> push 9
pop
9
empty
1</lang> pop and empty ignore their arguments. In this implementation. push returns an empty list.
Java
The collections framework includes a Stack class. Let's test it: <lang Java>import java.util.Stack;
public class StackTest {
public static void main( final String[] args ) {
final Stack<String> stack = new Stack<String>();
System.out.println( "New stack empty? " + stack.empty() );
stack.push( "There can be only one" );
System.out.println( "Pushed stack empty? " + stack.empty() );
System.out.println( "Popped single entry: " + stack.pop() );
stack.push( "First" );
stack.push( "Second" );
System.out.println( "Popped entry should be second: " + stack.pop() );
// Popping an empty stack will throw...
stack.pop();
stack.pop();
}
}</lang>
- Output:
New stack empty? true Pushed stack empty? false Popped single entry: There can be only one Popped entry should be second: Second Exception in thread "main" java.util.EmptyStackException at java.util.Stack.peek(Stack.java:85) at java.util.Stack.pop(Stack.java:67) at StackTest.main(StackTest.java:21)
Alternatively, you might implement a stack yourself... <lang java>public class Stack{
private Node first = null;
public boolean isEmpty(){
return first == null;
}
public Object Pop(){
if(isEmpty())
throw new Exception("Can't Pop from an empty Stack.");
else{
Object temp = first.value;
first = first.next;
return temp;
}
}
public void Push(Object o){
first = new Node(o, first);
}
class Node{
public Node next;
public Object value;
public Node(Object value){
this(value, null);
}
public Node(Object value, Node next){
this.next = next;
this.value = value;
}
}
}</lang>
<lang java5>public class Stack<T>{
private Node first = null;
public boolean isEmpty(){
return first == null;
}
public T Pop(){
if(isEmpty())
throw new Exception("Can't Pop from an empty Stack.");
else{
T temp = first.value;
first = first.next;
return temp;
}
}
public void Push(T o){
first = new Node(o, first);
}
class Node{
public Node next;
public T value;
public Node(T value){
this(value, null);
}
public Node(T value, Node next){
this.next = next;
this.value = value;
}
}
}</lang>
JavaScript
The built-in Array class already has stack primitives. <lang javascript>var stack = []; stack.push(1) stack.push(2,3); print(stack.pop()); // 3 print(stack.length); // 2, stack empty if 0</lang> Here's a constructor that wraps the array: <lang javascript>function Stack() {
this.data = new Array();
this.push = function(element) {this.data.push(element)}
this.pop = function() {return this.data.pop()}
this.empty = function() {return this.data.length == 0}
this.peek = function() {return this.data[this.data.length - 1]}
}</lang> Here's an example using the revealing module pattern instead of prototypes. <lang javascript> function makeStack() {
var stack = [];
var popStack = function () {
return stack.pop();
};
var pushStack = function () {
return stack.push.apply(stack, arguments);
};
var isEmpty = function () {
return stack.length === 0;
};
var peekStack = function () {
return stack[stack.length-1];
};
return {
pop: popStack,
push: pushStack,
isEmpty: isEmpty,
peek: peekStack,
top: peekStack
};
} </lang>
Jsish
From Javascript entry. Being ECMAScript, Jsi supports stack primitives as part of the Array methods. <lang javascript>/* Stack, is Jsish */ var stack = []; puts('depth:', stack.length);
stack.push(42); stack.push('abc'); puts('depth:', stack.length);
puts('popped:', stack.pop()); if (stack.length) printf('not '); printf('empty\n'); puts('top:', stack[stack.length-1]); puts('popped:', stack.pop()); if (stack.length) printf('not '); printf('empty\n');
puts('depth:', stack.length);</lang>
- Output:
prompt$ jsish stack.jsi depth: 0 depth: 2 popped: abc not empty top: 42 popped: 42 empty depth: 0
Julia
The built-in Array class already has efficient (linear amortized time) stack primitives.
<lang julia>stack = Int[] # []
@show push!(stack, 1) # [1]
@show push!(stack, 2) # [1, 2]
@show push!(stack, 3) # [1, 2, 3]
@show pop!(stack) # 3
@show length(stack) # 2
@show empty!(stack) # []
@show isempty(stack) # true</lang>
K
<lang K>stack:() push:{stack::x,stack} pop:{r:*stack;stack::1_ stack;r} empty:{0=#stack}
/example: stack:()
push 3 stack
,3
push 5 stack
5 3
pop[]
5
stack
,3
empty[]
0
pop[]
3
stack
!0
empty[]
1 </lang>
Kotlin
Rather than use the java.util.Stack<E> class, we will write our own simple Stack<E> class for this task: <lang scala>// version 1.1.2
class Stack<E> {
private val data = mutableListOf<E>()
val size get() = data.size
val empty get() = size == 0
fun push(element: E) = data.add(element)
fun pop(): E {
if (empty) throw RuntimeException("Can't pop elements from an empty stack")
return data.removeAt(data.lastIndex)
}
val top: E
get() {
if (empty) throw RuntimeException("Empty stack can't have a top element")
return data.last()
}
fun clear() = data.clear()
override fun toString() = data.toString()
}
fun main(args: Array<String>) {
val s = Stack<Int>()
(1..5).forEach { s.push(it) }
println(s)
println("Size of stack = ${s.size}")
print("Popping: ")
(1..3).forEach { print("${s.pop()} ") }
println("\nRemaining on stack: $s")
println("Top element is now ${s.top}")
s.clear()
println("After clearing, stack is ${if(s.empty) "empty" else "not empty"}")
try {
s.pop()
}
catch (e: Exception) {
println(e.message)
}
}</lang>
- Output:
[1, 2, 3, 4, 5] Size of stack = 5 Popping: 5 4 3 Remaining on stack: [1, 2] Top element is now 2 After clearing, stack is empty Can't pop elements from an empty stack
lang5
<lang lang5>: cr "\n" . ;
- empty? dup execute length if 0 else -1 then swap drop ;
- pop dup execute length 1 - extract swap drop ;
- push dup execute rot append over ;
- s. stack execute . ;
[] '_ set
- stack '_ ;
stack # local variable
1 swap push set 2 swap push set s. cr # [ 1 2 ] pop . s. cr # 2 [ 1 ] pop drop empty? . # -1</lang>
Lasso
Lasso Arrays natively supports push and pop.
<lang Lasso>local(a) = array
- a->push('a')
- a->push('b')
- a->push('c')
- a->pop // c
- a->pop // b
- a->pop // a
- a->pop // null</lang>
Liberty BASIC
<lang lb> global stack$ stack$=""
randomize .51 for i = 1 to 10
if rnd(1)>0.5 then
print "pop => ";pop$()
else
j=j+1
s$ = chr$(j + 64)
print "push ";s$
call push s$
end if
next
print print "Clean-up" do
print "pop => ";pop$()
loop while not(empty()) print "Stack is empty"
end
'------------------------------------ sub push s$
stack$=s$+"|"+stack$ 'stack
end sub
function pop$()
if stack$="" then pop$="*EMPTY*": exit function pop$=word$(stack$,1,"|") stack$=mid$(stack$,instr(stack$,"|")+1)
end function
function empty()
empty =(stack$="")
end function </lang>
Lingo
<lang lingo>-- parent script "Stack"
property _tos
on push (me, data)
me._tos = [#data:data, #next:me._tos]
end
on pop (me)
if voidP(me._tos) then return VOID data = me._tos.data me._tos = me._tos.next return data
end
on peek (me)
if voidP(me._tos) then return VOID return me._tos.data
end
on empty (me)
return voidP(me.peek())
end</lang>
Logo
UCB Logo has built-in methods for treating lists as stacks. Since they are destructive, they take the name of the stack rather than the list itself. <lang logo>make "stack [] push "stack 1 push "stack 2 push "stack 3 print pop "stack ; 3 print empty? :stack ; false</lang>
Logtalk
A stack can be trivially represented using the built-in representation for lists: <lang logtalk>
- - object(stack).
:- public(push/3). push(Element, Stack, [Element| Stack]).
:- public(pop/3). pop([Top| Stack], Top, Stack).
:- public(empty/1) empty([]).
- - end_object.
</lang>
LOLCODE
<lang lolcode>HAI 2.3 HOW IZ I Init YR Stak
Stak HAS A Length ITZ 0
IF U SAY SO
HOW IZ I Push YR Stak AN YR Value
Stak HAS A SRS Stak'Z Length ITZ Value Stak'Z Length R SUM OF Stak'Z Length AN 1
IF U SAY SO
HOW IZ I Top YR Stak
FOUND YR Stak'Z SRS DIFF OF Stak'Z Length AN 1
IF U SAY SO
HOW IZ I Pop YR Stak
I HAS A Top ITZ I IZ Top YR Stak MKAY Stak'Z Length R DIFF OF Stak'Z Length AN 1 FOUND YR Top
IF U SAY SO
HOW IZ I Empty YR Stak
FOUND YR BOTH SAEM 0 AN Stak'Z Length
IF U SAY SO
I HAS A Stak ITZ A BUKKIT I IZ Init YR Stak MKAY I IZ Push YR Stak AN YR "Fred" MKAY I IZ Push YR Stak AN YR "Wilma" MKAY I IZ Push YR Stak AN YR "Betty" MKAY I IZ Push YR Stak AN YR "Barney" MKAY
IM IN YR Loop UPPIN YR Dummy TIL I IZ Empty YR Stak MKAY
VISIBLE I IZ Pop YR Stak MKAY
IM OUTTA YR Loop
KTHXBYE</lang>
- Output:
Barney Betty Wilma Fred
Lua
Tables have stack primitives by default: <lang lua>stack = {} table.insert(stack,3) print(table.remove(stack)) --> 3</lang>
M2000 Interpreter
A Stack object can be used as LIFO or FIFO. Push statement push to top of stack. Read pop a value to a variable from top of stack. StackItem(1) read top item without modified stack. Data statement append items to bottom. <lang M2000 Interpreter> Module Checkit {
a=Stack
Stack a {
Push 100, 200, 300
}
Print StackItem(a, 1)=300
Stack a {
Print StackItem(1)=300
While not empty {
Read N
Print N
}
}
} Checkit </lang>
Every module and function has a "current" stack. Number is a read only variable, which pop a value from current stack (or raise error if not number is in top of stack).
User functions get a new stack, and drop it at return. Modules take parent stack, and return stack to parent. So a Module can return values too. In M2000 a call happen without checkig signatures (except for special events calls). We have to leave stack at a proper state, when return from a module. Return/Execution stack is hidden and different from stack of values.
<lang M2000 Interpreter> Module Checkit {
Read a, b
Print a, b
} \\ add parameters in a FIFO, and this FIFO merged to current stack Push 100 Checkit 10, 20 Print StackItem(1)=100 Module Checkit {
Read a, b
Print a=20, b=100
} Checkit 20
Function alfa {
k=0
n=0
while not empty {
k+=number
n++
}
if n=0 then Error "No parameters found"
=k/n
}
Print alfa(1,2,3,4)=2.5
</lang>
Maple
<lang Maple>with(stack): # load the package, to allow use of short command names
s := stack:-new(a, b):
push(c, s):
- The following statements terminate with a semicolon and print output.
top(s); pop(s); pop(s); empty(s); pop(s); empty(s);</lang>
- Output:
c
c
b
false
a
true
Mathematica/Wolfram Language
<lang Mathematica>EmptyQ[a_] := If[Length[a] == 0, True, False] SetAttributes[Push, HoldAll];[a_, elem_] := AppendTo[a, elem] SetAttributes[Pop, HoldAllComplete]; Pop[a_] := If[EmptyQ[a], False, b = Last[a]; Set[a, Most[a]]; b] Peek[a_] := If[EmptyQ[a], False, Last[a]]
Example use: stack = {};Push[stack, 1]; Push[stack, 2]; Push[stack, 3]; Push[stack, 4]; Peek[stack] ->4 Pop[stack] ->4 Peek[stack] ->3</lang>
MATLAB / Octave
Here is a simple implementation of a stack, that works in Matlab and Octave. It is closely related to the queue/fifo example. <lang matlab>mystack = {};
% push mystack{end+1} = x;
%pop x = mystack{end}; mystack{end} = [];
%peek,top x = mystack{end};
% empty isempty(mystack)</lang> Below is another solution, that encapsulates the fifo within the object-orientated "class" elements supported by Matlab. The given implementation is exactly the same as the MATLAB FIFO example, except that the "push()" function is modified to add stuff to the end of the queue instead of the beginning. This is a naive implementation, for rigorous applications this should be modified to initialize the LIFO to a buffered size, so that the "pop()" and "push()" functions don't resize the cell array that stores the LIFO's elements, every time they are called.
To use this implementation you must save this code in a MATLAB script file named "LIFOQueue.m" which must be saved in a folder named @LIFOQueue in your MATLAB directory. <lang MATLAB>%This class impliments a standard LIFO queue. classdef LIFOQueue
properties
queue
end
methods
%Class constructor
function theQueue = LIFOQueue(varargin)
if isempty(varargin) %No input arguments
%Initialize the queue state as empty
theQueue.queue = {};
elseif (numel(varargin) > 1) %More than 1 input arg
%Make the queue the list of input args
theQueue.queue = varargin;
elseif iscell(varargin{:}) %If the only input is a cell array
%Make the contents of the cell array the elements in the queue
theQueue.queue = varargin{:};
else %There is one input argument that is not a cell
%Make that one arg the only element in the queue
theQueue.queue = varargin;
end
end
%push() - pushes a new element to the end of the queue
function push(theQueue,varargin)
if isempty(varargin)
theQueue.queue(end+1) = {[]};
elseif (numel(varargin) > 1) %More than 1 input arg
%Make the queue the list of input args
theQueue.queue( end+1:end+numel(varargin) ) = varargin;
elseif iscell(varargin{:}) %If the only input is a cell array
%Make the contents of the cell array the elements in the queue
theQueue.queue( end+1:end+numel(varargin{:}) ) = varargin{:};
else %There is one input argument that is not a cell
%Make that one arg the only element in the queue
theQueue.queue{end+1} = varargin{:};
end
%Makes changes to the queue permanent
assignin('caller',inputname(1),theQueue);
end
%pop() - pops the first element off the queue
function element = pop(theQueue)
if empty(theQueue)
error 'The queue is empty'
else
%Returns the first element in the queue
element = theQueue.queue{end};
%Removes the first element from the queue
theQueue.queue(end) = [];
%Makes changes to the queue permanent
assignin('caller',inputname(1),theQueue);
end
end
%empty() - Returns true if the queue is empty
function trueFalse = empty(theQueue)
trueFalse = isempty(theQueue.queue);
end
end %methods
end</lang> Sample Usage: <lang MATLAB>>> myLIFO = LIFOQueue(1,'fish',2,'fish','red fish','blue fish')
myLIFO =
LIFOQueue
>> myLIFO.pop()
ans =
blue fish
>> myLIFO.push('Cat Fish') >> myLIFO.pop()
ans =
Cat Fish
>> myLIFO.pop()
ans =
red fish
>> empty(myLIFO)
ans =
0</lang>
Maxima
<lang maxima>/* lists can be used as stacks; Maxima provides pop and push */
load(basic)$
a: []$ push(25, a)$ push(7, a)$ pop(a);
emptyp(a); length(a);</lang>
Mercury
Efficient, generic stacks are provided as part of the standard library in Mercury. For sake of illustration, here is how a simple stack could be implemented.
<lang mercury>:- module sstack.
- - interface.
% We're going to call the type sstack (simple stack) because we don't want to get it % accidentally confused with the official stack module in the standard library.
- - type sstack(T).
- - func sstack.new = sstack(T).
- - pred sstack.is_empty(sstack(T)::in) is semidet.
- - func sstack.push(sstack(T), T) = sstack(T).
- - pred sstack.pop(T::out, sstack(T)::in, sstack(T)::out) is semidet.
- - implementation.
- - import_module list.
- - type sstack(T)
---> sstack(list(T)).
sstack.new = sstack([]).
sstack.is_empty(sstack([])).
sstack.push(Stack0, Elem) = Stack1 :-
Stack0 = sstack(Elems), Stack1 = sstack([Elem | Elems]).
sstack.pop(Elem, !Stack) :-
!.Stack = sstack([Elem | Elems]), !:Stack = sstack(Elems).
- - end_module sstack.</lang>
It should be noted that this is purely an illustrative example of a very simple stack. A real implementation would have predicate (:- pred) versions of the functions (:- func), for example, for consistency's sake with either the functions implemented in terms of the predicates or vice versa. The real library implementation also features more functionality including both semi-deterministic and deterministic versions of some functions/predicates as well as the ability to push a list of values in one operation.
Some of the implementation decisions above need an explanation. new/0 and push/2 were implemented as functions both for pedagogical reasons (a desire to show function syntax) and because they are a natural fit for functional thought: 0 or more inputs, one output, deterministic. is_empty/1 was implemented as a predicate because it's a single, simple succeed/fail test which is precisely what a predicate is in logic. pop/3 was implemented as a predicate because it has two outputs (the element and the new stack) and because it is semi-deterministic (it will fail if the stack is empty).
Note also that while pop/3 has three parameters, the function implementation looks like it has two. This is because the !Stack "parameter" is actually a pair of parameters using Mercury's state variable notation. !Stack is, in effect, two variables: !.Stack and !:Stack, input and output respectively. Using state variable notation here is a bit of overkill but again was brought in for pedagogical reasons to show the syntax.
MIPS Assembly
<lang MIPS>addi sp,sp,-4 sw t0,0(sp) ;push
lw t0,0(sp) addi sp,sp,4 ;pop
lw t0,0(sp) ;top</lang>
"Empty" requires you to know the starting value of SP. Since it's hardware-dependent, there's no one answer for this part of the task.
MiniScript
<lang MiniScript>// Note in Miniscript, a value of zero is false, // and any other number is true. // therefore the .len function works as the inverse of a .empty function stack = [2, 4, 6] stack.push 8 print "Stack is " + stack print "Adding '9' to stack " + stack.push(9) print "Top of stack is " + stack.pop print "Stack is " + stack if stack.len then
print "Stack is not empty"
else
print "Stack is empty"
end if</lang>
- Output:
Stack is [2, 4, 6, 8] Adding '9' to stack [2, 4, 6, 8, 9] Top of stack is 9 Stack is [2, 4, 6, 8] Stack is not empty
Nanoquery
<lang nanoquery>class Stack
declare internalList
// constructor
def Stack()
internalList = list()
end
def push(val)
internalList.append(val)
end
def pop()
val = internalList[int(len($internalList) - 1)]
internalList.remove(val)
return val
end
def empty()
return len(internalList) = 0
end
end</lang>
Nemerle
Mutable stacks are available in System.Collections, System.Collections.Generic and Nemerle.Collections depending on what functionality beyond the basics you want. An immutable stack could be implemented fairly easily, as, for example, this quick and dirty list based implementation. <lang Nemerle>public class Stack[T] {
private stack : list[T];
public this()
{
stack = [];
}
public this(init : list[T])
{
stack = init;
}
public Push(item : T) : Stack[T]
{
Stack(item::stack)
}
public Pop() : T * Stack[T]
{
(stack.Head, Stack(stack.Tail))
}
public Peek() : T
{
stack.Head
}
public IsEmpty() : bool
{
stack.Length == 0
}
}</lang>
NetRexx
<lang netrexx>/* NetRexx ************************************************************
- 13.08.2013 Walter Pachl translated from REXX version 2
- /
options replace format comments java crossref savelog symbols nobinary
stk = create_stk
say push(stk,123) 'from push' say empty(stk) say peek(stk) 'from peek' say pull(stk) 'from pull' say empty(stk) Say pull(stk) 'from pull'
method create_stk static returns Rexx
stk = stk[0] = 0 return stk
method push(stk,v) static
stk[0]=stk[0]+1 stk[stk[0]]=v Return v
method peek(stk) static
x=stk[0] If x=0 Then Return 'stk is empty' Else Return stk[x]
method pull(stk) static
x=stk[0] If x=0 Then Return 'stk is empty' Else Do stk[0]=stk[0]-1 Return stk[x] End
method empty(stk) static
Return stk[0]=0</lang>
- Output:
123 from push 0 123 from peek 123 from pull 1 stk is empty from pull
Nim
In Nim, the sequences offer all the functionalities of a stack. Procedure add appends an item at the end, procedure pop returns the last element and removes it from the sequence. And it’s easy to check if if the sequence is empty with the procedure len which returns its length.
If we want a stack type limited to the four or five functions of the task, it is possible to define a distinct generic type Stack[T] derived from seq[T]. The code will be typically as follows. Note that we have defined a procedure top to get the value of the top item, another mtop to get a mutable reference to the top item and also a procedure mtop= to assign directly a value to the top item.
<lang Nim>type Stack[T] = distinct seq[T]
func initStack[T](initialSize = 32): Stack[T] =
Stack[T](newSeq[T](initialSize))
func isEmpty[T](stack: Stack[T]): bool =
seq[T](stack).len == 0
func push[T](stack: var Stack[T]; item: sink T) =
seq[T](stack).add(item)
func pop[T](stack: var Stack[T]): T =
if stack.isEmpty: raise newException(IndexDefect, "stack is empty.") seq[T](stack).pop()
func top[T](stack: Stack[T]): T =
if stack.isEmpty: raise newException(IndexDefect, "stack is empty.") seq[T](stack)[^1]
func mtop[T](stack: var Stack[T]): var T =
if stack.isEmpty: raise newException(IndexDefect, "stack is empty.") seq[T](stack)[^1]
func `mtop=`[T](stack: var Stack[T]; value: T) =
if stack.isEmpty: raise newException(IndexDefect, "stack is empty.") seq[T](stack)[^1] = value
when isMainModule:
var s = initStack[int]() s.push 2 echo s.pop s.push 3 echo s.top s.mtop += 1 echo s.top s.mtop = 5 echo s.top</lang>
- Output:
2 3 4 5
Oberon-2
<lang oberon2> MODULE Stacks; IMPORT
Object, Object:Boxed, Out := NPCT:Console;
TYPE
Pool(E: Object.Object) = POINTER TO ARRAY OF E; Stack*(E: Object.Object) = POINTER TO StackDesc(E); StackDesc*(E: Object.Object) = RECORD pool: Pool(E); cap-,top: LONGINT; END;
PROCEDURE (s: Stack(E)) INIT*(cap: LONGINT); BEGIN NEW(s.pool,cap);s.cap := cap;s.top := -1 END INIT;
PROCEDURE (s: Stack(E)) Top*(): E; BEGIN RETURN s.pool[s.top] END Top;
PROCEDURE (s: Stack(E)) Push*(e: E); BEGIN INC(s.top); ASSERT(s.top < s.cap); s.pool[s.top] := e; END Push;
PROCEDURE (s: Stack(E)) Pop*(): E; VAR resp: E; BEGIN ASSERT(s.top >= 0); resp := s.pool[s.top];DEC(s.top); RETURN resp END Pop;
PROCEDURE (s: Stack(E)) IsEmpty(): BOOLEAN; BEGIN RETURN s.top < 0 END IsEmpty;
PROCEDURE (s: Stack(E)) Size*(): LONGINT; BEGIN RETURN s.top + 1 END Size;
PROCEDURE Test;
VAR
s: Stack(Boxed.LongInt);
BEGIN
s := NEW(Stack(Boxed.LongInt),100);
s.Push(NEW(Boxed.LongInt,10));
s.Push(NEW(Boxed.LongInt,100));
Out.String("size: ");Out.Int(s.Size(),0);Out.Ln;
Out.String("pop: ");Out.Object(s.Pop());Out.Ln;
Out.String("top: ");Out.Object(s.Top());Out.Ln;
Out.String("size: ");Out.Int(s.Size(),0);Out.Ln
END Test;
BEGIN
Test
END Stacks. </lang>
- Output:
size: 2 pop: 100 top: 10 size: 1
<lang oberon2> MODULE Stacks; (** AUTHOR ""; PURPOSE ""; *)
IMPORT Out := KernelLog;
TYPE Object = OBJECT END Object;
Stack* = OBJECT VAR top-,capacity-: LONGINT; pool: POINTER TO ARRAY OF Object;
PROCEDURE & InitStack*(capacity: LONGINT); BEGIN SELF.capacity := capacity; SELF.top := -1; NEW(SELF.pool,capacity) END InitStack;
PROCEDURE Push*(a:Object); BEGIN INC(SELF.top); ASSERT(SELF.top < SELF.capacity,100); SELF.pool[SELF.top] := a END Push;
PROCEDURE Pop*(): Object; VAR r: Object; BEGIN ASSERT(SELF.top >= 0); r := SELF.pool[SELF.top]; DEC(SELF.top);RETURN r END Pop;
PROCEDURE Top*(): Object; BEGIN ASSERT(SELF.top >= 0); RETURN SELF.pool[SELF.top] END Top;
PROCEDURE IsEmpty*(): BOOLEAN; BEGIN RETURN SELF.top < 0 END IsEmpty;
END Stack;
BoxedInt = OBJECT (Object) VAR val-: LONGINT;
PROCEDURE & InitBoxedInt*(CONST val: LONGINT); BEGIN SELF.val := val END InitBoxedInt;
END BoxedInt;
PROCEDURE Test*; VAR s: Stack; bi: BoxedInt; obj: Object; BEGIN NEW(s,10); (* A new stack of ten objects *) NEW(bi,100);s.Push(bi); NEW(bi,102);s.Push(bi); NEW(bi,104);s.Push(bi); Out.Ln; Out.String("Capacity:> ");Out.Int(s.capacity,0);Out.Ln; Out.String("Size:> ");Out.Int(s.top + 1,0);Out.Ln; obj := s.Pop(); obj := s.Pop(); WITH obj: BoxedInt DO Out.String("obj:> ");Out.Int(obj.val,0);Out.Ln ELSE Out.String("Unknown object...");Out.Ln; END (* with *) END Test; END Stacks. </lang>
- Output:
Capacity:> 10 Size:> 3 obj:> 102
Objeck
Class library support for Stack/IntStack/FloatStack <lang objeck>stack := IntStack->New(); stack->Push(13); stack->Push(7); (stack->Pop() + stack->Pop())->PrintLine(); stack->IsEmpty()->PrintLine();</lang>
Objective-C
Using a NSMutableArray: <lang objc>NSMutableArray *stack = [NSMutableArray array]; // creating
[stack addObject:value]; // pushing
id value = [stack lastObject]; [stack removeLastObject]; // popping
[stack count] == 0 // is empty?</lang>
OCaml
Implemented as a singly-linked list, wrapped in an object: <lang ocaml>exception Stack_empty
class ['a] stack =
object (self) val mutable lst : 'a list = []
method push x =
lst <- x::lst
method pop =
match lst with
[] -> raise Stack_empty
| x::xs -> lst <- xs;
x
method is_empty =
lst = []
end</lang>
Oforth
Stack is already defined at startup.
<lang Oforth>ListBuffer Class new: Stack Stack method: push self add ; Stack method: pop self removeLast ; Stack method: top self last ;</lang>
Usage : <lang Oforth>: testStack | s |
Stack new ->s s push(10) s push(11) s push(12) s top println s pop println s pop println s pop println s isEmpty ifTrue: [ "Stack is empty" println ] ;</lang>
- Output:
12 12 11 10 Stack is empty
Ol
Simplest stack can be implemented using 'cons' and 'uncons' primitives. <lang scheme> (define stack #null) (print "stack is: " stack) (print "is stack empty: " (eq? stack #null))
(print "* pushing 1") (define stack (cons 1 stack)) (print "stack is: " stack) (print "is stack empty: " (eq? stack #null))
(print "* pushing 2") (define stack (cons 2 stack)) (print "stack is: " stack) (print "is stack empty: " (eq? stack #null))
(print "* pushing 3") (define stack (cons 3 stack)) (print "stack is: " stack) (print "is stack empty: " (eq? stack #null))
(print "* poping") (define-values (value stack) (uncons stack #f)) (print "value: " value) (print "stack: " stack) (print "is stack empty: " (eq? stack #null))
(print "* poping") (define-values (value stack) (uncons stack #f)) (print "value: " value) (print "stack: " stack) (print "is stack empty: " (eq? stack #null))
(print "* poping") (define-values (value stack) (uncons stack #f)) (print "value: " value) (print "stack: " stack) (print "is stack empty: " (eq? stack #null))
(print "* poping") (define-values (value stack) (uncons stack #f)) (print "value: " value) (print "stack: " stack) (print "is stack empty: " (eq? stack #null)) </lang>
- Output:
stack is: () is stack empty: #true * pushing 1 stack is: (1) is stack empty: #false * pushing 2 stack is: (2 1) is stack empty: #false * pushing 3 stack is: (3 2 1) is stack empty: #false * poping value: 3 stack: (2 1) is stack empty: #false * poping value: 2 stack: (1) is stack empty: #false * poping value: 1 stack: () is stack empty: #true * poping value: #false stack: () is stack empty: #true
But in real programs may be useful a more complex stack implementation based on coroutines (ol is a purely functional lisp, so it does not support mutators like 'set!').
<lang scheme> (fork-server 'stack (lambda ()
(let this ((me '()))
(let*((envelope (wait-mail))
(sender msg envelope))
(case msg
(['empty]
(mail sender (null? me))
(this me))
(['push value]
(this (cons value me)))
(['pop]
(cond
((null? me)
(mail sender #false)
(this me))
(else
(mail sender (car me))
(this (cdr me))))))))))
(define (push value)
(mail 'stack ['push value]))
(define (pop)
(await (mail 'stack ['pop])))
(define (empty)
(await (mail 'stack ['empty])))
(for-each (lambda (n)
(print "pushing " n)
(push n))
(iota 5 1)) ; '(1 2 3 4 5)
(let loop ()
(print "is stack empty: " (empty))
(unless (empty)
(begin
(print "popping value, got " (pop))
(loop))))
(print "done.") </lang>
- Output:
pushing 1 pushing 2 pushing 3 pushing 4 pushing 5 is stack empty: #false popping value, got 5 is stack empty: #false popping value, got 4 is stack empty: #false popping value, got 3 is stack empty: #false popping value, got 2 is stack empty: #false popping value, got 1 is stack empty: #true done.
ooRexx
The ooRexx queue class functions as a stack as well (it is a dequeue really). <lang ooRexx> stack = .queue~of(123, 234) -- creates a stack with a couple of items stack~push("Abc") -- pushing value = stack~pull -- popping value = stack~peek -- peeking -- the is empty test if stack~isEmpty then say "The stack is empty" </lang>
OxygenBasic
The real stack is freely available! <lang oxygenbasic> function f()
sys a=1,b=2,c=3,d=4 push a push b push c push d print a "," b "," c "," d 'result 1,2,3,4 a=10 b=20 c=30 d=40 print a "," b "," c "," d 'result 10,20,30,40 pop a pop b pop c pop d print a "," b "," c "," d 'result 4,3,2,1
end function
f </lang>
Oz
A thread-safe, list-based stack. Implemented as a module: <lang oz>functor export
New Push Pop Empty
define
fun {New}
{NewCell nil}
end
proc {Push Stack Element}
NewStack
%% Use atomic swap for thread safety
OldStack = Stack := NewStack
in
NewStack = Element|OldStack
end
proc {Pop Stack ?Result}
NewStack
%% Use atomic swap for thread safety
OldStack = Stack := NewStack
in
Result|NewStack = OldStack
end
fun {Empty Stack}
@Stack == nil
end
end</lang> There is also a stack implementation in the standard library.
PARI/GP
<lang parigp>push(x)=v=concat(v,[x]);; pop()={
if(#v,
my(x=v[#v]);
v=vecextract(v,1<<(#v-1)-1);
x
,
error("Stack underflow")
)
}; empty()=v==[]; peek()={
if(#v,
v[#v]
,
error("Stack underflow")
)
};</lang>
Pascal
This implements stacks of integers in standard Pascal (should work on all existing Pascal dialects). <lang pascal>{ tStack is the actual stack type, tStackNode a helper type } type
pStackNode = ^tStackNode;
tStackNode = record
next: pStackNode;
data: integer;
end;
tStack = record
top: pStackNode;
end;
{ Always call InitStack before using a stack } procedure InitStack(var stack: tStack);
begin stack.top := nil end;
{ This function removes all content from a stack; call before disposing, or before a local stack variable goes out of scope } procedure ClearStack(var stack: tStack);
var node: pStackNode; begin while stack.top <> nil do begin node := stack.top; stack.top := stack.top^.next; dispose(node); end end;
function StackIsEmpty(stack: tStack):Boolean;
begin StackIsEmpty := stack.top = nil end;
procedure PushToStack(var stack: tStack; value: integer);
var node: pStackNode; begin new(node); node^.next := stack.top; node^.data := value; stack.top := node end;
{ may only be called on a non-empty stack! } function PopFromStack(var stack: tStack): integer;
var node: pStackNode; begin node := stack.top; stack.top := node^.next; PopFromStack := node^.data; dispose(node); end;</lang>
Perl
Perl comes prepared to treat its arrays as stacks, giving us the push and pop functions for free. To add empty, we basically give a new name to "not": <lang perl>sub empty{ not @_ }</lang>
Phix
with javascript_semantics -- comparing a simple implementation against using the builtins: sequence stack = {} procedure push_(object what) stack = append(stack,what) end procedure function pop_() object what = stack[$] stack = stack[1..$-1] return what end function function empty_() return length(stack)=0 end function ?empty_() -- 1 push_(5) ?empty_() -- 0 push_(6) ?pop_() -- 6 ?pop_() -- 5 ?empty_() -- 1 ?"===builtins===" requires("1.0.2") -- (latest bugfixes, plus top renamed as peep, for p2js) integer sid = new_stack() ?stack_empty(sid) -- 1 push(sid,5) ?stack_empty(sid) -- 0 push(sid,6) --?peep(sid) -- 6 (leaving it there) ?pop(sid) -- 6 ?pop(sid) -- 5 ?stack_empty(sid) -- 1
Note you get true/false rather than 1/0 under pwa/p2js (use printf(%t) for consistent results)
PHP
PHP arrays behave like a stack: <lang php>$stack = array();
empty( $stack ); // true
array_push( $stack, 1 ); // or $stack[] = 1; array_push( $stack, 2 ); // or $stack[] = 2;
empty( $stack ); // false
echo array_pop( $stack ); // outputs "2" echo array_pop( $stack ); // outputs "1"</lang>
PicoLisp
The built-in functions push and pop are used to maintain a stack (of any type). <lang PicoLisp>(push 'Stack 3) (push 'Stack 2) (push 'Stack 1)</lang>
: Stack -> (1 2 3) : (pop 'Stack) -> 1 : Stack -> (2 3) : (set 'Stack) # empty -> NIL : Stack -> NIL
Pike
Pike has a built in module ADT (Abstract Data Types) which among other things contains a stack.
<lang Pike> object s = ADT.Stack(); s->push("a"); s->push("b"); write("top: %O, pop1: %O, pop2: %O\n",
s->top(), s->pop(), s->pop());
s->reset(); // Empty the stack </lang>
- Output:
top: "b", pop1: "b", pop2: "a"
PL/I
<lang PL/I>/* Any controlled variable may behave as a stack. */
declare s float controlled;
/* to push a value on the stack. */ allocate s; s = 10;
/* To pop a value from the stack. */ put (s); free s;
/* to peek at the top of stack> */ put (s);
/* To see whether the stack is empty */ if allocation(s) = 0 then ...
/* Note: popping a value from the stack, or peeking, */ /* would usually require a check that the stack is not empty. */
/* Note: The above is a simple stack for S. */ /* S can be any kind of data structure, an array, etc. */
/* Example to push ten values onto the stack, and then to */ /* remove them. */
/* Push ten values, obtained from the input, onto the stack: */ declare S float controlled; do i = 1 to 10;
allocate s; get list (s);
end; /* To pop those values from the stack: */ do while (allocation(s) > 0);
put skip list (s); free s;
end; /* The values are printed in the reverse order, of course. */</lang>
PostScript
<lang postscript>% empty? is already defined. /push {exch cons}. /pop {uncons exch pop}. [2 3 4 5 6] 1 push = [1 2 3 4 5 6] [1 2 3 4 5 6] pop =[2 3 4 5 6] [2 3 4 5 6] empty? =false [] empty? =true</lang>
PowerShell
A new stack: <lang PowerShell> $stack = New-Object -TypeName System.Collections.Stack
- or
$stack = [System.Collections.Stack] @() </lang> Push some stuff on the stack: <lang PowerShell> 1, 2, 3, 4 | ForEach-Object {$stack.Push($_)} </lang> Show stack as a string: <lang PowerShell> $stack -join ", " </lang>
- Output:
4, 3, 2, 1
Pop the top level of the stack: <lang PowerShell> $stack.Pop() </lang>
- Output:
4
Show stack as a string: <lang PowerShell> $stack -join ", " </lang>
- Output:
3, 2, 1
Get a copy of the top level of the stack: <lang PowerShell> $stack.Peek() </lang>
- Output:
3
The stack: <lang PowerShell> $stack </lang>
- Output:
3 2 1
Prolog
Prolog is a particularly silly language to implement stack functions in, as the built-in lists can be treated as stacks in an ad hoc manner. Nonetheless, in the name of completeness: <lang prolog>% push( ELEMENT, STACK, NEW ) % True if NEW is [ELEMENT|STACK] push(ELEMENT,STACK,[ELEMENT|STACK]).
% pop( STACK, TOP, NEW ) % True if TOP and NEW are head and tail, respectively, of STACK pop([TOP|STACK],TOP,STACK).
% empty( STACK ) % True if STACK is empty empty([]).</lang>
PureBasic
For LIFO function PureBasic normally uses linked lists. Usage as described above could look like; <lang PureBasic>Global NewList MyStack()
Procedure Push_LIFO(n)
FirstElement(MyStack()) InsertElement(MyStack()) MyStack() = n
EndProcedure
Procedure Pop_LIFO()
If FirstElement(MyStack()) Topmost = MyStack() DeleteElement(MyStack()) EndIf ProcedureReturn Topmost
EndProcedure
Procedure Empty_LIFO()
Protected Result If ListSize(MyStack())=0 Result = #True EndIf ProcedureReturn Result
EndProcedure
Procedure Peek_LIFO()
If FirstElement(MyStack()) Topmost = MyStack() EndIf ProcedureReturn Topmost
EndProcedure
- ---- Example of implementation ----
Push_LIFO(3) Push_LIFO(1) Push_LIFO(4) While Not Empty_LIFO()
Debug Pop_LIFO()
Wend</lang>
- Output:
4 1 3
Python
The faster and Pythonic way is using a deque (available from 2.4). A regular list is a little slower. <lang python>from collections import deque stack = deque() stack.append(value) # pushing value = stack.pop() not stack # is empty?</lang> If you need to expose your stack to the world, you may want to create a simpler wrapper: <lang python>from collections import deque
class Stack:
def __init__(self):
self._items = deque()
def append(self, item):
self._items.append(item)
def pop(self):
return self._items.pop()
def __nonzero__(self):
return bool(self._items)</lang>
Here is a stack implemented as linked list - with the same list interface. <lang python>class Stack:
def __init__(self):
self._first = None
def __nonzero__(self):
return self._first is not None
def append(self, value):
self._first = (value, self._first)
def pop(self):
if self._first is None:
raise IndexError, "pop from empty stack"
value, self._first = self._first
return value</lang>
Notes:
Using list interface - append, __nonzero__ make it easier to use, cleanup the client code, and allow changing the implementation later without affecting the client code. For example, instead of: <lang python>while not stack.empty():</lang> You can write: <lang python>while stack:</lang> Quick testing show that deque is about 5 times faster then the wrapper linked list implementations. This may be important if your stack is used in tight loops.
Quackery
Quackery is a stack based language. In addition to the stack (i.e. the Quackery data stack) and the call stack, named ancillary stacks can be created with [ stack ] is <name-of-stack>. Pushing to and popping from ancillary stacks is done with the words put and take. A word to test if an ancillary stack is empty can be defined as [ size 1 = ] is isempty. (The word empty already has a meaning in Quackery.) The word share returns the topmost element of an ancillary stack without changing the ancillary stack. Other ancillary stack operations are also available.
<lang Quackery>[ size 1 = ] is isempty ( s --> b )
[ stack ] is mystack ( --> s )
mystack isempty if [ say "mystack is empty" cr cr ] 23 mystack put mystack share echo say " is on the top of mystack" cr cr
mystack mystack put ( you can put anything on an ancillary stack, even itself! )
mystack share echo say " is on the top of mystack" cr cr mystack take echo say " has been removed from mystack" cr cr mystack take echo say " has been removed from mystack" cr cr mystack isempty if [ say "mystack is empty" cr cr ] say "you are in a maze of twisty little passages, all alike"</lang>
- Output:
mystack is empty 23 is on the top of mystack mystack is on the top of mystack mystack has been removed from mystack 23 has been removed from mystack mystack is empty you are in a maze of twisty little passages, all alike
R
See FIFO for functional and object oriented implementations of a First-In-First-Out object, with similar code. <lang R>library(proto)
stack <- proto(expr = {
l <- list()
empty <- function(.) length(.$l) == 0
push <- function(., x)
{
.$l <- c(list(x), .$l)
print(.$l)
invisible()
}
pop <- function(.)
{
if(.$empty()) stop("can't pop from an empty list")
.$l1 <- NULL
print(.$l)
invisible()
}
})
stack$empty()
- [1] TRUE
stack$push(3)
- 1
- [1] 3
stack$push("abc")
stack$push(matrix(1:6, nrow=2))
stack$empty()
- [1] FALSE
stack$pop()
[1] "abc"
- 2
- [1] 3
stack$pop()
- 1
- [1] 3
stack$pop()
- list()
stack$pop()
- Error in get("pop", env = stack, inherits = TRUE)(stack, ...) :
- can't pop from an empty list</lang>
Racket
Quick functional version:
<lang Racket>
- lang racket
(define stack '()) (define (push x stack) (cons x stack)) (define (pop stack) (values (car stack) (cdr stack))) (define (empty? stack) (null? stack)) </lang>
And a destructive object:
<lang Racket> (struct stack ([items #:auto]) #:mutable #:auto-value '()) (define (push! x stack)
(set-stack-items! stack (cons x (stack-items stack))))
(define (pop! stack)
(begin0 (car (stack-items stack)) (set-stack-items! stack (cdr (stack-items stack)))))
(define (empty? stack)
(null? (stack-items stack)))
</lang>
Raku
(formerly Perl 6)
Raku still has the stack functions from Perl 5, but now they also can be accessed by object notation: <lang perl6>my @stack; # just a array @stack.push($elem); # add $elem to the end of @stack $elem = @stack.pop; # get the last element back @stack.elems == 0 # true, because the stack is empty not @stack # also true because @stack is false</lang>
Raven
Use built in stack type: <lang raven>new stack as s 1 s push s pop</lang> Word empty is also built in: <lang raven>s empty if 'stack is empty' print</lang>
REBOL
<lang rebol>REBOL [ Title: "Stack" URL: http://rosettacode.org/wiki/Stack ]
stack: make object! [ data: copy []
push: func [x][append data x] pop: func [/local x][x: last data remove back tail data x] empty: does [empty? data]
peek: does [last data] ]
- Teeny Tiny Test Suite
assert: func [code][print [either do code [" ok"]["FAIL"] mold code]]
print "Simple integers:" s: make stack [] s/push 1 s/push 2 ; Initialize.
assert [2 = s/peek] assert [2 = s/pop] assert [1 = s/pop] assert [s/empty]
print [lf "Symbolic data on stack:"] v: make stack [data: [this is a test]] ; Initialize on instance.
assert ['test = v/peek] assert ['test = v/pop] assert ['a = v/pop] assert [not v/empty]</lang> Sample run:
Simple integers: ok [2 = s/peek] ok [2 = s/pop] ok [1 = s/pop] ok [s/empty] Symbolic data on stack: ok ['test = v/peek] ok ['test = v/pop] ok ['a = v/pop] ok [not v/empty]
Retro
<lang Retro>: stack ( n"- ) create 0 , allot ;
- push ( na- ) dup ++ dup @ + ! ;
- pop ( a-n ) dup @ over -- + @ ;
- top ( a-n ) dup @ + @ ;
- empty? ( a-f ) @ 0 = ;
10 stack st
1 st push 2 st push 3 st push st empty? putn st top putn st pop putn st pop putn st pop putn st empty? putn</lang>
REXX
version 1
<lang rexx>y=123 /*define a REXX variable, value is 123 */ push y /*pushes 123 onto the stack. */ pull g /*pops last value stacked & removes it. */ q=empty() /*invokes the EMPTY subroutine (below)*/ exit /*stick a fork in it, we're done. */
empty: return queued() /*subroutine returns # of stacked items.*/</lang>
version 2
<lang rexx>/* REXX ***************************************************************
- supports push, pull, and peek
- 11.08.2013 Walter Pachl
- /
stk.=0 Call push 123 Say empty() say peek() say pull() Say empty() say peek() say push(456) say peek() Exit
push: Procedure Expose stk.
Parse Arg v z=stk.0+1 stk.z=v stk.0=z Return v
peek: Procedure Expose stk.
If stk.0=0 Then Return 'stack is empty' Else Do z=stk.0 Return stk.z End
pull: Procedure Expose stk.
If stk.0=0 Then Return 'stack is empty' Else Do z=stk.0 res=stk.z stk.0=stk.0-1 Return res End
empty: Procedure Expose stk.
Return stk.0=0</lang>
- Output:
0 123 123 1 stack is empty 456 456
Ring
<lang ring>
- Project : Stack
load "stdlib.ring" ostack = new stack for n = 5 to 7
see "Push: " + n + nl
ostack.push(n)
next see "Pop:" + ostack.pop() + nl see "Push: " + "8" + nl ostack.push(8) while len(ostack) > 0
see "Pop:" + ostack.pop() + nl
end if len(ostack) = 0
see "Pop: stack is empty" + nl
ok </lang> Output:
Push: 5 Push: 6 Push: 7 Pop:7 Push: 8 Pop:8 Pop:6 Pop:5 Pop: stack is empty
Ruby
Using an Array, there are already methods Array#push, Array#pop and Array#empty?. <lang ruby>stack = [] stack.push(value) # pushing value = stack.pop # popping stack.empty? # is empty?</lang> If you need to expose your stack to the world, you may want to create a simpler wrapper. Here is a wrapper class Stack that wraps Array but only exposes stack methods. <lang ruby>require 'forwardable'
- A stack contains elements in last-in, first-out order.
- Stack#push adds new elements to the top of the stack;
- Stack#pop removes elements from the top.
class Stack
extend Forwardable
# Creates a Stack containing _objects_.
def self.[](*objects)
new.push(*objects)
end
# Creates an empty Stack.
def initialize
@ary = []
end
# Duplicates a Stack.
def initialize_copy(obj)
super
@ary = @ary.dup
end
# Adds each object to the top of this Stack. Returns self.
def push(*objects)
@ary.push(*objects)
self
end
alias << push
##
# :method: pop
# :call-seq:
# pop -> obj or nil
# pop(n) -> ary
#
# Removes an element from the top of this Stack, and returns it.
# Returns nil if the Stack is empty.
#
# If passing a number _n_, removes the top _n_ elements, and returns
# an Array of them. If this Stack contains fewer than _n_ elements,
# returns them all. If this Stack is empty, returns an empty Array.
def_delegator :@ary, :pop
##
# :method: top
# :call-seq:
# top -> obj or nil
# top(n) -> ary
# Returns the topmost element without modifying the stack.
def_delegator :@ary, :last, :top
##
# :method: empty?
# Returns true if this Stack contains no elements.
def_delegator :@ary, :empty?
##
# :method: size
# Returns the number of elements in this Stack.
def_delegator :@ary, :size
alias length size
# Converts this Stack to a String.
def to_s
"#{self.class}#{@ary.inspect}"
end
alias inspect to_s
end</lang>
<lang ruby>p s = Stack.new # => Stack[] p s.empty? # => true p s.size # => 0 p s.top # => nil p s.pop # => nil p s.pop(1) # => [] p s.push(1) # => Stack[1] p s.push(2, 3) # => Stack[1, 2, 3] p s.top # => 3 p s.top(2) # => [2, 3] p s # => Stack[1, 2, 3] p s.size # => 3 p s.pop # => 3 p s.pop(1) # => [2] p s.empty? # => false
p s = Stack[:a, :b, :c] # => Stack[:a, :b, :c] p s << :d # => Stack[:a, :b, :c, :d] p s.pop # => :d</lang>
Just meeting the requirements of a push, pop and empty method: <lang ruby>require 'forwardable'
class Stack
extend Forwardable
def initialize @stack = [] end
def_delegators :@stack, :push, :pop, :empty?
end </lang> (push takes multiple arguments; pop takes an optional argument which specifies how many to pop)
Run BASIC
<lang runbasic>dim stack$(10) ' stack of ten global stack$ global stackEnd
for i = 1 to 5 ' push 5 values to the stack
a$ = push$(chr$(i + 64)) print "Pushed ";chr$(i + 64);" stack has ";stackEnd
next i
print "Pop Value:";pop$();" stack has ";stackEnd ' pop last in print "Pop Value:";pop$();" stack has ";stackEnd ' pop last in
e$ = mt$() ' MT the stack print "Empty stack. stack has ";stackEnd
' ------ PUSH the stack FUNCTION push$(val$) stackEnd = stackEnd + 1 ' if more than 10 then lose the oldest if stackEnd > 10 then
for i = 0 to 9
stack$(i) = stack$(i+1)
next i
stackEnd = 10
end if stack$(stackEnd) = val$ END FUNCTION
' ------ POP the stack ----- FUNCTION pop$() if stackEnd = 0 then
pop$ = "Stack is MT" else pop$ = stack$(stackEnd) ' pop last in stackEnd = max(stackEnd - 1,0)
end if END FUNCTION
' ------ MT the stack ------ FUNCTION mt$()
stackEnd = 0
END FUNCTION</lang>
- Output:
Pushed A stack has 1 Pushed B stack has 2 Pushed C stack has 3 Pushed D stack has 4 Pushed E stack has 5 Pop Value:E stack has 4 Pop Value:D stack has 3 Empty stack. stack has 0
Rust
Using the standard library
One could just use a vector (Vec<T>) which is part of the standard library
<lang rust>fn main() {
let mut stack = Vec::new();
stack.push("Element1");
stack.push("Element2");
stack.push("Element3");
assert_eq!(Some(&"Element3"), stack.last());
assert_eq!(Some("Element3"), stack.pop());
assert_eq!(Some("Element2"), stack.pop());
assert_eq!(Some("Element1"), stack.pop());
assert_eq!(None, stack.pop());
}</lang>
Simple implementation
Simply uses a singly-linked list. <lang rust>type Link<T> = Option<Box<Frame<T>>>;
pub struct Stack<T> {
head: Link<T>,
} struct Frame<T> {
elem: T, next: Link<T>,
}
/// Iterate by value (consumes list) pub struct IntoIter<T>(Stack<T>); impl<T> Iterator for IntoIter<T> {
type Item = T;
fn next(&mut self) -> Option<Self::Item> {
self.0.pop()
}
}
/// Iterate by immutable reference pub struct Iter<'a, T: 'a> {
next: Option<&'a Frame<T>>,
} impl<'a, T> Iterator for Iter<'a, T> { // Iterate by immutable reference
type Item = &'a T;
fn next(&mut self) -> Option<Self::Item> {
self.next.take().map(|frame| {
self.next = frame.next.as_ref().map(|frame| &**frame);
&frame.elem
})
}
}
/// Iterate by mutable reference pub struct IterMut<'a, T: 'a> {
next: Option<&'a mut Frame<T>>,
} impl<'a, T> Iterator for IterMut<'a, T> {
type Item = &'a mut T;
fn next(&mut self) -> Option<Self::Item> {
self.next.take().map(|frame| {
self.next = frame.next.as_mut().map(|frame| &mut **frame);
&mut frame.elem
})
}
}
impl<T> Stack<T> {
/// Return new, empty stack
pub fn new() -> Self {
Stack { head: None }
}
/// Add element to top of the stack
pub fn push(&mut self, elem: T) {
let new_frame = Box::new(Frame {
elem: elem,
next: self.head.take(),
});
self.head = Some(new_frame);
}
/// Remove element from top of stack, returning the value
pub fn pop(&mut self) -> Option<T> {
self.head.take().map(|frame| {
let frame = *frame;
self.head = frame.next;
frame.elem
})
}
/// Get immutable reference to top element of the stack
pub fn peek(&self) -> Option<&T> {
self.head.as_ref().map(|frame| &frame.elem)
}
/// Get mutable reference to top element on the stack
pub fn peek_mut(&mut self) -> Option<&mut T> {
self.head.as_mut().map(|frame| &mut frame.elem)
}
/// Iterate over stack elements by value
pub fn into_iter(self) -> IntoIter<T> {
IntoIter(self)
}
/// Iterate over stack elements by immutable reference
pub fn iter<'a>(&'a self) -> Iter<'a,T> {
Iter { next: self.head.as_ref().map(|frame| &**frame) }
}
/// Iterate over stack elements by mutable reference
pub fn iter_mut(&mut self) -> IterMut<T> {
IterMut { next: self.head.as_mut().map(|frame| &mut **frame) }
}
}
// The Drop trait tells the compiler how to free an object after it goes out of scope. // By default, the compiler would do this recursively which *could* blow the stack for // extraordinarily long lists. This simply tells it to do it iteratively. impl<T> Drop for Stack<T> {
fn drop(&mut self) {
let mut cur_link = self.head.take();
while let Some(mut boxed_frame) = cur_link {
cur_link = boxed_frame.next.take();
}
}
}</lang>
Sather
This one uses a builtin linked list to keep the values pushed onto the stack. <lang sather>class STACK{T} is
private attr stack :LLIST{T};
create:SAME is
res ::= new;
res.stack := #LLIST{T};
return res;
end;
push(elt: T) is stack.insert_front(elt); end;
pop: T is
if ~stack.is_empty then
stack.rewind;
r ::= stack.current;
stack.delete;
return r;
else
raise "stack empty!\n";
end;
end;
top: T is stack.rewind; return stack.current; end;
is_empty: BOOL is return stack.is_empty; end;
end;</lang>
<lang sather>class MAIN is
main is
s ::= #STACK{INT};
#OUT + "push values...\n";
s.push(3);
s.push(2);
s.push(1);
s.push(0);
#OUT + "retrieving them...\n";
loop
#OUT + s.pop + "\n";
until!(s.is_empty); end;
end;
end;</lang>
Sather library has the abstract class $STACK{T}, but using this forces us to implement other methods too.
Scala
The Do it yourself approach: <lang Scala>class Stack[T] {
private var items = List[T]()
def isEmpty = items.isEmpty
def peek = items match {
case List() => error("Stack empty")
case head :: rest => head
}
def pop = items match {
case List() => error("Stack empty")
case head :: rest => items = rest; head
}
def push(value: T) = items = value +: items
}</lang> Or use the standard Scala library. Slightly modified to meet to requirements of this task. <lang scala>import collection.mutable.{ Stack => Stak }
class Stack[T] extends Stak[T] {
override def pop: T = {
if (this.length == 0) error("Can't Pop from an empty Stack.")
else super.pop
}
def peek: T = this.head
}</lang>A test could be:<lang Scala>object StackTest extends App {
val stack = new Stack[String]
stack.push("Peter Pan")
stack.push("Suske & Wiske", "Alice in Wonderland")
assert(stack.peek == "Alice in Wonderland")
assert(stack.pop() == "Alice in Wonderland")
assert(stack.pop() == "Suske & Wiske")
assert(stack.pop() == "Peter Pan")
println("Completed without errors")
}</lang>
Scheme
This version uses primitive message passing. <lang scheme>(define (make-stack)
(let ((st '()))
(lambda (message . args)
(case message
((empty?) (null? st))
((top) (if (null? st)
'empty
(car st)))
((push) (set! st (cons (car args) st)))
((pop) (if (null? st)
'empty
(let ((result (car st)))
(set! st (cdr st))
result)))
(else 'badmsg)))))</lang>
Seed7
<lang seed7>$ include "seed7_05.s7i";
const func type: stack (in type: baseType) is func
result var type: stackType is void; begin stackType := array baseType;
const proc: push (inout stackType: aStack, in baseType: top) is func
begin
aStack := [] (top) & aStack;
end func;
const func baseType: pop (inout stackType: aStack) is func
result
var baseType: top is baseType.value;
begin
if length(aStack) = 0 then
raise RANGE_ERROR;
else
top := aStack[1];
aStack := aStack[2 ..];
end if;
end func;
const func boolean: empty (in stackType: aStack) is
return length(aStack) = 0;
end func;
const type: intStack is stack(integer);
const proc: main is func
local var intStack: s is intStack.value; begin push(s, 10); push(s, 20); writeln(pop(s) = 20); writeln(pop(s) = 10); writeln(empty(s)); end func;</lang>
SenseTalk
<lang sensetalk>put () into stack repeat with each item of 1 .. 10 push it into stack end repeat
repeat while stack is not empty pop stack put it end repeat</lang>
Sidef
Using a built-in array: <lang ruby>var stack = []; stack.push(42); # pushing say stack.pop; # popping say stack.is_empty; # is_emtpy?</lang>
Creating a Stack class: <lang ruby>class Stack(stack=[]) {
method pop { stack.pop };
method push(item) { stack.push(item) };
method empty { stack.is_empty };
}
var stack = Stack(); stack.push(42); say stack.pop; # => 42 say stack.empty; # => true</lang>
Slate
From Slate's standard library: <lang slate>collections define: #Stack &parents: {ExtensibleArray}. "An abstraction over ExtensibleArray implementations to follow the stack protocol. The convention is that the Sequence indices run least-to-greatest from bottom to top."
s@(Stack traits) push: obj [s addLast: obj].
s@(Stack traits) pop [s removeLast].
s@(Stack traits) pop: n [s removeLast: n].
s@(Stack traits) top [s last].
s@(Stack traits) top: n [s last: n].
s@(Stack traits) bottom [s first].</lang>
Smalltalk
Smalltalk has a built-in Stack class, instances of which you can send messages: <lang smalltalk> s := Stack new. s push: 1. s push: 2. s push: 3. s pop. s top. "2" </lang>
Standard ML
The signature for a module supplying a stack interface, with a couple added functions.
<lang sml>signature STACK = sig
type 'a stack exception EmptyStack
val empty : 'a stack val isEmpty : 'a stack -> bool
val push : ('a * 'a stack) -> 'a stack
val pop : 'a stack -> 'a stack
val top : 'a stack -> 'a
val popTop : 'a stack -> 'a stack * 'a
val map : ('a -> 'b) -> 'a stack -> 'b stack
val app : ('a -> unit) -> 'a stack -> unit
end</lang>
An implementation of the STACK signature, using immutable lists.
<lang sml>structure Stack :> STACK = struct
type 'a stack = 'a list exception EmptyStack
val empty = []
fun isEmpty st = null st
fun push (x, st) = x::st
fun pop [] = raise EmptyStack
| pop (x::st) = st
fun top [] = raise EmptyStack
| top (x::st) = x
fun popTop st = (pop st, top st)
fun map f st = List.map f st fun app f st = List.app f st
end</lang>
Stata
Swift
Generic stack. <lang Swift>struct Stack<T> {
var items = [T]()
var empty:Bool {
return items.count == 0
}
func peek() -> T {
return items[items.count - 1]
}
mutating func pop() -> T {
return items.removeLast()
}
mutating func push(obj:T) {
items.append(obj)
}
}
var stack = Stack<Int>() stack.push(1) stack.push(2) println(stack.pop()) println(stack.peek()) stack.pop() println(stack.empty)</lang>
- Output:
2 1 true
Tailspin
<lang tailspin> processor Stack
@: $;
sink push ..|@Stack: $; end push
source peek $@Stack(last) ! end peek
source pop ^@Stack(last) ! end pop
source empty $@Stack::length -> # <=0> 1 ! <> 0 ! end empty
end Stack
def myStack: [1] -> Stack;
2 -> !myStack::push
'$myStack::empty; $myStack::pop; ' -> !OUT::write '$myStack::empty; $myStack::pop; ' -> !OUT::write '$myStack::empty; ' -> !OUT::write
3 -> !myStack::push '$myStack::empty; $myStack::peek; ' -> !OUT::write '$myStack::empty; $myStack::pop; ' -> !OUT::write '$myStack::empty;' -> !OUT::write </lang>
- Output:
0 2 0 1 1 0 3 0 3 1
Tcl
Here's a simple implementation using a list: <lang tcl>proc push {stackvar value} {
upvar 1 $stackvar stack lappend stack $value
} proc pop {stackvar} {
upvar 1 $stackvar stack set value [lindex $stack end] set stack [lrange $stack 0 end-1] return $value
} proc size {stackvar} {
upvar 1 $stackvar stack llength $stack
} proc empty {stackvar} {
upvar 1 $stackvar stack
expr {[size stack] == 0}
} proc peek {stackvar} {
upvar 1 $stackvar stack lindex $stack end
}
set S [list] empty S ;# ==> 1 (true) push S foo empty S ;# ==> 0 (false) push S bar peek S ;# ==> bar pop S ;# ==> bar peek S ;# ==> foo</lang>
<lang tcl>package require struct::stack struct::stack S S size ;# ==> 0 S push a b c d e S size ;# ==> 5 S peek ;# ==> e S pop ;# ==> e S peek ;# ==> d S pop 4 ;# ==> d c b a S size ;# ==> 0</lang>
UnixPipes
<lang bash>init() { if [ -e stack ]; then rm stack; fi } # force pop to blow up if empty push() { echo $1 >> stack; } pop() { tail -1 stack; x=`head -n -1 stack | wc -c` if [ $x -eq '0' ]; then rm stack; else truncate -s `head -n -1 stack | wc -c` stack fi } empty() { head -n -1 stack |wc -l; } stack_top() { tail -1 stack; }</lang> test it: <lang bash>% push me; push you; push us; push them % pop;pop;pop;pop them us you me</lang>
UNIX Shell
Here's a simple single-stack solution: <lang sh>init() {
if -n $KSH_VERSION ; then set -A stack else stack=(); # this sets stack to '()' in ksh fi
}
push() {
stack=("$1" "${stack[@]}")
}
stack_top() {
# this approach sidesteps zsh indexing difference
set -- "${stack[@]}"
printf '%s\n' "$1"
}
pop() {
stack_top
stack=("${stack[@]:1}")
}
empty() {
(( ${#stack[@]} == 0 ))
}
- Demo
push fred; push wilma; push betty; push barney printf 'peek(stack)==%s\n' "$(stack_top)" while ! empty; do
pop
done</lang>
- Output:
peek(stack)==barney barney betty wilma fred
You can generalize it to multiple stacks with some judicious use of the twin evils of pass-by-name and eval:
<lang sh>init_stack() {
if -n $KSH_VERSION ; then eval 'set -A '"$1" else eval "$1=()" fi
}
push() {
eval "$1"'=("$2" "${'"$1"'[@]}")'
}
stack_top() {
eval 'set -- "${'"$1"'[@]}"';
printf '%s\n' "$1"
}
pop() {
stack_top "$1";
eval "$1"'=("${'"$1"'[@]:1}")'
}
empty() {
eval '(( ${#'"$1"'[@]} == 0 ))'
}
init_stack mystack push mystack fred; push mystack wilma; push mystack betty; push mystack barney printf 'peek(mystack)==%s\n' "$(stack_top mystack)" while ! empty mystack; do
pop mystack
done</lang>
- Output:
peek(mystack)==barney barney betty wilma fred
VBA
Define a class Stack in a class module with that name. <lang vb>'Simple Stack class
'uses a dynamic array of Variants to stack the values 'has read-only property "Size" 'and methods "Push", "Pop", "IsEmpty"
Private myStack() Private myStackHeight As Integer
'method Push Public Function Push(aValue)
'increase stack height myStackHeight = myStackHeight + 1 ReDim Preserve myStack(myStackHeight) myStack(myStackHeight) = aValue
End Function
'method Pop Public Function Pop()
'check for nonempty stack If myStackHeight > 0 Then Pop = myStack(myStackHeight) myStackHeight = myStackHeight - 1 Else MsgBox "Pop: stack is empty!" End If
End Function
'method IsEmpty Public Function IsEmpty() As Boolean
IsEmpty = (myStackHeight = 0)
End Function
'property Size Property Get Size() As Integer
Size = myStackHeight
End Property</lang> Usage example: <lang vb>'stack test Public Sub stacktest()
Dim aStack As New Stack
With aStack
'push and pop some value
.Push 45
.Push 123.45
.Pop
.Push "a string"
.Push "another string"
.Pop
.Push Cos(0.75)
Debug.Print "stack size is "; .Size
While Not .IsEmpty
Debug.Print "pop: "; .Pop
Wend
Debug.Print "stack size is "; .Size
'try to continue popping
.Pop
End With
End Sub</lang>
- Output:
stacktest stack size is 3 pop: 0,731688868873821 pop: a string pop: 45 stack size is 0
(after wich a message box will pop up)
VBScript
Stack class
<lang vb>class stack dim tos dim stack() dim stacksize
private sub class_initialize stacksize = 100 redim stack( stacksize ) tos = 0 end sub
public sub push( x ) stack(tos) = x tos = tos + 1 end sub
public property get stackempty stackempty = ( tos = 0 ) end property
public property get stackfull stackfull = ( tos > stacksize ) end property
public property get stackroom stackroom = stacksize - tos end property
public function pop() pop = stack( tos - 1 ) tos = tos - 1 end function
public sub resizestack( n ) redim preserve stack( n ) stacksize = n if tos > stacksize then tos = stacksize end if end sub end class
dim s set s = new stack s.resizestack 10 wscript.echo s.stackempty dim i for i = 1 to 10 s.push rnd wscript.echo s.stackroom if s.stackroom = 0 then exit for next for i = 1 to 10 wscript.echo s.pop if s.stackempty then exit for next</lang>
- Output:
(changes every time)
-1 9 8 7 6 5 4 3 2 1 0 0.7090379 0.81449 0.7607236 1.401764E-02 0.7747401 0.301948 0.2895625 0.5795186 0.533424 0.7055475
Using an ArrayList.
<lang vb>' Stack Definition - VBScript
Option Explicit
Dim stack, i, x Set stack = CreateObject("System.Collections.ArrayList") If Not empty_(stack) Then Wscript.Echo stack.Count push stack, "Banana" push stack, "Apple" push stack, "Pear" push stack, "Strawberry" Wscript.Echo "Count=" & stack.Count ' --> Count=4 Wscript.Echo pop(stack) & " - Count=" & stack.Count ' --> Strawberry - Count=3 Wscript.Echo "Tail=" & stack.Item(0) ' --> Tail=Banana Wscript.Echo "Head=" & stack.Item(stack.Count-1) ' --> Head=Pear Wscript.Echo stack.IndexOf("Apple", 0) ' --> 1 For i=1 To stack.Count Wscript.Echo join(stack.ToArray(), ", ") x = pop(stack) Next 'i
Sub push(s, what)
s.Add what
End Sub 'push
Function pop(s) Dim what
If s.Count > 0 Then
what = s(s.Count-1)
s.RemoveAt s.Count-1
Else
what = ""
End If
pop = what
End Function 'pop
Function empty_(s)
empty_ = s.Count = 0
End Function 'empty_ </lang>
- Output:
Count=4 Strawberry - Count=3 Tail=Banana Head=Pear 1 Banana, Apple, Pear Banana, Apple Banana
Vlang
<lang go>const (
max_depth = 256
)
struct Stack { mut:
data []f32 = []f32{len: max_depth}
depth int
}
fn (mut s Stack) push(v f32) {
if s.depth >= max_depth {
return
}
println('Push: ${v:3.2f}')
s.data[s.depth] = v
s.depth++
}
fn (mut s Stack) pop() ?f32 {
if s.depth > 0 {
s.depth--
result := s.data[s.depth]
println('Pop: top of stack was ${result:3.2f}')
return result
}
return error('Stack Underflow!!')
}
fn (s Stack) peek() ?f32 {
if s.depth > 0 {
result := s.data[s.depth - 1]
println('Peek: top of stack is ${result:3.2f}')
return result
}
return error('Out of Bounds...')
}
fn (s Stack) empty() bool {
return s.depth == 0
}
fn main() {
mut stack := Stack{}
println('Stack is empty? ' + if stack.empty() { 'Yes' } else { 'No' })
stack.push(5.0)
stack.push(4.2)
println('Stack is empty? ' + if stack.empty() { 'Yes' } else { 'No' })
stack.peek() or { return }
stack.pop() or { return }
stack.pop() or { return }
} </lang>
- Output:
Stack is empty? Yes Push: 5.00 Push: 4.20 Stack is empty? No Peek: top of stack is 4.20 Pop: top of stack was 4.20 Pop: top of stack was 5.00
Wart
Stacks as user-defined objects backed by a list.
<lang wart>def (stack)
(tag 'stack nil)
mac (push! x s) :qcase `(isa stack ,s)
`(push! ,x (rep ,s))
mac (pop! s) :qcase `(isa stack ,s)
`(pop! (rep ,s))
def (empty? s) :case (isa stack s)
(empty? rep.s)</lang>
Example usage:
s <- (stack) => (object stack nil) push! 3 s => (object stack (3)) push! 4 s => (object stack (4 3)) push! 5 s => (object stack (5 4 3)) pop! s => 5 (empty? s) => nil pop! s => 4 pop! s => 3 (empty? s) => 1 # true
Wren
This uses the Stack class in the above module. <lang ecmascript>import "/seq" for Stack
var s = Stack.new() s.push(1) s.push(2) System.print("Stack contains %(s.toList)") System.print("Number of elements in stack = %(s.count)") var item = s.pop() System.print("'%(item)' popped from the stack") System.print("Last element is now %(s.peek())") s.clear() System.print("Stack cleared") System.print("Is stack now empty? %((s.isEmpty) ? "yes" : "no")")</lang>
- Output:
Stack contains [1, 2] Number of elements in stack = 2 '2' popped from the stack Last element is now 1 Stack cleared Is stack now empty? yes
X86 Assembly
<lang x86asm>
- x86_64 linux nasm
struc Stack
maxSize: resb 8 currentSize: resb 8 contents:
endStruc
section .data
soError: db "Stack Overflow Exception", 10 seError: db "Stack Empty Error", 10
section .text
createStack:
- IN
- max number of elements (rdi)
- OUT
- pointer to new stack (rax)
push rdi xor rdx, rdx mov rbx, 8 mul rbx mov rcx, rax mov rax, 12 mov rdi, 0 syscall push rax mov rdi, rax add rdi, rcx mov rax, 12 syscall pop rax pop rbx mov qword [rax + maxSize], rbx mov qword [rax + currentSize], 0 ret
push:
- IN
- stack to operate on (stack argument), element to push (rdi)
- OUT
- void
mov rax, qword [rsp + 8] mov rbx, qword [rax + currentSize] cmp rbx, qword [rax + maxSize] je stackOverflow lea rsi, [rax + contents + 8*rbx] mov qword [rsi], rdi add qword [rax + currentSize], 1 ret
pop:
- pop
- IN
- stack to operate on (stack argument)
- OUT
- element from stack top
mov rax, qword [rsp + 8] mov rbx, qword [rax + currentSize] cmp rbx, 0 je stackEmpty sub rbx, 1 lea rsi, [rax + contents + 8*rbx] mov qword [rax + currentSize], rbx mov rax, qword [rsi] ret
- stack operation exceptions
stackOverflow:
mov rsi, soError mov rdx, 25 jmp errExit
stackEmpty:
mov rsi, seError mov rdx, 18
errExit:
mov rax, 1 mov rdi, 1 syscall mov rax, 60 mov rdi, 1 syscall
</lang>
XLISP
This is a fairly straightforward implementation, representing a stack as a linked list inside an object. <lang lisp>(define-class stack
(instance-variables vals))
(define-method (stack 'initialize)
(setq vals '()) self)
(define-method (stack 'push x)
(setq vals (cons x vals)))
(define-method (stack 'pop)
(define tos (car vals)) (setq vals (cdr vals)) tos)
(define-method (stack 'emptyp)
(null vals))</lang>
A sample REPL session: <lang lisp>; Loading 'stack.lsp' [1] (define st (stack 'new))
ST [2] (st 'push 1)
(1) [3] (st 'push 2)
(2 1) [4] (st 'emptyp)
() [5] (st 'pop)
2 [6] (st 'pop)
1 [7] (st 'emptyp)
- T
[8] </lang>
XPL0
<lang XPL0>include c:\cxpl\codes; \intrinsic 'code' declarations int Stack(100), SP;
proc Push(I); \Push an integer onto the Stack int I; [SP:= SP+1; Stack(SP):= I; ]; \Push
func Pop; \Pop an integer from the Stack int I; [I:= Stack(SP); SP:= SP-1; return I; ]; \Pop
func Empty; \Return 'true' if Stack is empty return SP<0;
func Top; \Return the integer at top of Stack return Stack(SP);
int I; [SP:= -1; \initialize stack pointer for I:= 0 to 10 do Push(I*I); IntOut(0, Top); CrLf(0); while not Empty do [IntOut(0, Pop); ChOut(0, ^ )]; CrLf(0); ]</lang>
- Output:
100 100 81 64 49 36 25 16 9 4 1 0
Yabasic
<lang Yabasic>limit = 1000 dim stack(limit)
top = 0
sub push(n)
if top < limit then
top = top + 1 : stack(top) = n
else
print "stack full - ";
end if
end sub
sub pop()
if top then
top = top - 1 : return stack(top + 1)
else
print "stack empty - ";
end if
end sub
sub empty()
return not top
end sub
// ======== test ========
for n = 3 to 5
print "Push ", n : push(n)
next
print "Pop ", pop()
print "Push ", 6 : push(6)
while(not empty())
print "Pop ", pop()
wend
print "Pop ", pop() </lang>
Z80 Assembly
The stack can be initialized by loading it directly with an immediate value. Z80-based home computers such as the Amstrad CPC and ZX Spectrum do this for you. Messing with the stack on those systems is a bad idea, since an assembly program stored on a floppy disk or cassette tape begins with the return address of BASIC on top of the stack. However, on systems like the Game Boy this step is a must. Unlike the 6502, the z80's stack does not have a fixed size, and is only limited by the address space of the CPU. From a practical standpoint, however, it's very unlikely you'll need more than 256 bytes.
<lang z80>LD SP,&FFFF</lang>
Registers must be pushed in pairs. If you push/pop the accumulator, the processor flags go with it.
<lang z80>push af
push bc
push de
push hl</lang>
Popping is very similar. To properly pop values, they must be popped in the reverse order they were pushed. <lang z80>pop hl pop de pop bc pop af</lang>
The stack is empty if its value equals the original starting value of the stack pointer. This is a little difficult, since the stack doesn't necessarily start in a fixed location like it does on the 6502. There are two ways to do this:
<lang z80>ld (&nnnn),SP ;&nnnn represents a memory location that the programmer will later read from to use as a
;comparison for the current stack pointer</lang>
<lang z80>ld hl,0 add hl,sp ;the z80 doesn't allow you to load SP directly into HL, so this is the quickest way</lang>
From there it's a matter of comparing this value to the current stack pointer, which in itself is tricky since the built-in compare instruction forces you to use the accumulator as one of the operands, and works natively in terms of 8-bit values.
Peek can be achieved with the EX (SP),HL command which exchanges HL with the top item of the stack.
On the Game Boy, the stack can also be manually adjusted by a signed 8-bit constant. A Zilog Z80 cannot do this in a single command. The code below only works on a Game Boy or any other hardware running on a Sharp LR35902 CPU: <lang z80> ADD SP,&FE ;subtract two from the stack pointer. Remember that the stack grows "down" in memory.</lang>
zkl
Lists have stack methods so this class is somewhat reduntant <lang zkl>class Stack{
var [const] stack=L();
fcn push(x){stack.append(x); self}
fcn pop {stack.pop()}
fcn empty {(not stack.len())}
var [proxy] isEmpty = empty;
}</lang>
- Output:
var s=Stack();
s.push(5).push("five");
s.isEmpty //-->False
s.pop() //-->"five"