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

JavaScript ES6, 9 characters

This is the only (golfy) way to do it in JS. ES6 just makes it take super less characters

Run this in latest Firefox's Web Console:

f=x=>f==x

Example usage:

f("check") // returns false
f("x=>f==x") // returns true

Answer 3

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 4

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 5

><>, 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 6

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:

print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(print<>==q(...

Answer 7

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 8

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 9

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 10

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 11

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 12

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 13

JavaScript (V8), 35

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

call i() and it will prompt for input

Answer 14

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 15

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 16

Python 2, 47 bytes

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

A simple quine with the added check.

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

05AB1E, 15 bytes

0"D34çýQ"D34çýQ

Modifies the default quine 0"D34çý"D34çý by adding Q (check for equality with the implicit input)

Try it online.

Explanation:

0                # Push 0 to the stack
                 #  STACK: [0]
 "D34çýQ"        # Push the string 'D34çýQ' to the stack
                 #  STACK: [0, 'D34çýIå']
         D       # Duplicate this string
                 #  STACK: [0, 'D34çýIå', 'D34çýIå']
          34ç    # Push '"' to the stack
                 #  STACK: [0, 'D34çýIå', 'D34çýIå', '"']
             ý   # Join the stack by this '"' delimiter
                 #  STACK: ['0"D34çýIå"D34çýIå']
              Q  # Check if it's equal to the (implicit) input
                 # (and output the top of the stack implicitly as result)

---

Cool 15 bytes alternative provided by @Grimy:

187745012D27BJQ

Try it online.

Explanation:

187745012        # Push integer 187745012 
                 #  STACK: [187745012]
         D       # Duplicate it
                 #  STACK: [187745012, 187745012]
          27     # Push integer 27
                 #  STACK: [187745012, 187745012, 27]
            B    # Convert 187745012 to base-27
                 #  STACK: [187745012, "D27BJQ"]
             J   # Join the values on the stack together
                 #  STACK: ["187745012D27BJQ"]
              Q  # Check if it's equal to the (implicit) input
                 # (and output the top of the stack implicitly as result)

Answer 24

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 25

Stax, 26 bytes

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

Run and debug it

Answer 26

q, 8 bytes

{x~.z.s}

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

Answer 27

Runic, 11 bytes

"3X4+kSqi=@

Try it online!

TIO got updated and there's no longer an issue reading input (and no longer requires a trailing whitespace).

Explanation

>                 Implicit entry
 "                Begin reading as string
  3X4+kSqi=@      Pushed to the stack as a string, loop around
 "                End reading as string
  3X4+            Push 3*10 and 4 to the stack, add them together
      k           Convert to character (")
       S          Swap the top two items on the stack
        q         Concatenate. This leaves only "3X4+kSqi=@ on the stack
         i        Read input
          =       Compare using .Equals, push 1 if equal, else 0
           @      Print and terminate

JoKing's solution:

"'<~qi=@|

Explanation

  <              Entry
 '               Read character (loop around)
"                Push "
         |       Mirror
"                Begin reading string (loop around)
 '<~ri=@|        Push the string '<~qi=@| (loop around)
"                End reading string
 '<~             Push the character < and then discard it
    q            Concatenate, stack contains only "'<~qi=@|
      i          Read input
       =         Compare
        @        Print and terminate

Answer 28

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 29

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 30

Keg, 36 28 26 bytes

`:&\`ⁿ^\`ⁿ&⅀=`:&\`ⁿ^\`ⁿ&⅀=

Try it online!

Same concept as before, but shorter.

Answer History

`:&\`ⁿ⅍^\`ⁿ⅍++&+¿=`:&\`ⁿ⅍^\`ⁿ⅍++&+¿=

Try it online!

Who knew generating a program that knows itself would be so unreadable!

Explained

The very first step here is to generate the string that will be checked against. At this point, we don't particularly know what the checking part will look like, but we do know that there will be a string that looks like so:

`\`ⁿ⅍^\`ⁿ⅍++`

Which would consequently be followed by:

 \`ⁿ⅍^\`ⁿ⅍++

This creates a string which contains itself quoted... Something needed for when input is taken, as the quotes are going to be present.

From here, string equality is quite easy, coming in at a trivial 2 bytes:

¿=

Incorporating this into the string gives:

`\`ⁿ⅍^\`ⁿ⅍++¿=`\`ⁿ⅍^\`ⁿ⅍++¿=

At this stage, one might think the quine is complete... It has the string part and it has the equality part... What else could be needed? Well. Let me tell you we ain't done yet.

Y'see, we have a quoted string on the stack, but we don't have it's unquoted equivalent present, meaning we need to account for that. The register works well here:

`:&\`ⁿ⅍^\`ⁿ⅍++&+¿=`:&\`ⁿ⅍^\`ⁿ⅍++&+¿=

And there you have it: a self identifying program written completely on a phone keyboard.

Answer 31

Perl 6, 33 bytes

<dd "<$_>~~.EVAL"eq slurp>~~.EVAL

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Outputs either Bool::True or Bool::False to STDERR. This is the basic quine format, with an added check against input (eq slurp).

Answer 32

V, 19 bytes

ñOÑ~"qpxØ¥^¨©î±¥$

Try it online!

Prints 1 or 0. Here is a hexdump:

00000000: f14f d11b 7e22 7170 78d8 a55e a881 a9ee  .O..~"qpx..^....
00000010: b1a5 24                                  ..$

Answer 33

Java 8, 288 bytes (Full Program with STDIN)

interface M{static void main(String[]a){String s="interface M{static void main(String[]a){String s=%c%s%1$c;System.out.print(s.format(s,34,s).equals(new java.util.Scanner(System.in).nextLine()));}}";System.out.print(s.format(s,34,s).equals(new java.util.Scanner(System.in).nextLine()));}}

Try it online.

Java 8, 210 bytes (Full Program with Console Arguments)

interface M{static void main(String[]a){String s="interface M{static void main(String[]a){String s=%c%s%1$c;System.out.print(s.format(s,34,s).equals(a[0]));}}";System.out.print(s.format(s,34,s).equals(a[0]));}}

Try it online.

Java 8, 108 bytes (Function)

i->{String s="i->{String s=%c%s%1$c;return s.format(s,34,s).equals(i);}";return s.format(s,34,s).equals(i);}

Try it online.

Explanation:

quine-part:

• The String s contains the unformatted source code.
• %s is used to input this String into itself with the s.format(...).
• %c, %1$c and the 34 are used to format the double-quotes.
• s.format(s,34,s) puts it all together

Challenge-part:

• .equals(...) checks if this formatted source code equals the input.
  • java.util.Scanner(System.in).nextLine() is used as this input for STDIN
  • a[0] is used as this input for Console arguments
  • i is used as this input for the lambda function

Answer 34

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 35

Pascal (FPC), 167 157 bytes

const b=#39';var s:string;begin read(s);write(s=a+b+#39#59#97#61#39+a+b)end.';a='const b=#39';var s:string;begin read(s);write(s=a+b+#39#59#97#61#39+a+b)end.

Try it online!

While substring can be easily extracted, like in my regular quine, unfortunately, they cannot be concatenated because FPC thinks it is an array in these circumstances. In a and b are characters before and after constant definitions. #39#59#97#61#39 is a replacement for ';a=' as this is not represented in the constants. #39 represents ' and is at the start of b, glued together to the rest of b, to shorten the comparison expression as much as possible.

Answer 36

Zsh, 33 bytes

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

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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 37

TeX, 55 bytes

The file has to be saved as a.tex and should be run using pdftex a.tex. The script doesn't terminate after returning 1 or 0, if that should be necessary append it by \end (+4 bytes). If it should work for arbitrary file names replace \openin0a with \openin0\jobname (+7 bytes).

\openin0=a\read0to\0\read1to~\message{\ifx\0~1\else0\fi}

Answer 38

Python 3.8 (pre-release), 42 bytes

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

Try it online!

Answer 39

PowerShell, 28 bytes

Very similar to this JavaScript answer, uses function provider.

filter f{"$Function:f"-eq$_}

Example

PS > '"$Function:f"-eq$_' | f
True

PS > 'xxx' | f
False