Assertions
![Task](http://static.miraheze.org/rosettacodewiki/thumb/b/ba/Rcode-button-task-crushed.png/64px-Rcode-button-task-crushed.png)
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>
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++
<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>
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
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
- GAP has assertions levels. An assertion is tested if its level
- is less then the global level.
- Set global level
SetAssertionLevel(10);
a := 1; Assert(20, a > 1, "a should be greater than one");
- nothing happens
a := 1; Assert(4, a > 1, "a should be greater than one");
- error
- Show current global level
AssertionLevel();
- 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
<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 %**--------------------------------------------------------------
PARI/GP
PARI can use any of the usual C methods for making assertions. GP has no built-in assertions.
<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;
- ...input or change $a here
$a == 42 or die "Error message\n";
- or, alternatively:
die "Error message\n" unless $a == 42;
- or:
die "Error message\n" if not $a == 42;
- 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';
- etc.</lang>
Perl 6
<lang perl6>my $a = (1..100).pick;
- with a (non-hygienic) macro
macro assert ($x) { "$x or die 'assertion failed: $x'" } assert('$a == 42');
- but usually we just say
$a == 42 or die "$x ain't 42";</lang>
PHP
<lang php><?php $a = 5
- ...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
<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
- ...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>
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
<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
<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>
- Programming Tasks
- Basic language learning
- Ada
- ALGOL 68
- AutoHotkey
- Brat
- C
- C sharp
- C++
- Clojure
- Common Lisp
- D
- Dart
- Delphi
- DWScript
- E
- Eiffel
- Erlang
- Euphoria
- Factor
- F Sharp
- GAP
- Go
- Groovy
- Haskell
- Icon
- Unicon
- J
- Java
- Lisaac
- Lua
- Mathematica
- MATLAB
- Metafont
- Modula-3
- Nimrod
- Objective-C
- OCaml
- Oz
- PARI/GP
- Pascal
- Perl
- Perl 6
- PHP
- PicoLisp
- PL/I
- Prolog
- PureBasic
- Python
- R
- Racket
- RLaB
- Ruby
- Sather
- Scala
- Scheme
- Slate
- SPARK
- Tcl
- Tcllib
- Gnuplot/Omit
- NSIS/Omit
- Vala
- VBScript
- Visual Basic