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

<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>

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++

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.

C#

<lang csharp>using System.Diagnostics;

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

Clojure

<lang Clojure> (let [i 42]

    (assert (= i 42)))

</lang>

Common Lisp

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

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

D

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

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

}</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.

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#

This example is incomplete. The task asks for an assertion that an integer variable is equal to 42. Please ensure that it meets all task requirements and remove this message.

F# provides an assert function that is only enabled when the program is compiled with DEBUG defined. <lang fsharp>let f x =

   assert (x > 1)
   x</lang>

Go

<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()

Groovy

This example is incomplete. The task asks for an assertion that an integer variable is equal to 42. Please ensure that it meets all task requirements and remove this message.

<lang groovy>def doSomething = { x, f ->

  assert f != null : "f was null!"
  assert f instanceof Closure : "f was not a closure! f: ${f}"
  f(x)

}</lang>

Test program: <lang groovy>println doSomething(-1) { x -> -2*Math.asin(x) } try { println doSomething(-1, 50) } catch (Throwable t) { println t.message } try { println doSomething(-1, null) } catch (Throwable t) { println t.message }</lang>

Output:

3.141592653589793
f was not a closure! f: 50. Expression: (f instanceof groovy.lang.Closure). Values: f = 50
f was null!. Expression: (f != null). Values: f = null

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

This example is incomplete. The task asks for an assertion that an integer variable is equal to 42. Please ensure that it meets all task requirements and remove this message.

<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>

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.

J

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

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>

MATLAB

<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.

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
%**--------------------------------------------------------------

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>

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>

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

Library: test/unit

<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

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>