Assertions

Assertions
You are encouraged to solve this task according to the task description, using any language you may know.

Assertions are a way of breaking out of code when there is an error or an unexpected input. Some languages throw exceptions and some treat it as a break point.

Show an assertion in your language by asserting that an integer variable is equal to 42.

Ada

Using pragma Assert: <lang ada>pragma Assert (A = 42, "Oops!");</lang> The behavior of pragma is controlled by pragma Assertion_Policy. Another way is to use the predefined package Ada.Assertions: <lang ada>with Ada.Assertions; use Ada.Assertions; ... Assert (A = 42, "Oops!");</lang> The procedure Assert propagates Assertion_Error when condition is false.

ALGOL 68

The "Revised Report on the Algorithmic Language - ALGOL 68" suggest that ASSERT may be made available by a particular implementation, quote: "Pragmats may ... convey to the implementation some piece of information affecting some aspect of the meaning of the program which is not defined by this Report,..."

Example given[1]:

INT a, b; read((a, b)) PR ASSERT a >= 0 & b > 0 PR;

This works with neither ELLA ALGOL 68 nor ALGOL 68G.

The standard alternative would be to implement the assertions as an exception as per the Exceptions sample code.

In ELLA ALGOL 68 the ASSERT is implemented as an operator in the environment prelude: <lang algol68>OP ASSERT = (VECTOR [] CHAR assertion,BOOL valid) VOID: IF NOT valid THEN type line on terminal(assertion);

       terminal error( 661 {invalid assertion } )

FI;</lang> And can be "USEd" as follows: <lang algol68>PROGRAM assertions CONTEXT VOID USE standard,environment BEGIN

 INT a := 43;
 "Oops!" ASSERT ( a = 42 )

END FINISH</lang>

AutoHotkey

Exceptions

Works with: AutoHotkey_L

<lang AHK>a := 42 Assert(a > 10) Assert(a < 42) ; throws exception

Assert(bool){

   If !bool
       throw Exception("Expression false", -1)

}</lang>

Legacy versions

<lang AutoHotkey>if (a != 42) { OutputDebug, "a != 42" ; sends output to a debugger if connected ListVars ; lists values of local and global variables Pause ; pauses the script, use ExitApp to exit instead }</lang>

BBC BASIC

<lang bbcbasic> PROCassert(a% = 42)

     END
     
     DEF PROCassert(bool%)
     IF NOT bool% THEN ERROR 100, "Assertion failed"
     ENDPROC</lang>

Brat

<lang brat>squish import :assert :assertions

assert_equal 42 42 assert_equal 13 42 #Raises an exception</lang>

C

<lang c>#include <assert.h>

int main(){

  int a;
  /* ...input or change a here */
  assert(a == 42); /* aborts program when a is not 42, unless the NDEBUG macro was defined */

  return 0;

}</lang> To turn off assertions, simply define the NDEBUG macro before where <assert.h> is included.

There is no mechanism to add a custom "message" with your assertion, like in other languages. However, there is a "trick" to do this, by simply logical-AND-ing your condition with a string constant message, like in the following. Since a string constant is guaranteed to be non-NULL (and hence evaluated as True), and since AND-ing with True is an identity operation for a boolean, it will not alter the behavior of the assertion, but it will get captured in the debug message that is printed: <lang c>assert(a == 42 && "Error message");</lang> This trick only works with messages written directly in the source code (i.e. cannot be a variable or be computed), however, since the assertion message is captured by the macro at compile-time.

C#

<lang csharp>using System.Diagnostics;

Debug.Assert(a == 42);</lang>

C++

Translation of: C

<lang cpp>#include <cassert> // assert.h also works

int main() {

 int a;
 // ... input or change a here

 assert(a == 42); // Aborts program if a is not 42, unless the NDEBUG macro was defined
                   // when including <cassert>, in which case it has no effect

}</lang> Note that assert does not get a std:: prefix because it's a macro.

Clojure

<lang Clojure> (let [i 42]

    (assert (= i 42)))

</lang>

Common Lisp

<lang lisp>(let ((x 42))

   (assert (and (integerp x) (= 42 x))))</lang>

Component Pascal

Works with BlackBox Component Builder <lang oberon2> MODULE Assertions; VAR x: INTEGER; PROCEDURE DoIt*; BEGIN x := 41; ASSERT(x = 42); END DoIt; END Assertions.

Assertions.DoIt </lang> Output:

TRAP 0

 Assertions.DoIt   [0000001DH] 
 Kernel.Call   [00001A7CH] 
	.adr	INTEGER	1685454913
	.kind	INTEGER	0
	.n	INTEGER	0
	.p	INTEGER	0
	.par	ARRAY 256 OF INTEGER	elements
	.r	REAL	8.70603013185328E+175
	.sig	POINTER	[64760018H]
	.size	INTEGER	2287288
	.sp	INTEGER	256
	.typ	POINTER	NIL
 Meta.Item.ParamCallVal   [00002B5EH] 
	.adr	INTEGER	1685454913
	.data	ARRAY 256 OF INTEGER	elements

D

<lang d>import std.string; void main() {

 int a = readln().chomp().atoi();
 assert(a == 42, "You did not input 42!");

}</lang>

Dart

Dart supplies a class Expect that works similar to the Assert methods of Junit <lang d>main() {

 int i=42;
 int j=41;

 Expect.equals(42,i);
 Expect.equals(42,j);

}</lang>

Delphi

<lang Delphi>Assert(a = 42);</lang>

If an assertion fails, EAssertionFailed exception is raised.

The generation of assertion code can be disabled by compiler directive

<lang Delphi>{$ASSERTIONS OFF}</lang>

Here is a simple console demo app which raises and handles assertion exception:

<lang Delphi>program TestAssert;

{$APPTYPE CONSOLE}

{.$ASSERTIONS OFF} // remove '.' to disable assertions

uses

 SysUtils;

var

 a: Integer;

begin

 try
   Assert(a = 42);
 except
   on E:Exception do
     Writeln(E.Classname, ': ', E.Message);
 end;
 Readln;

end.</lang>

DWScript

Simple assertion, with a custom (optional) message <lang Delphi>Assert(a = 42, 'Not 42!');</lang> Other specialized assertions can be used in contracts, for instance this function check that the parameter (passed by reference ofr the purpose of illustration) is 42 when entering the function and when leaving the function <lang Delphi>procedure UniversalAnswer(var a : Integer); require

  a = 42;

begin

  // code here

ensure

  a = 42;

end;</lang>

E

E does not have the specific feature of assertions which may be disabled by a global option. But it does have a utility to throw an exception if a condition is false:

<lang e>require(a == 42) # default message, "Required condition failed"

require(a == 42, "The Answer is Wrong.") # supplied message

require(a == 42, fn { `Off by ${a - 42}.` }) # computed only on failure</lang>

Eiffel

Works with: SmartEiffel

version 2.4

There are many assertion types in Eiffel, one is the following:

File called main.e: <lang eiffel>class MAIN

   creation main
   feature main is
       local
           test: TEST;
       do
           create test;

           io.read_integer;
           test.assert(io.last_integer);
       end

end</lang> Another file called test.e: <lang eiffel>class TEST

   feature assert(val: INTEGER) is
       require
           val = 42;
       do
           print("Thanks for the 42!%N");
       end

end</lang>

Erlang

Erlang doesn't have an assert statement. However, it is single assignment, and its assignment operator won't complain if you reassign the exact same value to an existing variable but will throw an exception otherwise. <lang erlang>1> N = 42. 42 2> N = 43.

• • exception error: no match of right hand side value 43

3> N = 42. 42 4> 44 = N.

• • exception error: no match of right hand side value 42

5> 42 = N. 42</lang>

As such, the behavior of Erlang's assignment operator is extremely similar to a regular assert in other languages.

Euphoria

<lang euphoria>type fourty_two(integer i)

   return i = 42

end type

fourty_two i

i = 41 -- type-check failure</lang>

Factor

Throw an exception if the value on the top of the stack is not equal to 42:

<lang factor>USING: kernel ; 42 assert=</lang>

F#

F# provides an assert function that is only enabled when the program is compiled with DEBUG defined. When an assertion fails, a dialog box is shown with the option to enter the debugger. <lang fsharp>let test x =

 assert (x = 42)

test 43</lang>

GAP

<lang gap># See section 7.5 of reference manual

1. GAP has assertions levels. An assertion is tested if its level
2. is less then the global level.

1. Set global level

SetAssertionLevel(10);

a := 1; Assert(20, a > 1, "a should be greater than one");

1. nothing happens

a := 1; Assert(4, a > 1, "a should be greater than one");

1. error

1. Show current global level

AssertionLevel();

1. 10</lang>

Go

Assertions are a feature consciously omitted from Go. For cases where you want feedback during development, the following code should provide a similar purpose. While it is simply an if statement and a panic, the technique does have some properties typical of assertions. For one, the predicate of an if statement in Go is required to be of boolean type. Specifically, ints are not tacitly tested for zero, pointers are not tested for nil: the expression must be boolean, as the WP article mentions is typical of assertions. Also, it provides a good amount of information should the predicate evaluate to true. First, a value of any type can be passed to the panic, and by default is displayed, followed by a stack trace which includes the location of the panic in the source code—function name, file name, and line number. <lang go>package main

func main() {

   x := 43
   if x != 42 {
       panic(42)
   }

}</lang> Output:

panic: 42

panic PC=0x2b772d1a1048
runtime.panic+0xa7 /pool/go/src/pkg/runtime/proc.c:1032
        runtime.panic(0x40e820, 0x2a)
main.main+0x48 /pool/test.go:8
        main.main()
runtime.mainstart+0xf /pool/go/src/pkg/runtime/amd64/asm.s:77
        runtime.mainstart()
runtime.goexit /pool/go/src/pkg/runtime/proc.c:148
        runtime.goexit()

Also see Monty_Hall_problem#Go for more examples of this assertion idiom.

Groovy

<lang groovy>def checkTheAnswer = {

  assert it == 42 : "This: " + it + " is not the answer!"

}</lang>

Test program: <lang groovy>println "before 42..." checkTheAnswer(42) println "before 'Hello Universe'..." checkTheAnswer("Hello Universe")</lang>

Output:

before 42...
before 'Hello Universe'...
java.lang.AssertionError: This: Hello Universe is not the answer!. Expression: (it == 42). Values: it = Hello Universe
	at ConsoleScript80$_run_closure1.doCall(ConsoleScript80:2)
	at ConsoleScript80.run(ConsoleScript80:8)

Haskell

<lang haskell>import Control.Exception

main = let a = someValue in

        assert (a == 42) -- throws AssertionFailed when a is not 42
               somethingElse -- what to return when a is 42</lang>

Icon and Unicon

<lang Icon>... runerr(n,( expression ,"Assertion/error - message.")) # Throw (and possibly trap) an error number n if expression succeeds. ... stop(( expression ,"Assertion/stop - message.")) # Terminate program if expression succeeds. ...</lang>

There are no 'assertions', which can be turned on/off by the compiler. We can emulate them by prefixing a stop statement with a check on a global variable:

<lang Icon> $define DEBUG 1 # this allows the assertions to go through

procedure check (a)

 if DEBUG then stop (42 = a, " is invalid value for 'a'")
 write (a)

end

procedure main ()

 check (10)
 check (42)
 check (12)

end </lang>

This produces the output:

10
42 is invalid value for 'a'

Changing the define to: $define DEBUG &fail turns off the assertion checking.

J

<lang j> assert n = 42</lang>

Java

<lang java5>public static void main(String[] args){

  int a;
  //...input or change a here
  assert a == 42;//throws an AssertionError when a is not 42
  assert a == 42 : "Error message"; //throws an AssertionError 
         //when a is not 42 with "Error message" for the message
         //the error message can be any non-void expression

}</lang> Note: assertion checking is disabled by default when you run your program with the java command. You must provide the -ea (short for -enableassertions) flag in order to enable them.

Lisaac

<lang Lisaac>? { n = 42 };</lang>

Lua

<lang lua>a = 5 assert (a == 42) assert (a == 42,'\..a..'\' is not the answer to life, the universe, and everything')</lang>

Mathematica

<lang Mathematica>Assert[var===42]</lang>

MATLAB / Octave

<lang MATLAB>assert(x == 42,'x = %d, not 42.',x);</lang>

Sample Output: <lang MATLAB>x = 3; assert(x == 42,'Assertion Failed: x = %d, not 42.',x); ??? Assertion Failed: x = 3, not 42. </lang>

Metafont

Metafont has no really an assert built in, but it can easily created:

<lang metafont>def assert(expr t) = if not (t): errmessage("assertion failed") fi enddef;</lang>

This assert macro uses the errmessage built in to show the "error". The errmessage gives the error message and asks the user what to do.

Usage example:

<lang metafont>n := 41; assert(n=42); message "ok";</lang>

Output (failed assertion):

This is METAFONT, Version 2.71828 (Web2C 7.5.5)
(./assert.mf
! assertion failed.
<to be read again> 
                   ;
l.4 assert(n=42);
                 
?

Modula-3

ASSERT is a pragma, that creates a run-time error if it returns FALSE. <lang modula3><*ASSERT a = 42*></lang>

Assertions can be ignored in the compiler by using the -a switch.

Nimrod

<lang Nimrod>var a = 42 assert(a == 42)</lang> Assertions may be disabled by compiling with --assertions:off.

Oberon-2

Oxford Oberon-2 <lang oberon2> MODULE Assertions; VAR a: INTEGER; BEGIN a := 40; ASSERT(a = 42); END Assertions. </lang> Output:

Runtime error: assertion failed (0) on line 6 in module Assertions
In procedure Assertions.%main
   called from MAIN

Objective-C

For use within an Objective-C method: <lang objc>NSAssert(a == 42, @"Error message");</lang>

If you want to use formatting arguments, you need to use the assertion macro corresponding to your number of formatting arguments: <lang objc>NSAssert1(a == 42, @"a is not 42, a is actually %d", a); # has 1 formatting arg, so use NSAssert"1"</lang>

Within a regular C function you should use NSCAssert or NSCAssertN instead.

To turn off assertions, define the NS_BLOCK_ASSERTIONS macro.

OCaml

<lang ocaml>let a = get_some_value () in

 assert (a = 42); (* throws Assert_failure when a is not 42 *)
 (* evaluate stuff to return here when a is 42 *)</lang>

It is possible to compile with the parameter -noassert then the compiler won't compile the assertion checks.

Oz

Oz does not have an assert statement. But if different values are assigned to the same dataflow variable, an exception will be thrown (similar to Erlang).

<lang oz>declare

 proc {PrintNumber N}
    N=42  %% assert
    {Show N}
 end

in

 {PrintNumber 42} %% ok
 {PrintNumber 11} %% throws </lang>

Output:

%***************************** failure **************************
%**
%** Tell: 11 = 42
%**
%** Call Stack:
%** procedure 'PrintNumber' in file "Oz<8>", line 3, column 0, PC = 18600220
%**--------------------------------------------------------------

PARI/GP

PARI can use any of the usual C methods for making assertions. GP has no built-in assertions.

Translation of: C

<lang C>#include <assert.h>

void test() {

 int a;
 // ... input or change a here

 assert(a == 42); // Aborts program if a is not 42, unless the NDEBUG macro was defined

}</lang>

Pascal

See Delphi

Perl

While not exactly an assertion, a common Perl idiom is to use ... or die ... to throw an exception when a certain statement is false. <lang perl>open my $fh, '<', 'file' or die "Cannot open file: $!\n"; # $! contains the error message from the last error</lang> Note: the above "or die" idiom is not needed when the "autodie" pragma is in use: <lang perl>use autodie; open my $fh, '<', 'file'; # automatically throws an exception on failure</lang>

Another example: <lang perl>my $a = 5;

1. ...input or change $a here

$a == 42 or die "Error message\n";

1. or, alternatively:

die "Error message\n" unless $a == 42;

1. or:

die "Error message\n" if not $a == 42;

1. or just:

die "Error message\n" if $a != 42;</lang>

Some third-party modules provide other ways of using assertions in Perl: <lang perl>use Carp::Assert; assert($a == 42);</lang>

There is also a number of ways to test assertions in test suites, for example: <lang perl>is $a, 42; ok $a == 42; cmp_ok $a, '==', 42, 'The answer should be 42';

1. etc.</lang>

Perl 6

Works with: pugs

<lang perl6>my $a = (1..100).pick;

1. with a (non-hygienic) macro

macro assert ($x) { "$x or die 'assertion failed: $x'" } assert('$a == 42');

1. but usually we just say

$a == 42 or die "$x ain't 42";</lang>

PHP

<lang php><?php $a = 5

1. ...input or change $a here

assert($a == 42) # when $a is not 42, take appropriate actions,

                # which is set by assert_options()

?></lang>

PicoLisp

The 'assert' function, in combination with the tilde read macro, generates code only in debug mode: <lang PicoLisp>... ~(assert (= N 42)) # Exists only in debug mode ...</lang> Other possibilities are either to break into an error handler: <lang PicoLisp>(let N 41

  (unless (= N 42) (quit "Incorrect N" N)) )  # 'quit' throws an error

41 -- Incorrect N ?</lang> or to stop at a debug break point, allowing to continue with the program: <lang PicoLisp>(let N 41

  (unless (= N 42) (! setq N 42)) )   # '!' is a breakpoint

(setq N 42) # Manually fix the value ! # Hit ENTER to leave the breakpoint -> 42</lang>

PL/I

<lang> /* PL/I does not have an assert function as such, */ /* but it is something that can be implemented in */ /* any of several ways. A straight-forward way */ /* raises a user-defined interrupt. */

on condition (assert_failure) snap

  put skip list ('Assert failure');

.... if a ^= b then signal condition(assert_failure);

/* Another way is to use the preprocessor, thus: */ %assert: procedure (a, b) returns (character);

  return ('if ' || a || '^=' || b ||
     ' then signal condition(assert_failure);');

%end assert; %activate assert;

assert(a, 42); </lang>

Prolog

Works with: SWI Prolog

<lang prolog> test(A):-

   assertion(A==42).

</lang>

PureBasic

PureBasic does not have a native function for assertion, but allows for the definition of one.

The Macro below will only be included in the code if is compiled in debug mode, if so it will test the condition and if it fails it will inform with the message defined by the programmer, the line where it happened and in which source code file.

<lang PureBasic>Macro Assert(TEST,MSG="Assert: ")

 CompilerIf #PB_Compiler_Debugger
   If Not (TEST)
     Debug MSG+" Line="+Str(#PB_Compiler_Line)+" in "+#PB_Compiler_File
     CallDebugger
   EndIf
 CompilerEndIf

EndMacro</lang>

A implementation as defined above could be; <lang PureBasic>A=42 Assert(A=42,"Assert that A=42") A=42-1 Assert(A=42)</lang> Where the second test would fail resulting in a message to the programmer with cause (if given by programmer), code line & file.

Python

<lang python>a = 5

1. ...input or change a here

assert a == 42 # throws an AssertionError when a is not 42 assert a == 42, "Error message" # throws an AssertionError

      # when a is not 42 with "Error message" for the message
      # the error message can be any expression</lang>

It is possible to turn off assertions by running Python with the -O (optimizations) flag.

R

<lang R>stopifnot(a==42)</lang>

Racket

Racket has higher-order assertions known as contracts that can protect any values including functions and objects. Contracts are typically applied on the imports or exports of a module.

<lang Racket>#lang racket

(define/contract x

 (=/c 42) ; make sure x = 42
 42)

(define/contract f

 (-> number? (or/c 'yes 'no)) ; function contract
 (lambda (x)
   (if (= 42 x) 'yes 'no)))

(f 42)  ; succeeds (f "foo") ; contract error! </lang>

If typical assertion checking (i.e. error unless some boolean condition holds) is needed, that is also possible:

<lang Racket>#lang racket

(define x 80) (unless (= x 42)

 (error "a is not 42")) ; will error

</lang>

Rexx

<lang REXX>/* REXX ***************************************************************

• There's no assert feature in Rexx. That's how I'd implement it
• 10.08.2012 Walter Pachl
  • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • /

x.=42 x.2=11 Do i=1 By 1

 Call assert x.i,42
 End

Exit assert:

 Parse Arg assert_have,assert_should_have
 If assert_have\==assert_should_have Then Do
   Say 'Assertion fails in line' sigl
   Say 'expected:' assert_should_have
   Say '   found:' assert_have
   Say sourceline(sigl)
   Say 'Look around'
   Trace ?R
   Nop
   Signal Syntax
   End
 Return

Syntax: Say 'program terminated'</lang> Output:

Assertion fails in line 8
expected: 42
   found: 11
  Call assert x.i,42
Look around
    Here I enter Say i
2
    and then I press just enter
program terminated    

RLaB

RLaB does not have a special function to deal with assertions. The following workaround will do the trick:

<lang RLaB> // test if 'a' is 42, and if not stop the execution of the code and print // some error message if (a != 42) {

 stop("a is not 42 as expected, therefore I stop until this issue is resolved!");

} </lang>

Ruby

This uses test/unit from the standard library.

<lang ruby>require "test/unit/assertions" include Test::Unit::Assertions

n = 5 begin

 assert_equal(42, n)

rescue Exception => e

 # Ruby 1.8: e is a Test::Unit::AssertionFailedError
 # Ruby 1.9: e is a MiniTest::Assertion
 puts e

end</lang>

Output:

<42> expected but was
<5>.

Sather

<lang sather>class MAIN is

 main is
   i ::= 41;
   assert i = 42; -- fatal
   -- ...
 end;

end;</lang>

(The current GNU Sather compiler v1.2.3 I am using to test the code seems to ignore the assertion and no fatal error is raised, despite Sather should, see e.g. here).

Scala

These two are the same thing, and are tagged @elidable(ASSERTION): <lang scala>assert(a == 42) assert(a == 42, "a isn't equal to 42") assume(a == 42) assume(a == 42, "a isn't equal to 42")</lang>

The next one does the same thing as above, but it is not tagged. Often used as a pre-condition checker on class constructors. <lang scala>require(a == 42) require(a == 42, "a isn't equal to 42")</lang>

This one checks a value and returns it for further use (here shown being printed). It uses assert, which, as explained, gets tagged. <lang scala>println(a.ensuring(a == 42)) println(a.ensuring(a == 42, "a isn't equal to 42")) println(a.ensuring(_ == 42)) println(a.ensuring(_ == 42, "a isn't equal to 42"))</lang>

Scheme

Works with: Scheme version R$^{6}$RS

Translation of: Common Lisp

<lang scheme>(let ((x 42))

 (assert (and (integer? x) (= x 42))))</lang>

Slate

<lang slate>load: 'src/lib/assert.slate'. define: #n -> 7. assert: n = 42 &description: 'That is not the Answer.'.</lang> raises an AssertionFailed condition (an Error).

SPARK

Works with SPARK GPL 2010

Assertions are analysed statically, before compilation or execution. They can appear in various places:

inline in the code, either

<lang ada>-# check X = 42;</lang>

or

<lang ada>-# assert X = 42;</lang>

as a precondition on an operation:

<lang ada>procedure P (X : in out Integer); --# derives X from *; --# pre X = 42;</lang>

or as a postcondition on an operation:

<lang ada>procedure P (X : in out Integer); --# derives X from *; --# post X = 42;</lang> Example: <lang ada>X := 7; --# check X = 42;</lang> produces the following output:

H1:    true .
       ->
C1:    false .

which is an unprovable theorem that tells you that there is a guaranteed failure.

Tcl

Library: Tcllib (Package: control)

<lang tcl>package require control

set x 5 control::assert {$x == 42}</lang> Produces the output:

assertion failed: $x == 42

Vala

<lang vala>int a = 42; int b = 33; assert (a == 42); assert (b == 42); // will break the program with "assertion failed" error </lang>

VBScript

Definition

<lang vb>sub Assert( boolExpr, strOnFail ) if not boolExpr then Err.Raise vbObjectError + 99999, , strOnFail end if end sub </lang>

Invocation

<lang vb>dim i i = 43 Assert i=42, "There's got to be more to life than this!"</lang>

Output

<lang VBScript>>cscript "C:\foo\assert.vbs" C:\foo\assert.vbs(3, 3) (null): There's got to be more to life than this!</lang>

Visual Basic

VB's Assert only fires when run from within the IDE. When compiled, all Debug lines are ignored.

<lang vb>Debug.Assert i = 42</lang>

XPL0

XPL0 does not have an assert command. The equivalent is usually synthesized something like this.

<lang XPL0>proc Fatal(Str); \Display error message and terminate program char Str; [\return; uncomment this if "assertions" are to be disabled SetVid(3); \set normal text display if program uses graphics Text(0, Str); \display error message ChOut(0, 7); \sound the bell exit 1; \terminate the program; pass optional error code to DOS ];

if X#42 then Fatal("X#42");</lang>