Write the shortest self-identifying program (a quine variant)

Question

Write a program that will generate a "true" output iff the input matches the source code of the program, and which generates a "false" output iff the input does not match the source code of the program.

This problem can be described as being related to quines, as the program must be able to somehow compute its own source code in the process.

This is code golf: standard rules apply. Your program must not access any special files, such as the file of its own source code.

Edit: If you so choose, true/false can be replaced with True/False or 1/0.

Example

If the source code of your program is bhiofvewoibh46948732));:/)4, then here is what your program must do:

Input (Stdin)

bhiofvewoibh46948732));:/)4

Output (Stdout)

true

Input

(Anything other than your source code)

Output

false

Answer 1

JavaScript : 26

function f(s){return s==f}

I don't know if a JavaScript file really qualifies as a "program".

Answer 2

Haskell, 72 characters

main=interact$show.(==s++show s);s="main=interact$show.(==s++show s);s="

Note: there is no end-of-line character at the end of the script.

$ runhaskell Self.hs < Self.hs
True

Answer 3

Perl, Infinity 41 38 Characters

$_=q(print<>eq"\$_=q($_);eval"|0);eval

Update: The program no longer ends with a newline, which means it will work correctly on multi-line files. You have to enter input from STDIN without hitting enter. On Windows I was only able to do this by reading from a file.

Original solution:

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Answer 4

><>, 68 bytes

Fishes love eating fish poop. Now we know they can distinguish theirs from their friends'.

00v      0+1~$^?)0~\;n0\
  >:@@:@gi:0(?\:a=?/=?!/$1+
  0n;n*=f$=2~~/

You can try it online !

Answer 5

GolfScript, 11 chars

{`".~"+=}.~

Without the =, this code would be a quine that generates its own source code as a string. The = makes it compare this string to its input and output 1 if they match and 0 if they don't. Note that the comparison is exact — in particular, a trailing newline at the end of the input will cause it to fail.

Explanation:

• { } is a code block literal in GolfScript;
• . duplicates this code block, and ~ executes the second copy (leaving the first on the stack);
• ` stringifies the code block, and ".~"+ appends .~ to it;
• finally, = compares the resulting string with the input (which is pushed on the stack as a string by the GolfScript interpreter before the program starts) and returns 1 if they match and 0 if they don't.

Answer 6

Python 2, 55

a='a=%r;print a%%a==raw_input()';print a%a==raw_input()

Tested:

a='a=%r;print a%%a==raw_input()';print a%a==raw_input() -> True

(anything else) -> False

Answer 7

JavaScript ES6, 16 14 bytes

$=_=>_==`$=`+$

Minus two bytes thanks to Neil.

31 bytes if we must take input via prompt.

$=_=>prompt()==`$=${$};$()`;$()

38 bytes if we must output via alert.

$=_=>alert(prompt()==`$=${$};$()`);$()

This is the proper way to do it, as Optimizer's answer does not accept the entire source code.

Answer 8

Node.js : 54

function f(){console.log(f+'f()'==process.argv[2])}f()

You test it by saving it into a file f.js (the exact name has no importance) and using

node f.js "test"

(which outputs false) or

node f.js "$(< f.js)"

(which outputs true)

I also made a different version based on eval :

eval(f="console.log('eval(f='+JSON.stringify(f)+')'==process.argv[2])")

It's now at 72 chars, I'll try to shorten that when I have time.

Answer 9

Smalltalk (Pharo 2.0 dialect), 41 bytes

Implement this 41 chars method in String (ugly formatting for code-golf):

isItMe^self=thisContext method sourceCode

Then evaluate this in a Workspace (printIt the traditional Smalltalk way)
The input is not read from stdin, it's just a String to which we send the message (what else a program could be in Smalltalk?):

'isItMe^self=thisContext method sourceCode' isItMe.

But we are cheating, sourceCode reads some source file...
Here is a variant with 51 chars which does not:

isItMe
    ^ self = thisContext method decompileString

And test with:

'isItMe
    ^ self = thisContext method decompileString' isItMe

If a String in a Workspace is not considered a valid input, then let's see how to use some Dialog Box in 116 chars
Just evaluate this sentence:

(UIManager default request: 'type me') = (thisContext method decompileString withSeparatorsCompacted allButFirst: 7)

Since decompile format includes CR and TAB, we change that withSeparatorsCompacted.
Then we skip the first 7 chars are 'doIt ^ '

Finally a 105 chars variant using stdin, just interpret this sentence from command line, just to feel more mainstream:

Pharo -headless Pharo-2.0.image eval "FileStream stdin nextLine = (thisContext method decompileString withSeparatorsCompacted allButFirst: 7)"

Answer 10

flex - 312 chars

Q \"
N \n
S " "
B \\
P "Q{S}{B}{Q}{N}N{S}{B}n{N}S{S}{Q}{S}{Q}{N}B{S}{B}{B}{N}P{S}{Q}{P}{Q}{N}M{S}{Q}{M}{Q}{N}%%{N}{P}{N}{M}{N} putchar('1');"
M "(.|{N})* putchar('0');"
%%
Q{S}{B}{Q}{N}N{S}{B}n{N}S{S}{Q}{S}{Q}{N}B{S}{B}{B}{N}P{S}{Q}{P}{Q}{N}M{S}{Q}{M}{Q}{N}%%{N}{P}{N}{M}{N} putchar('1');
(.|{N})* putchar('0');

Can probably be made shorter, but it works with multi-line input (necessary since the source code is multiple lines) and even for inputs that contain the program as a substring. It seems many of the answers so far fail on one or both of these.

Compile command: flex id.l && gcc -lfl lex.yy.c

Answer 11

D (133 chars)

enum c=q{import std.stdio;import std.algorithm;void main(){auto i=readln();writeln(equal("auto c=q{"~c~"};mixin(c);",i));}};mixin(c);

Answer 12

JavaScript (V8), 35

function i(){alert(prompt()==i+[])}

call i() and it will prompt for input

Answer 13

GolfScript - 26

":@;[34]@+2*=":@;[34]@+2*=

Inspired from http://esolangs.org/wiki/GolfScript#Examples

Another version:

"[34].@@;+2*="[34].@@;+2*=

Too bad that \ is both swap and escape...

Answer 14

Python 2, 47 bytes

_='_=%r;print _%%_==input()';print _%_==input()

A simple quine with the added check.

Answer 15

CJam, 12 bytes

{s"_~"+q=}_~

Try it online!

Explanation

This just uses the standard CJam quine framework.

{s"_~"+q=}    e# Push this block (function literal).
          _~  e# Copy and run it.

What the block does:

 s            e# Stringify the top element (this block itself).
  "_~"+       e# Append "_~". Now the source code is on the stack.
       q      e# Read the input.
        =     e# Check if it equals the source code.

Answer 16

Python, 187 bytes

import sys;code="import sys;code=!X!;print(sys.stdin.read()==code.replace(chr(33),chr(34)).replace(!X!,code,1))";print(sys.stdin.read()==code.replace(chr(33),chr(34)).replace("X",code,1))

Careful not to add newline at the end. Someone with better Python-fu might be able to shorten it.

Answer 17

Tcl, 111 chars

set c {set c {$c};puts [expr {[read stdin] eq [subst -noc \$c]}]};puts [expr {[read stdin] eq [subst -noc $c]}]

Answer 18

Perl, 52 char

$_='$/=$\;$_="\$_=\47$_\47;eval";print<>eq$_|0';eval

Answer 19

Husk, 11 bytes

=hS+s"=hS+s

Try it online!

Explanation

The explanation uses ¨ to delimit strings (to avoid unreadable escaping):

     "=hS+s  -- string literal: ¨=hS+s¨
  S+         -- join itself with
    s        -- | itself "showed": ¨"=hS+s"¨
             -- : ¨=hS+s"=hS+s"¨
 h           -- init: ¨=hS+s"=hS+s¨
=            -- is the input equal?

By removing the function = you can verify that it will indeed only match the source itself.

Answer 20

Python 2, 40 bytes

s="print's=%r;exec s'%s==input()";exec s

Try it online!

Answer 21

><>, 24 bytes

'1rd3*i={*}50l3-?.~i)*n;

Try it online!

Wrapping string literal followed by checking whether the input is identical to the stack, with a final check that there is no more input.

Answer 22

Jelly, 10 bytes

“Ṿ;$⁼”Ṿ;$⁼

Try it online!

“Ṿ;$⁼”Ṿ;$⁼
“Ṿ;$⁼”      String literal: 'Ṿ;$⁼'
        $   Next two links act on the string literal
      Ṿ     Uneval: '“Ṿ;$⁼”'
       ;    Append string: '“Ṿ;$⁼”Ṿ;$⁼' (source code)
         ⁼  Is the string above equal to the input?

Answer 23

Zsh, 33 bytes

f () {
	[ "`type -f f`" = $1 ]
}

Try it online!

The extra spaces, tab, and newlines are required. type -f f does not print the original source, but the function formatted in a particular way, with indentation and a trailing newline.

Answer 24

R, 54 bytes

f=function(s)s==paste0("f=function(s)s==", body(f)[3])

Try it online!

body gets the body of the function (splitting it a bit, so that body(f)[3] is everything from paste0 onwards). Interestingly, body reformats the code, adding spaces after commas, etc. This is thus a rare case of an R golf answer with a space after a comma.

This works because body(f) is an object of type language, and there exists an as.character method for this type. On the other hand, f and args(f) are of type closure, and cannot be converted to character type as far as I can tell. Please don't ask me what the language type is for…

Answer 25

R (version-dependent), 46 bytes

f=function(x)x==paste0('f=',capture.output(f))

capture.output - as the function name suggests - retrieves the text string that results from entering the function name f directly to R, which should output the body of the function. We manually prepend this with f= so that it exactly* matches the program code, and check whether the function argument x is the same as this.
*See below

There are a couple of caveats, though, that depend on the R version used:

1. Early versions of R (like my own version 3.2.1) return the originally-defined function body as-is when the function name is entered to R. However, later versions add additional formatting, like inserting spaces and newlines, which breaks this code: this unfortunately includes the installation (3.5.2) on 'Try it online'. Later still, much newer R versions (I tested 4.0.3) return to the as-is output formatting and everything is Ok again... except...
2. The most recent versions of R compile functions at the first run, and thereafter append a 'byte code' specific for the compiled function to the outputted function body. This means that capture.output returns two strings, beginning at the second time the function is run. Luckily, the function body is still the first one (the byte code is the second one), so the result of supplying the program code as input is TRUE FALSE, which evaluates to 'truthy' in R and so counts as a 'true' output for this challenge.

Special thanks to Robin Ryder for helping to uncover the version-dependence of R's function outputting.

Answer 26

Vyxal, 12 bytes

`:q$+=`:q$+=

Try it Online!

I finished making this, and then realized that it is almost identical to the Vyxal quine that @Lyxal wrote, but with the addition of = to check if the input is equal.

Explanation:

              # Implicit input
`:q$+=`       # Push the string ':q$+='
       :      # Duplicate the string
        q     # Quotify; put backticks around the topmost string
         $    # Swap the top two values on the stack
          +   # Concatenate the two strings
           =  # Check if the input is equal to the string
              # Implicit output

Answer 27

C - 186 176 characters

One liner:

 *a="*a=%c%s%c,b[999],c[999];main(){sprintf(b,a,34,a,34);gets(c);putchar(strcmp(b,c)?'0':'1');}",b[999],c[999];main(){sprintf(b,a,34,a,34);gets(c);putchar(strcmp(b,c)?'0':'1');}

With whitespace (note that this breaks the program):

*a="*a=%c%s%c,b[999],c[999];main(){sprintf(b,a,34,a,34);gets(c);putchar(strcmp(b,c)?'0':'1');}",b[999],c[999];
main() {
  sprintf(b,a,34,a,34);
  gets(c);
  putchar(strcmp(b,c)?'0':'1');
}

Answer 28

Stax, 26 bytes

"34bLN26)s:f="34bLN26)s:f=

Run and debug it

Answer 29

JavaScript ES6, 14 bytes

Like the other javascript answer but includes it's entire source code and not just the function

a=q=>'a='+a==q

Example usage:

a("q=>'a='+a==q") // returns false
a("a=q=>'a='+a==q") // returns true

Answer 30

q, 8 bytes

{x~.z.s}

Return boolean on input matching the self-referential .z.s