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{{TaskDeprecatedTask}}
[[Category:Maintenance]]In this former task, we document loop structures offered by different languages.
{{Control Structures}}
 
;What are loops?:
In this task, we document loop structures offered by different languages.
 
Loops are control structures that allow sections of code to be executed repeatedly according to the controlling conditions of the loop.
==[[Ada]]==
 
[[Category:Ada]]
There are two types of loops:
===Simple Loop===
 
The simple loop in Ada produces an infinite loop.
;Repetition:
loop
Additionally, there is the overly simple repetitive loop: [[repeat|repetition]]. The simplistic construct executes a block of code, or a procedure, a given number of times, without explicitly exposing any state change to the looped procedure.
-- do something
 
end loop;
;Iterative loops:
Any loop can be terminated by the ''exit'' reserved word.
 
loop
An [[:Category:Iteration|iterative loop]] repeatedly executes a set of instructions as the iterator steps through a series of values. Types of iterative loops include [[for loop]]s and [[foreach]] loops. An iterative loop is a repetition but with a variable dependent on the current iteration. This allows the looped procedure to vary slightly between iterations. For example, the same operation can be carried out on each iteration, but each time on a different object.
-- do something
 
if some-condition then
;Conditional loops:
exit;
A [[conditional loop]] tests for a condition around the loop, and repeatedly executes a block of [[instruction]]s whilst the [[condition]] is true. Types of [[conditional loop]]s include [[while loop]]s and [[do-while loop]]s.
end if;
 
-- do somthing else
'''Examples here should be migrated to an appropriate [[:Category:Iteration|Iteration]] page and removed from here. If a page does not exist demonstrating a particular loop structure, discuss it [[:Category talk:Iteration|here]].'''
end loop;
<br><br>
The short-hand notation for expressing the exit condition is:
=={{header|68000 Assembly}}==
loop
'''NOT COVERED IN LOOP PAGES'''
-- do something
 
exit when some-conditon;
The 68000 uses <code>DBxx Dn, label</code> for loop counting. "Dn" refers to a chosen data register. The "xx" is replaced with the condition code of your choice (<code>DBRA</code> stands for Decrement, Branch Always which is most commonly used). Execution will jump to the labeled line of code unless Dn's lower two bytes equal #$FFFF or the specified condition code is true, whichever occurs first. Keep in mind that the condition code has nothing to do with the value stored in Dn; rather, it represents the outcome of the operation just before the branch. This is similar to a "repeat until" construct in some other languages.
-- do something else
 
end loop;
The below code snippet represents a loop that continues until a value greater than 3500 is read. However, it will also end after the 2000th iteration automatically, regardless of whether the condition is ever met.
===While loop===
 
while some-condition loop
<syntaxhighlight lang="68000devpac">
-- do something
MOVE.W #1999,D1 ;DBxx loop counters need to be pre-decremented to work properly, since they terminate at $FFFF rather than 0
end loop;
LOOP:
===For loop===
MOVE.W (A0)+,D0
The ''for'' loop in Ada iterates over a specified range of discrete values. Ranges are an important concept in Ada. Ranges are always expressed in the form of lowest..highest. The loop control variable always takes on the type of the specified range. The loop control variable is read-only within the loop, and has a scope only within the loop.
CMP.W #3501,D0 ;COMPARE TO #3501
DBCC D1,LOOP ;DECREMENT, BRANCH UNTIL CARRY CLEAR OR D1 = #$FFFF
</syntaxhighlight>
 
=={{header|AmbientTalk}}==
 
===doTimes===
<syntaxhighlight lang="ambienttalk">
// print 1 2 3 ... 20
20.doTimes { |i| system.print(" "+i); }
 
</syntaxhighlight>
 
===each===
 
Iterate over a collection:
<syntaxhighlight lang="ambienttalk">
[ "foo", "bar", "baz" ].each: { |e|
system.print(" "+e);
}
// prints: foo bar baz
 
</syntaxhighlight>
 
Iteration over a literal range:
for I in 1..10 loop
Put(I);
end loop;
You can also employ a range defined earlier in your code.
subtype Month_Number is Integer range 1..12;
for I in Month_Number loop
Put(I);
end loop;
Ranges are also defined for the indices for arrays. Ada array indices can begin at any value, not just 0 or 1.
type Balanced_Index is range -10..10;
type My_Array_Type is array(Balanced_Index) of Integer;
My_Array : My_Array_Type;
for I in My_Array'Range loop
Put(My_Array(I));
end loop;
==[[AppleScript]]==
'''NOT COVERED IN LOOP PAGES'''
[[Category:AppleScript]]
===repeat-until===
<syntaxhighlight lang="applescript>
set i to 5
set i to 5
repeat until i is less than 0
repeat setuntil i tois iless -than 10
set i to i - 1
end repeat
end repeat
 
repeat
--endless loop
end repeat
</syntaxhighlight>
 
===repeat-with===
<syntaxhighlight lang="applescript>
repeat with i from 1 to 20
repeat with i from 1 to 20
--do something
end--do repeatsomething
end repeat
 
set array to {1,2,3,4,5}
repeat with i in array
display dialog i
end repeat
</syntaxhighlight>
 
==[[BefungeAssemblyScript]]==
'''NOT COVERING ALL POSSIBLE LOOP OPTIONS'''
[[Category:Befunge]]
Befunge does not have specific loop statements, but any type of loop construct can be created using the branch (_|) skip (#) and direction (<>v^) commands. For example, the following is one possible structure for a while loop:
enter > test v
^ body _ exit
 
This program is like a for-loop in other languages, printing the digits from nine down to zero.
9>:.:#v_@
^ -1 <
 
==[[C]]==
[[Category:C]]
===while===
<syntaxhighlight lang="javascript">
'''Compiler:''' [[GCC]] 4.1.2
let done = false
while (!done) {
done = true
}
</syntaxhighlight>
 
===do while===
int main (int argc, char ** argv) {
<syntaxhighlight lang="javascript">
int condition = 1;
let done = false
do {
while ( condition ) {
done //= Do somethingtrue
} while (!done)
// Don't forget to change the value of condition.
</syntaxhighlight>
// If it remains nonzero, we'll have an infinite loop.
}
}
 
===do-while===
int main (int argc, char ** argv) {
int condition = ...;
do {
// Do something
// The difference with the first loop is that the
// code in the loop will be executed at least once,
// even if the condition is 0 at the beginning,
// because it is only checked at the end.
// Don't forget to change the value of condition.
// If it remains nonzero, we'll have an infinite loop.
} while ( condition );
}
 
===for===
<syntaxhighlight lang="javascript">
int main (int argc, char ** argv) {
for (let i = 0; i < 10000; i++) {
int i;
i += i
}
for {i=0; i<10; ++i) {
</syntaxhighlight>
// The code here will be performed 10 times.
// The first part in the for-statement (i=0) is the initialization,
// and is executed once before the loop begins.
// The second part is the end condition (i<10), which is checked
// every time the loop is started, also the first time;
// the loop ends if it is false.
// The third part (++i) is performed every time the code in the loop
// is at the end, just before the end condition is checked.
}
}
 
==[[Brainf***]]==
===while with continue===
'''NOT EXPLAINED THIS MUCH IN LOOP PAGES'''
The continue statement allows you to continue execution
atBF's theonly beginningcontrol flow construct of theany kind is a loop,. skippingTwo of the resteight ofcommands define the start and end of a conditional loop.
* '''[''' - branch forward past matching ']' if the current cell is zero
In C you can only do this with the most inner loop.
* ''']''' - branch back to the matching '[' if the current cell is non-zero
You can also do this with do-while and for.
In practice this is equivalent to a "while not zero" loop in other languages.
[-] set current cell to zero
 
[->+>+<<]>>[-<<+>>] copy cell 0 to cell 1, using cell 2 as temporary storage
int main (int argc, char ** argv) {
int condition = 1;
while ( condition ) {
// Do something
if (other condition)
continue; // Continue at the beginning of the loop
// Do something else
// This part is not executed if other condition was true
}
}
 
==[[C++]]==
===while with break===
=== Compile-Time Control Structures ===
The break statement allows you to stop a loop.
'''Necessary?'''
In C you can only break from most inner loop.
A compile-time for loop can be generated with template metaprogramming. Example:
You can also do this with do-while and for.
 
<syntaxhighlight lang="cpp">
int main (int argc, char ** argv) {
// the loop
int condition = 1;
template<int start, int finish, template<int n, typename T> class X> struct loop
{
typedef typename X<start, typename loop<start+1, finish, X>::type>::type type;
};
// the termination of the loop
while ( condition ) {
template<int finish, template<int n, typename T> class X> struct loop<finish, finish, X>
// Do something
{
typedef typename X<finish, void>::type type;
};
 
// example usage: This implements just a very complicated way of building a multi-dimensional array
// the loop body
if (other condition)
template<int n, typename T> struct build_array
break; // Continue after the the loop
{
typedef T type[n];
};
template<int n> struct build_array<n, void>
// Do something else
{
// This part is not executed if other condition was true
typedef double type;
}
};
// the loop execution: This is equivalent to
// typedef double array_type[2][3][4][5];
typedef loop<2,6,build_array>::type array_type;
</syntaxhighlight>
 
==[[C plus plus|C++Clojure]]==
'''NOT COVERED IN LOOP PAGES'''
[[Category:C plus plus]]
===loop===
=== Run-Time Control Structures ===
<syntaxhighlight lang="clojure">
;; loop/recur is the most general looping construct
(loop [lst [1 3 5 7]
ret []]
(if lst
(recur (rest lst) (conj ret (* (first lst) (first lst))))
ret))
==> [1 9 25 49]
</syntaxhighlight>
 
=={{header|Crack}}==
==== for ====
===For===
'''Compiler:''' [[GCC]] 3.3.4
<syntaxhighlight lang="crack">
#include <iostream>
for( i=0; i<9; i++)
cout ` $i\n`;
int main()
</syntaxhighlight>
{
int i = 1;
// Loops forever:
for(; i == 1;)
std::cout << "Hello, World!\n";
}
 
=={{header|Curto}}==
====do-while====
===HACER-BUCLE===
'''Compiler:''' [[GCC]] 4.1.2
<syntaxhighlight lang="curto">
int main (void) {
\ limite inicio HACER sentencias iteradas BUCLE
int condition = 1;
\ limite inicio HACER sentencias iteradas incremento +BUCLE
\ SALIR \ abandona bucle HACER
do {
\ DBUCLE SALIR \ limpia contadores de la pila de retorno antes de abandonar la palabra actual
// Do something
</syntaxhighlight>
// Don't forget to change the value of condition.
// If it remains nonzero, we'll have an infinite loop.
} while ( condition );
}
 
ejemplo: Dos iteraciones típicas
[[Category:C plus plus]]
<syntaxhighlight lang="curto">
10 0 hacer i . bucle \ Imprime números de 0 a 9
10 0 hacer i . 2 +bucle \ Imprime números pares de 0 a 8
</syntaxhighlight>
===EMPEZAR-HASTA===
<syntaxhighlight lang="curto">
\ EMPEZAR sentencias iteradas condicional HASTA
</syntaxhighlight>
 
ejemplo: Cuenta hacia abajo desde un número dado a cero
====while====
<syntaxhighlight lang="curto">
'''Compiler:''' [[GCC]] 4.1.2
: cuenta-abajo ( n -- ) empezar dup rc . 1- dup 0< hasta soltar ;
int main (void) {
</syntaxhighlight>
int condition = 1;
 
===EMPEZAR-DENUEVO===
while ( condition ) {
<syntaxhighlight lang="curto">
// Do something
\ EMPEZAR sentencias iteradas DENUEVO
// Don't forget to change the value of condition.
</syntaxhighlight>
// If it remains nonzero, we'll have an infinite loop.
 
}
ejemplo: repetir entrada de usuario (solo funciona en cli, no en la interface gráfica)
}
<syntaxhighlight lang="curto">
: porsiempre ( -- ) empezar tecla emitir denuevo ;
</syntaxhighlight>
 
===EMPEZAR-MIENTRAS-REPETIR===
<syntaxhighlight lang="curto">
\ EMPEZAR sentencias iteradas incondicionales condicional MIENTRAS sentencias iteradas condicionales repetir
</syntaxhighlight>
ejemplo: cuenta hacia abajo desde un número dado a uno
<syntaxhighlight>
: cuenta-abajo ( n -- ) empezar dup mientras rc dup . 1- repetir soltar ;
</syntaxhighlight>
 
=={{header|Dafny}}==
<syntaxhighlight lang="dafny">
var i: int := 0;
while i < n
invariant 0 <= i <= n
decreases n - i
{
i := i + 1;
}
assert i == n;
</syntaxhighlight>
 
=={{header|Dao}}==
===For===
<syntaxhighlight lang="java">
for( i=0; i<9; ++i) io.writeln( i );
for( i = 0 : 8 ) io.writeln( i );</syntaxhighlight>
 
===For In===
<syntaxhighlight lang="java">
items = { 1, 2, 3 }
for( item in items ) io.writeln( item )
</syntaxhighlight>
 
===While===
<syntaxhighlight lang="java">
i = 0
while( i < 5 ) { i += 1 }
</syntaxhighlight>
 
===Do While===
<syntaxhighlight lang="java">
i = 0
do { i += 1 } while( i < 9 )
</syntaxhighlight>
 
=={{header|Déjà Vu}}==
===For===
Déjà Vu has a for-loop protocol, so you can write your own iterators. The most commonly used iterators are <code>in</code> and <code>range</code>. The first iterates over a list, the second takes two arguments and goes from the first to the second, like a classic for-loop.
<syntaxhighlight lang="dejavu">
for i range 1 3:
!print i # prints 1, 2 and 3
</syntaxhighlight>
===While===
<syntaxhighlight lang="dejavu">
while true:
!print "This is the song that never ends..."
</syntaxhighlight>
===Repeat===
<syntaxhighlight lang="dejavu">
repeat 3:
!print "This sentence is printed three times."
</syntaxhighlight>
 
==[[Factor]]==
'''NOT COVERED IN LOOP PAGES'''
===Looping===
Most looping is done with recursion. Tail recursion is properly optimized.
<syntaxhighlight lang="factor">
: forever ( quot -- ) dup slip forever ; inline
[ "A hungry raptor stalks you..." print flush 2000 random sleep ] forever
</syntaxhighlight>
 
===Iteration===
<syntaxhighlight lang="factor">
Most indices are implicit or not present at all.
3 [ "pint" drink ] times
{ "high" "level" "language" } [ print ] each
high
level
language
10 [ sq ] map
{ 0 1 4 9 16 25 36 49 64 81 }
{ 1 2 3 } { 4 5 6 } [ * ] 2map .
{ 4 10 18 }
10 [ even? ] subset .
V{ 0 2 4 6 8 }
0 10 3 <range> >array .
{ 0 3 6 9 }
10 1 -2 <range> >array .
{ 10 8 6 4 2 }
2222 [ dup 0 > ] [ 2/ dup ] [ ] unfold nip .
{ 1111 555 277 138 69 34 17 8 4 2 1 0 }a
</syntaxhighlight>
 
Iterating with an index:
<syntaxhighlight lang="factor">
: indexed-alphabet. ( -- )
"abcdefghijklmnopqrstuvwxyz"
[ [ 1string ] [ number>string ] bi* ": " glue print ] each-index ;
</syntaxhighlight>
 
==[[Forth]]==
[[Category:Forth]]
===DO-LOOP===
<syntaxhighlight lang="forth">
( limit start ) DO ( iterated statements ) LOOP
( limit start ) DO ( iterated statements ) ( increment ) +LOOP
( limit start ) DO ( iterated statements ) ( increment ) +LOOP
LEAVE \ exits a DO loop
LEAVE \ exits a DO loop
UNLOOP EXIT \ cleans up loop counters from return stack before returning from the current word
</syntaxhighlight>
 
example: Two standard iterations
<syntaxhighlight lang="forth">
10 0 DO I . LOOP \ Prints the numbers from 0 to 9
10 0 DO I . 2 +LOOP \ Prints the even numbers from 0 to 89
10 0 DO I . 2 +LOOP \ Prints the even numbers from 0 to 8
</syntaxhighlight>
 
===BEGIN-UNTIL===
<syntaxhighlight lang="forth">
BEGIN ( iterated statements ) ( conditional ) UNTIL
BEGIN ( iterated statements ) ( conditional ) UNTIL
</syntaxhighlight>
 
example: Counts down from a given number to zero
<syntaxhighlight lang="forth">
: COUNTDOWN ( n -- ) BEGIN DUP CR . 1- DUP 0< UNTIL DROP ;
: COUNTDOWN ( n -- ) BEGIN DUP CR . 1- DUP 0< UNTIL DROP ;
</syntaxhighlight>
 
===BEGIN-AGAIN===
<syntaxhighlight lang="forth">
BEGIN ( iterated statements ) AGAIN
BEGIN ( iterated statements ) AGAIN
</syntaxhighlight>
 
example: echo user's input
<syntaxhighlight lang="forth">
: FOREVER ( -- ) BEGIN KEY EMIT AGAIN ;
: FOREVER ( -- ) BEGIN KEY EMIT AGAIN ;
</syntaxhighlight>
 
===BEGIN-WHILE-REPEAT===
<syntaxhighlight lang="forth">
BEGIN ( conditional ) WHILE ( iterated statements ) REPEAT
BEGIN ( unconditional iterated statements ) ( conditional ) WHILE ( conditional iterated statements ) REPEAT
example: counts down from a given number to one
: COUNTDOWN ( n -- ) BEGIN DUP WHILE CR DUP . 1- REPEAT DROP ;
</syntaxhighlight>
Additional WHILE clauses may be added to a loop, but each extra WHILE requires a matching THEN after the REPEAT.
 
===Mixed Structures===
Because Forth's compiler is laid bare to the programmer, it is quite easy to both define your own looping structures or combine existing structures in interesting ways. The rules for such combining are somewhat involved, though discussions can be found in the gforth user's manual, among other places. These more complex looping constructs can make up for Forth's lack of a "break" word, and can allow expressing complex loops without resorting to boolean variables. A practical example is also found in the [[Binary search]] task.
 
A good example of a useful combination is this complex loop:
<syntaxhighlight lang="forth">
BEGIN
BEGIN
( condition 1 )
( condition 1 )
WHILE
WHILE
( condition 2 )
( condition 2 )
UNTIL
UNTIL
( condition 2 succeeded )
( condition 2 succeeded )
ELSE
ELSE
( condition 1 failed )
( condition 1 failed )
THEN
THEN
</syntaxhighlight>
 
An example of using this idiom in practice might be this pseudo-Forth
<syntaxhighlight lang="forth">
BEGIN
BEGIN
read-next-record
read-next-record
WHILE
WHILE
found-record
found-record
UNTIL
UNTIL
process-record
process-record
ELSE
ELSE
error" Ran out of records looking for the right one!"
error" Ran out of records looking for the right one!"
THEN
THEN
</syntaxhighlight>
 
=={{header|FreeBASIC}}==
<h3>[[While_loop|While..Wend]]</h3>
Executes a block of statements while a condition is met.<br>
<syntaxhighlight lang="vbnet">While [ condition ]
[ statement block ]
Wend</syntaxhighlight>
 
<h3>[[For_loop|For..Next]]</h3>
Executes a block of statements while an iterator is less than or greater than an expression.<br>
<syntaxhighlight lang="vbnet">For iterator [ As datatype ] = startvalue To endvalue [ Step stepvalue ]
[ statement block ]
Next [ iterator ]
</syntaxhighlight>
 
<h3>[[While_loop|Do..Loop]]</h3>
Executes a block of statements while or until a condition is met.
<syntaxhighlight lang="vbnet">Do [ { Until | While } condition ]
[ statement block ]
Loop</syntaxhighlight>
or
<syntaxhighlight lang="vbnet">Do
[ statement block ]
Loop [ { Until | While } condition ]
</syntaxhighlight>
 
<h3>Intra-loop control</h3>
Continue While, Continue For and Continue Do
Prematurely re-enters a loop.<br>
<syntaxhighlight lang="vbnet">Continue {Do | For | While}</syntaxhighlight>
Exit While, Exit For and Exit Do
Prematurely breaks out of a loop.
<syntaxhighlight lang="vbnet">Exit {Do | For | While | Select }</syntaxhighlight>
<syntaxhighlight lang="vbnet">Exit {Sub | Function | Operator | Constructor | Destructor | Property }</syntaxhighlight>
 
<syntaxhighlight lang="vbnet">Exit {Do [, Do [ , ...] ] |
For [, For [ , ...] ] |
While [, While, [...] ] |
Select [, Select [ , ...] ] }</syntaxhighlight>
 
 
=={{header|Frink}}==
In all of the loops below, the curly braces can be omitted if the body is a single statement.
 
===For Loop===
A <CODE>for</CODE> loop is really a <CODE>foreach</CODE> loop that can work with range operators or iterate through various data structures. The <CODE>to</CODE> operator creates an enumerating expression that lazily steps through its range.
<syntaxhighlight lang="frink">
for i = 1 to 1000000
{
println[i]
}
 
</syntaxhighlight>
 
The <CODE>to</CODE> operator can be combined with a <CODE>step</CODE> statement:
<syntaxhighlight lang="frink">
for i = 1 to 1000000 step 3
println[i]
</syntaxhighlight>
 
As a <CODE>foreach</CODE> statement. The <CODE>for</CODE> construct can iterate over the elements of an array, set, dictionary, or enumerating expression.
<syntaxhighlight lang="frink">
for i = [2,3,7,9]
println[i]
</syntaxhighlight>
 
===Do...While Loop===
<syntaxhighlight lang="frink">
i=0
do
{
i = i+1
} while i<1000
</syntaxhighlight>
 
==[[Groovy]]==
[[Category:Groovy]]
 
===While Loops===
Line 303 ⟶ 496:
</pre>
 
===ClosuresEach===
<pre>
def stringList = [ "java", "perl", "python", "ruby" ];
Line 338 ⟶ 531:
// 6: Fr=Friday
</pre>
 
==[[Haskell]]==
 
Most of the usual applications for loops are realized in Haskell by operations on (lazy) lists, like '''map''', '''fold''' or '''filter'''. It's unusual to use loops in an imperative style. However, if one insists on it, it's easy to make your own implementation of any loop variant.
 
Here are a few examples:
 
===Pre-checked loop (while)===
 
whileM :: Monad m => m Bool -> m a -> m ()
whileM cond body =
cond >>= \b -> if b then body >> untilM cond body else return ()
 
===Post-checked loop (repeat-until)===
 
untilM :: Monad m => m Bool -> m a -> m ()
untilM cond body =
body >> cond >>= \b -> if b then return () else untilM cond body
 
===For-style loop===
 
Simplest done by iterating over a list:
 
forM :: Monad m => [a] -> (a -> m ()) -> m ()
forM [] f = return ()
forM (x:xs) f = f x >> forM xs f
 
==[[IDL]]==
[[Category:IDL]]
 
It should be noted that IDL programmers tend to avoid loops -- most of the time loops are used to access the elements of arrays or vectors, and since IDL is an array language the same purpose can almost always be served in a faster, more elegant and more readable way though any of the array operations.
Line 387 ⟶ 605:
The <tt>break</tt> statement will immediately terminate the current innermost <tt>for</tt>, <tt>while</tt>, <tt>repeat</tt>, <tt>if</tt>, <tt>case</tt> or <tt>switch</tt> without having to resort to a <tt>goto</tt>.
 
==[[JavaJinja]]==
[[Category:Java]]
===while===
 
===for===
while(true)
<lang jinja>
{
print(Template("""{% for lang in ["Jinja", "Python", "Swift", "Nim"] %}
foo();
{{ loop.index }}) {{ lang }}
}
{%- endfor %}""").render())
</lang>
 
===do-while=[[Kabap]]==
 
There is no native loop command in Kabap, but labels, variables, jumps and conditional execution are supported which is enough to create a basic loop structure. Support for native loops is being prepared for the next major release.
do
{
foo();
}
while (true)
===for===
 
===Basic loop===
for(int i = 0; i < 5; i++)
<lang Kabap>
{
$i = foo()0;
}:start;
// Your loop code here
$i = $i + 1;
if $i < 20;
goto start;
</lang>
 
===foreach=[[Logo]]==
forever [right random 360 forward 10]
'''Platform:''' [[J2SE]] 1.5.0
 
repeat 5 [right 180-36 forward 100]
Object[] objects;
// ...
for (Object current : objects[]) {
// ...
}
 
Repeat and forever also have access to a loop counter, starting at 1.
int[] numbers;
repeat 10 [print repcount]
// ...
for (int i : numbers) {
// ...
}
 
while [:x > 0] [make "x :x / 2]
==[[JavaScript]]==
do.while [make "x :x / 2] [:x > 0]
[[Category:JavaScript]]
===while===
 
until [:x = 0] [make "x :x / 2]
while(true) {
do.until [make "x :x / 2] [:x = foo();0]
}
 
===do while=[[LSE64]]==
LSE's loop words all work via tail recursion, like [[Scheme]], by re-executing the current word. If used interactively, "repeat" works upon the current line. Exiting words, like "&&", "||", and "exit" can be used to exit an otherwise infinite loop (see other tasks).
infinite : " again and " ,t repeat
limited : continue? &repeat
 
Counted loops execute a specified word ''n'' times. Within that word, "count" accesses a loop counter, counted down to zero.
do {
body : count , sp
foo();
10 body iterate # 10 9 8 7 6 5 4 3 2 1
} while(test);
body? : count , sp count 5 >
===for===
10 body? &iterate # 10 9 8 7 6 5
 
==[[Make]]==
for(var i = 0; i < 5; i++) {
Make does looping through recursion.
foo();
SUCC=`expr $* + 1`
}
MAX=10
 
===for in===
all: 0-n;
 
//iterate through property names of an object
%-n: %-echo
@-make -f loop.mk $(SUCC)-n MAX=$(MAX)
%-echo:
@echo $*
$(MAX)-n: $(MAX)-echo;
var obj = {prop1:"a", prop2:"b", prop3:"c"};
for (var key in obj)
alert( key + ' is set to ' + obj[key] );
 
Invoking it
=== for each in===
|make -f loop.mk MAX=2
 
0
//iterate through property values of an object
1
2
var obj = {prop1:"a", prop2:"b", prop3:"c"};
for each(var element in obj)
alert( element );
 
==[[newLISP]]==
[[Category:{{works with|newLISP]]|v.9.0}}
 
===dotimes===
 
'''Interpreter:''' [[newLISP]] v.9.0
 
(dotimes (x 10) (println (+ x 1)))
 
===do-until===
 
'''Interpreter:''' [[newLISP]] v.9.0
 
(set 'x 1)
Line 479 ⟶ 687:
 
===do-while===
 
'''Interpreter:''' [[newLISP]] v.9.0
 
(set 'x 1)
Line 486 ⟶ 692:
 
===for===
 
'''Interpreter:''' [[newLISP]] v.9.0
 
(for (x 1 10) (println x))
==[[OCaml]]==
===let rec===
The for and while loops are imperative features of OCaml, and most often it is rather recommended to prefer using functional loop designed with recursive functions, or better when iterating over a list or an array using a built-in iterator from the standard library or the extLib library.
<lang ocaml>let rec loop i =
Printf.printf "%d\n" i;
if i <= 4 then loop (i + 1)
in
loop 0</lang>
 
===Built-in Iterators===
<lang ocaml>List.iter
List.fold_left
Array.iter
Array.iteri</lang>
 
=={{header|Prolog}}==
==[[Pascal]]==
[[Category:Pascal]]
 
There are three primitive methods of looping in Prolog: recursion, fail-driven loops, and repeat-driven loops.
===while===
 
<lang Prolog>% recursion as loop
'''Compiler:''' [[Turbo Pascal]] 7.0
print_each_element([]).
print_each_element([E|T]) :- writeln(E), print_each_element(T).
 
% fail-driven loop
WHILE condition1 DO
fact(foo).
BEGIN
fact(bar).
procedure1;
fact(baz).
procedure2;
print_each_fact :-
END;
( fact(X), writeln(X), fail
; true ).
 
% equivalently
===repeat-until===
%print_each_fact :- fact(X), writeln(X), fail.
%print_each_fact.
 
% repeat-driven loop
'''Compiler:''' [[Turbo Pascal]] 7.0
print_each_fact_again :-
repeat,
REPEAT
procedure1; fact(X),
procedure2; writeln(X),
X = baz,
UNTIL condition1;
!.
 
go :-
===for===
print_each_element([foo, bar, baz]),
print_each_fact,
print_each_fact_again.</lang>
 
Of the three recursion is the favoured approach as it requires no non-logical predicates and is thus easy to read in its declarative form.
'''Compiler:''' [[Turbo Pascal]] 7.0
 
The fail-driven loop form is a(n ab)use of the built-in backtracking mechanism of Prolog's reasoning engine. In the specific example provided, fact(X) will first succeed, binding "foo" to X. It will then write "foo" to the output (as a side effect of the writeln/1 predicate). It then hits the call to fail/0 which is a non-logical predicate which always fails and thus always triggers backtracking. On backtracking, the runtime will try fact(X) again and will find that it is true when X is bound to "bar". This will then print and backtrack again. A third time binds to and prints "baz". A fourth time will fail because there is no more solution to the goal "fact(X)". This triggers a further backtrack and a try on the second branch of the disjunction. That second branch invokes the true/0 predicate which always succeeds. This exits the query with an overall success.
FOR counter := 1 TO 10 DO
BEGIN
procedure1;
procedure2;
END;
 
The repeat-driven loop uses similar (ab)use of the backtracking mechanism. Instead of employing a predicate that always fails, however, it employs one that will always succeed: repeat/0. Thus, in this sample, fact(X) works as before, as does writeln(X), but the attempt to unify X with "baz" will fail for the first two attempts, causing the system to backtrack until it hits repeat. Since repeat always succeeds it drives the engine forward again, testing each fact in succession. Once X is unified with "baz" (which is to say once X contains the value "baz") the predicate carries on. The cut operator !/0, guarantees that the predicate won't be re-entered later.
'''Compiler:''' [[Delphi]] 1.x and above
 
As with any language permitting higher-order invocations, using the looping primitives directly as above is often not a desirable thing. Instead higher-order features would be used.
Var
 
<lang Prolog>% using maplist/2 to replace explicit recursion on a list
Counter : char ;
print_each_element(L) :- maplist(writeln, L).
 
% using forall/2 to replace an explicit fail-driven loop
FOR counter := 'A' TO 'Z' DO
fact(foo).
BEGIN
fact(bar).
procedure1;
fact(baz).
procedure2;
print_each_fact() :- forall(fact(X), writeln(X)).</lang>
END;
 
==[[Perl]]==
[[Category:Perl]]
 
===while===
 
'''Interpreter:''' [[Perl]] 5.8.8
#!/usr/bin/perl -w
use strict;
my $condition1 = 0;
while ( $condition1 ) {
# Do something.
# Remember to change the value of condition1 at some point.
}
 
===do-while===
 
'''Interpreter:''' [[Perl]] 5.8.8
#!/usr/bin/perl -w
use strict;
my $condition1 = 0;
do {
# Do something.
# Remember to change the value of condition1 at some point.
} while ( $condition1 );
===until===
 
'''Interpreter:''' [[Perl]] 5.8.8
#!/usr/bin/perl -w
use strict;
my $condition1 = 1;
until ( $condition1 ) {
# Do something.
# Remember to change the value of condition1 at some point.
}
 
 
===do-until===
'''Interpreter:''' [[Perl]] 5.8.8
 
#!/usr/bin/perl -w
use strict;
my $condition1 = 1;
do {
# Do something.
# Remember to change the value of condition1 at some point.
} until ( $condition1 );
 
===for===
'''Interpreter:''' [[Perl]] 5.8.8
 
#!/usr/bin/perl -w
use strict;
my $limit = 5;
for ( my $iterator = 0; $iterator < $limit; $iterator++ ) {
# Do something
}
# for-variant, implicit iteration
for (0..$limit) {
# Do something
}
do_something() for 0..$limit;
 
===foreach===
'''Interpreter:''' [[Perl]] 5.8.8
 
#!/usr/bin/perl -w
use strict;
my @numbers = (1, 2, 3);
my %names = (first => "George", last => "Jetson");
foreach my $number (@numbers) {
# Do something with $number
}
foreach my $key (keys %names) {
# Do something with $key (values are accessible as %names{$key} )
}
 
===map===
'''Interpreter:''' [[Perl]] 5.8.8
 
#!/usr/bin/perl -w
use strict;
my @numbers = (1, 2, 3);
my @target;
@target = map {
# Do something with $_
} @numbers;
@target = map($_ + 1, @numbers);
sub a_sub {
# Do something with $_
}
@target = map a_sub @numbers;
 
===grep===
'''Interpreter:''' [[Perl]] 5.8.8
 
#!/usr/bin/perl -w
use strict;
my @people = qw/Bobbie Charlie Susan/;
my @target;
@target = grep {
# Discriminate based on $_
} @people;
# Feed grep into map, this picks out elements 1, 3, 5, etc.
@target = map($people[$_], grep($_ & 1, 0..$#people));
# Pick out the diminutive names
@target = grep(/ie$/, @people);
sub a_sub {
# Do something with $_, and return a true or false value
}
@target = grep a_sub @people;
 
==[[PHP]]==
[[Category:PHP]]
 
===while===
while(ok()) {
foo();
bar();
baz();
}
 
===do-while===
$i = 0;
do {
echo $i;
} while ($i > 0);
===for===
for($i = 0; $i < 10; ++$i) {
echo $i;
}
===foreach===
foreach(range(0, 9) as $i) {
echo $i;
}
foreach is only used for arrays, which is not obvious from the above example
foreach($array as $key => $value) {
echo $key.' is '.$value;
}
 
There are a myriad of such predicates available in a useful Prolog implementation (SWI-Prolog provides, non-exhaustively: include/3, exclude/3, partition/4-5, maplist/2-5, foldl/4-7, scanl/4-6, aggregate/3-4, aggregate_all/3-4, forall/2, findall/3-4, findnsols/4-5, bagof/3, setof/3, … just as the more fundamental wrappings.) If the provided predicates do not permit the kinds of functionality desired for common patterns, it is trivial to make a new one. As an illustration, this is the source code for forall/2:
==[[Pop11]]==
[[Category:Pop11]]
 
<lang Prolog>:- meta_predicate forall(0,0).
=== while ===
forall(A, B) :- \+ (call(A), \+ call(B)).</lang>
Pop11 offers multiple looping constructs. Basic one is while loop:
 
=={{header|Pop11}}==
5 -> n;
while n > 0 do
printf(n, '%p\n');
n - 1 -> n;
endwhile;
 
=== until ===
Line 729 ⟶ 772:
is equivalent to:
 
while motnot(condition) do /* Action */ endwhile;
 
=== for ===
Line 740 ⟶ 783:
for x y in [a b c], [1 2 3] do [^x ^y] => endfor;
 
in first iteration sets x to "a" and y to 1, in the second x is "b" and y is 2, in the third (last) iteration x is "c" and y is 3. The iteration ends when the shortest list is exhausted.
and y is 2, in the third (last) iteration x is "c" and y is 3.
The iteration ends when the shortest list is exhausted.
 
SomtimesSometimes one wants to process tails of the list, to do this use on keyword instead of in keyword:
on keyword instead of in keyword:
 
for x on [a b c] do x => endfor;
Line 772 ⟶ 812:
 
=== quitloop quitif quitunless ===
Inside loops one can use control transfers to prematuralyprematurely exit the loop or end current iteration and start the next one:
loop or end current iteration and start the next one:
 
while true do n - 1 -> n; quitif(n=0); endwhile;
Line 781 ⟶ 820:
 
=== nextloop nextif nextunless ===
Similarely to quitloop nextloop unconditionally ends current iteration and starts the new one, nextif(x) ends current iteration when x is true, nextunless(x) is equivalent to nextif(not(x)). The loop control transfers can be also used inside for (and until) loops.
and starts the new one, nextif(x) ends current iteration when x is true,
nextunless(x) is equivalent to nextif(not(x)). The loop control transfers
can be also used inside for (and until) loops.
 
Finally, it is frequently possible to avoid explicit iteration using higher order map functions (like appdata and mapdata).
using higher order map functions (like appdata and mapdata).
 
==[[PostScript]]==
[[Category:PostScript]]
 
The "<tt>for</tt>" operator expects three numbers and a procedure on the stack. It will consume all four arguments then it will push the first number on the stack, execute the procedure, increase the first number by the second number, push it on the stack, execute the procedure etc until the third number is reached. For example
Line 802 ⟶ 836:
 
==[[Python]]==
[[Category:Python]]
 
===for===
Frequently one wants to both iterate over a list and increment a counter:
 
<lang python>
Typically for is used to iterate over items in an iteratable:
mylist = ["foo", "bar", "baz"]
for i, x in enumerate(mylist):
print "Element no.", i, " is", x
</lang>
 
for x in ["foo", "bar", "baz"]:
print x
 
Iterating over more than one list + incrementing a counter:
It is also used to create indexes:
 
<lang python>
for i in xrange(10):
for counter, [x, y, z] in enumerate(zip(lst1, lst2, lst3)):
print numbers[i]
print counter, x, y, z
</lang>
 
=== list comprehension expressions ===
Optional else is executed unless break was called during iteration:
 
Typically used when you want to create a list and there is little logic involved. Faster than for loop:
for i in numbers:
if item < 0:
break
else:
print 'all numbers are positive'
 
positives = [n for n in numbers if n > 0]
=== list comprehension ===
 
A list comprehension is an expression rather than a statement. This allows them to be used as arguments to a function:
Typically used when you want to create a list and there is little logic involved. Faster then for loop:
 
<lang python>
positives = [n for n in numbers if n > 0]
def square_each(n):
results = []
for each in n:
results.append(each * each)
return results
squares_3x5 = square_each([x for x in range(100) if (x%3)==0 and (x%5)==0])
# Return a list of all the squares of numbers from 1 up to 100 those numbers are
# multiples of both 3 and 5.
</lang>
 
===while===
Line 834 ⟶ 877:
Typical use:
 
<lang python>
while 1:
while True:
# Do stuff...
if condition:
break
</lang>
 
You can add optional ''else'', which is executed only if the tested expression istested was false. Typically not used for searches.
 
<lang python>
while running:
while True:
# Do stuff...
if conditionfound:
runningresults = 0...
break
else:
print 'stoppedNot found'
</lang>
 
Since Python has no "bottom-tested" loop construct (such as "do ... until") ... it's common Python practice to either rethink the design in terms of iteration or to use an ''while 1'' (infinite loop) and ''break'' out of it as appropriate.
==[[Ruby]]==
[[Category:Ruby]]
 
===while=[[Raven]]==
 
===each===
while true do
foo
end
 
10 each as i # counted loop
===for===
"index is %(i)d\n" print
'hello world' each as c # string characters
"%(c)c" print
[ 'a' 'b' 'c' ] each as item # list items
"%(item)s" print
{ 'a' 1 'b' 2 } each pair as k, v # hash key/val pairs
"%(k)s => %(v)d\n" print
'SELECT * FROM table' mysql query each as row # iterable resource
row print
 
===repeat while/until===
for i in 0..4 do
foo
end
 
repeat <some_condition> while
=== each ===
<some_process>
repeat <come_condition> until
<some_process>
 
=={{header|REXX}}==
['foo', 'bar', 'baz'].each do |x|
===repeat===
puts x
This example shows how to perform a loop for one million times.
end
<lang rexx>n= 1000000
 
x= 1
=== collect ===
y= 12
 
z= 0
array = ['foo', 'bar', 'baz'].collect do |x|
do n
foo x
z=someFunction(z, x, y)
end
end /*n*/</lang> <br><br>
 
=== map ===
 
array = ['foo', 'bar', 'baz'].map {|x| foo x }
 
=== inject ===
 
string = ['foo', 'bar', 'baz'].inject("") do |s,x|
s << x
end
 
sum = ['foo', 'bar', 'baz'].inject(0) do |s,x|
s + x.size
end
 
product = ['foo', 'bar', 'baz'].inject(1) do |p,x|
p * x.size
end
 
boolean = ['foo', 'bar', 'baz'].inject(true) do |b,x|
b &&= x != 'bar'
end
 
==[[Seed7]]==
In Seed7 new statements can be declared easily. This feature is not explained here. Here are examples of the predefined loops:
[[Category:Seed7]]
In Seed7 new statements can be declared easily.
This feature is not explained here.
Here are examples of the predefined loops:
 
===while===
Line 929 ⟶ 965:
end for;
 
==[[Smalltalk]]SETL4==
<lang setl4>
[[Category:Smalltalk]]
define('prime(n)set.this') :(prime.end)
 
* Tests if _n_ is a prime integer.
===timesRepeat===
 
prime
10 timesRepeat: [ expression ].
 
n = integer(n) +n
===iterating over a collection===
eq(n,2) :s(return)
even(n) :s(freturn)
exists(new('iter 3 ' square.root(n) ' 2'), 'multiple(n,this)') :s(freturn)f(return)
 
prime.end
( collection ) do: [ expression ].
 
define('primes(n)set.this') :(primes.end)
( collection ) collect: [:element | element sendMessageToElement].
 
* Returns set of primes less than _n_.
#(1 2 3 4) inject: 5 into: [:sum :number | sum + number]
 
primes
===whileTrue/whileFalse===
 
primes = filter(new('iter 2 ' (n - 1)),'prime(this)') :(return)
x := 0.
[ x <primes 100= ]new('set')
iter = new('iter whileTrue:2 [ x :=' x(n +- 10.].1))
loop(iter)
 
primes.loop
[ x = 0 ]
whileFalse: [ x := x - 20.].
this = next(iter) :f(return)
prime(this) add(primes,this) :(primes.loop)
 
primes.end
===simple looping===
</lang>
 
==[[SIMPOL]]==
1 to: 5 do: [:number | Transcript show: 'Here is a number: ' number printString ; cr]
In [[SIMPOL]] there is only one looping construct. It is very flexible and can be used as a while, a repeat, or a combination of both.
 
===while===
(-50 to: 50 by: 4) do: [:number | Transcript show: number ; cr]
 
<lang simpol>while [condition]
==[[Tcl]]==
// Actions taken here
[[Category:Tcl]]
end while [condition]</lang>
=== foreach ===
foreach i {foo bar baz} {
puts "$i"
}
Note that <tt>foreach</tt> also accepts multiple variables:
foreach {x y} {1 2 3 4} {
puts "$x,$y"
}
And also multiple lists:
foreach i {1 2 3} j {a b c} {
puts "$i,$j"
}
Or any combination of variables/list:
foreach i {1 2 3} {x y} {a b c d e f} {
puts "$i,$x,$y"
}
 
Either condition or both can be present. Not setting either condition will result in an endless loop. The first condition will continue the loop if the result is true, the second will exit the loop if it is true. In [[SIMPOL]] code it is common to see both present, with the initial condition being the standard method of exiting the loop, and the end while condition used for exiting because of an error.
=== for ===
for {set i 0} {$i < 10} {incr i} {
puts $i
}
 
In this abbreviated example, the code will read records from a database table in a loop, placing each in record object in an array element, until it reaches the end of the table.
It bears noting that the three parts of the for loop do not have to consist of "initialize variable", "test value of variable" and "increment variable". This is a common way to think of it as it resembles the "for" loop in other languages, but many other things make sense. For example this for-loop will read a file line-by-line:
 
<lang simpol>function readrecs(sbme1table table)
set line ""
sbme1record r
for { set io [open test.txt r] } { ![eof $io] } { gets $io line } {
array recs
if { $line != "" } { ...do something here... }
integer e, i
}
 
e = 0
(This is a somewhat awkward example; just to show what is possible)
i = 0
recs =@ array.new()
r =@ table.select(error=e)
while r !@= .nul
i = i + 1
recs[i] =@ r
r =@ r.select(error=e)
end while e != 0
end function recs</lang>
 
The final condition should best be read as: "end the while, if this is true".
=== while ===
 
set i 0
Another typical use of the loop is the traversing of rings that are commonly used in [[SIMPOL]].
while {$i < 10} {
 
puts [incr i]
<lang simpol>function getfieldnames(sbme1table table)
}
sbme1field field
integer i
array fieldnames
 
fieldnames =@ array.new()
i = 0
field =@ table.firstfield
while field !@= .nul
i = i + 1
fieldnames[i] = field.name
field =@ field.next
end while field =@= table.firstfield
end function fieldnames</lang>
 
=={{header|Sing}}==
<lang Sing>fn loops() void
{
// while: the condition must be strictly of boolean type
var idx = 0;
while (idx < 10) {
++idx;
}
 
// for in an integer range, the last value is excluded
// it is local to the loop and must not be previously declared
for (it in 0 : 10) {
}
 
// reverse direction
for (it in 10 : 0) {
}
 
// configurable step. The loop stops when it >= the final value
for (it in 0 : 100 step 3) {
}
 
// with an auxiliary counter. The counter start always at 0 and increments by one at each iteration
for (counter, it in 3450 : 100 step -22) {
}
 
// value assumes in turn all the values from array
var array [*]i32 = {0, 10, 100, 1000};
for (value in array) {
}
 
// as before with auxiliary counter
for (counter, value in array) {
}
}</lang>
 
==[[SNUSP]]==
==in==!/==body==?\==out==
\==ydob===/
 
The basic loop structure can be modified in many ways. You can have a pre-loop test by including '''?''' before the loop.
 
A unique feature of SNUSP is the ability to write ''bi-directional'' loops, designed for different effect depending on which direction the flow of control is running around the loop. The two entry points into this subroutine will move a value either up or down one cell, determined by the direction the loop is circulating.
# #
up1====?!/->+<\
? ?
down1==?!\<+>-/
# #
 
 
==[[Sparkling]]==
 
While loop (test-first loop):
 
<lang sparkling>var n = -1;
print("Enter 0 to exit");
 
while n != 0 {
n = toint(getline());
print(n);
}</lang>
 
For loop (alternate form of test-first loop, idiomatic approach to counting):
 
<lang sparkling>var i;
for i = 0; i < 10; i++ {
print(i);
}</lang>
 
Do-while loop (test-last loop):
 
<lang sparkling>print("enter empty line to exit");
 
var s;
do {
s = getline();
print(s);
} while s != "";</lang>
 
==[[Toka]]==
[[Category:Toka]]
 
=== countedLoop ===
Line 1,017 ⟶ 1,144:
0 [ dup . 1 + dup 101 = ] whileFalse
 
==[[UNIX ShellTern]]==
[[Category:UNIX Shell]]
===for===
'''Interpreter:''' [[Bourne Again SHell]]
 
Tern has several distinct loop statements.
#!/bin/bash
ARRAY="VALUE1 VALUE2 VALUE3 VALUE4 VALUE5"
for ELEMENT in $ARRAY
do
echo $ELEMENT # Print $ELEMENT
done
 
===whileInfinite Loop===
 
<lang tern>let v = 0;
'''Interpreter:''' [[bourne SHell]]
loop {
println(v++);
}</lang>
 
===While Loop===
<code> <pre>
 
#!/bin/sh
<lang tern>let v = 0;
x=1
while(v [< $x100) -lt 101 ]{
println(v++);
do
}</lang>
echo $x
 
x=`expr "$x" + 1`
===For Loop===
done
 
</pre> </code>
<lang tern>for(let v = 0; v < 100; v++) {
println(v);
}</lang>
 
===For In Loop===
 
<lang tern>for(v in 0 to 99) {
println(v);
}</lang>
 
=={{header|Woma}}==
 
Woma is limited to for loops.
 
==Infinte Loop==
 
<lang woma>i<@>iter(int, 1)
print(i)
</lang>
 
===For Loop===
'''Interpreter:''' [[Bourne Again SHell]]
 
<lang woma>i<@>range(100)
<code> <pre>
print(i)
#!/bin/bash
</lang>
while true
do
...
done
</pre> </code>
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