Palindrome Golf

Question

The goal: Any language. The smallest function which will return whether a string is a palindrome. Here is mine in Python:

R=lambda s:all(a==b for a,b in zip(s,reversed(s)))

50 characters.

The accepted answer will be the current smallest one - this will change as smaller ones are found. Please specify the language your code is in.

Answer 1

Here's mine; it's written in a domain-specific language I invented, called 'palindrome'.

p

Edit: Less flippant version (i386 asm, AT&T syntax)

xor %eax, %eax
mov %esi, %edi
#cld    not necessary, assume DF=0 as per x86 ABI
repne scasb
scan:
    dec %edi
    cmpsb
    .byte 0x75, 6    #jnz (short) done
    dec %edi
    cmp %esi, %edi
    .byte 0x72, -9    #jb (short) scan
inc %eax
done:

16 bytes, string pointer goes in ESI, result is in EAX.

Answer 2

My attempt in C (70 chars):

P(char*s){char*e=s+strlen(s)-1;while(s<e&&*s==*e)s++,e--;return s>=e;}

[Edit] Now actually working
[Edit 2] Reduced from 74 to 70 by using default int return

In response to some of the comments: I'm not sure if that preprocessor abuse counts - you could just define the whole thing on the command line and make the function one character.

Answer 3

Another python version that is rather shorter (21 chars):

R=lambda s:s==s[::-1]

Answer 4

Perl (27 chars):

sub p{$_[0]eq reverse$_[0]}

Ruby (24 chars):

def p(a)a==a.reverse end

Answer 5

73 clean, readable, chars written in java

boolean p(String s){return s.equals(""+new StringBuffer(s).reverse());}

peace :)

Answer 6

(equal p (reverse p))

lisp. 18 characters.

ok, this is a special case. This would work if typed directly into a lisp interpreter and p was already defined.

otherwise, this would be necessary:

(defun g () (equal p (reverse p)))

28 characters.

Answer 7

Haskell, 15 chars:

p=ap(==)reverse

More readable version, 16 chars:

p x=x==reverse x

Answer 8

PHP:

function p($s){return $s==strrev($s);} // 38 chars

or, just

$s==strrev($s); // 15 chars

Answer 9

CFScript, 39 characters:

function y(x){return(x is reverse(x));}

I was never very good at golf.

Answer 10

Java:

boolean y(StringBuffer x){return x.equals(x.reverse());}

The above doesn't work, oops!

boolean y(StringBuffer x){return x.toString().equals(x.reverse().toString()); }

Ew.

Answer 11

Haskell, 28 chars, needs Control.Arrow imported.

p=uncurry(==).(id&&&reverse)

Answer 12

Lua aims more at readability than conciseness, yet does an honest 37 chars:

function p(s)return s==s:reverse()end

variant, just for fun (same size):

p=function(s)return s==s:reverse''end

The JavaScript version is more verbose (55 chars), because it doesn't has a string reverse function:

function p(s){return s==s.split('').reverse().join('')}

Answer 13

Straightforward implementation in C using standard library functions, inspired by the strlen in the other C answer.

Number of characters: 57

p(char*s){char*r=strdup(s);strrev(r);return strcmp(r,s);}

Confession: I'm being the bad guy by not freeing r here. My current attempt at being good:

p(char*s){char*r=strdup(s);s[0]=strcmp(strrev(r),s);free(r);return s[0];}

brings it to 73 characters; I'm thinking of any ways to do it shorter.

Answer 14

Without using any library functions (because you should really add in the '#include' cost as well), here's a C++ version in 96:

int p(char*a,char*b=0,char*c=0){return c?b<a||p(a+1,--b,c)&&*a==*b:b&&*b?p(a,b+1):p(a,b?b:a,b);}

Skizz

Answer 15

Shell-script (sed + tac + tr):

test "`echo $1|sed -e 's/\(.\)/\1\n/g'|tac|tr -d '\n'`" == "$1"

Answer 16

7 characters in J: Not sure if this is the best way, I'm somewhat new to J :)

p=:-:|.

explanation: |. reverses the input. -: compares. the operands are implicit.

p 'radar'
1

p 'moose'
0

Answer 17

Isn't using the reverse function in your language kind of cheating a bit? I mean, looking at the Ruby solution give as

def p(a)a==a.reverse end

you could easily rewrite that as

def p(a)a==a.r end

and just say that you made an extension method in your code so that "r" called reverse. I'd like to see people post solutions that don't contain calls to other functions. Of course, the string length function should be permitted.

Ruby without reverse - 41 characters

def m(a)a==a.split('').inject{|r,l|l+r}end

VB.Net - 173 Chars

Function P(ByVal S As String) As Boolean
    For i As Integer = 0 To S.Length - 1
     If S(i) <> S(S.Length - i - 1) Then
      Return False
     End If
    Next
    Return True
End Function

Answer 18

C# Without Reverse Function 84 chars

int p(char[]s){int i=0,l=s.Length,t=1;while(++i<l)if(s[i]!=s[l-i-1])t&=0;return t;}

C# Without Reverse Function 86 chars

int p(char[]s){int i=0;int l=s.Length;while(++i<l)if(s[i]!=s[l-i-1])return 0;return 1;}

VBScript 41 chars

function p:p=s=strreverse(s):end function

Answer 19

Definitely not the smallest, but I still wanted to add a entry:

sub p{return @_==reverse split//;}

My perl's rusty tho and this is untested.

Answer 20

Groovy 17B:

p={it==it[-1..0]}

Downside is that it doesn't work with emptry string.

On second thought, throwing exception for empty string is reasonable since you can't tell if nothing is palindrome or not.

Answer 21

At the risk of getting down votes, most all of these just call a command reverse of some sort that hides all the real programming logic.

I wonder what the shortest manual way to do this is in each of these languages.

Answer 22

F# (a lot like the C# example)

let p s=let i=0;let l=s.Length;while(++i<l)if(s[i]!=[l-i-1]) 0; 1;;

Answer 23

Josh's Java snippet above will return true every time.

Answer 24

Pointless Haskell version (15 chars, though doesn't really work unless you include Control.Arrow and Control.Monad and ignore the monomorphism restriction):

p=ap(==)reverse

Answer 25

I'll take it a little bit further: full c code, compile and go.

90 characters

main(int n,char**v){char*b,*e;b=e=v[1];while(*++e);for(e--;*b==*e&&b++<e--;);return b>e;}

Answer 26

Common Lisp, short-and-cheating version (23 chars):

#L(equal !1(reverse !1))

#L is a reader macro character implemented by SHARPL-READER in the iterate package. It's basically equivalent to (lambda (!1) ...).

Common Lisp, long version using only primitives (137 including whitespace, compressible down to 108):

(defun p (s)
  (let ((l (1- (length s))))
    (iter (for i from l downto (/ l 2))
          (always (equal (elt s i) (elt s (- l i)))))))

Again, it uses iterate, which is basically a cleaner version of the builtin LOOP facility, so I tend to treat it as being in the core language.

Answer 27

With C# and LINQ operators:

public bool IsPalindrome(string s)
{
    return s.Reverse().SequenceEqual(s);
}

If you consider Reverse as cheating, you can do the entire thing with a reduction:

public bool IsPalindrome(string s)
{
    return s.Aggregate(new StringBuilder(),
                       (sb, c) => sb.Insert(0, c),
                       (sb) => sb.ToString() == s);
}

Answer 28

Clojure using 37 characters:

user=> (defn p[s](=(seq s)(reverse(seq s))))
#'user/p
user=> (p "radar")
true
user=> (p "moose")
false

Answer 29

C, no libraries, 70 characters:

p(char*a){char*b=a-1;while(*++b);while(a<b&&*a++==*--b);return!(a<b);}

As one of the comments on another C solution mentioned, prototypes are completely optional in C, int is assumed everywhere a type would go but isn't mentioned. Has nobody ever programmed in pre-ANSI C?

Edit: shorter and handles empty strings.

Answer 30

24 characters in Perl.

sub p{$_[0]eq+reverse@_}

Answer 31

Inspired by previous post, 69 characters

p(char*a){char*b=a,q=0;while(*++b);while(*a)q|=*a++!=*--b;return!q;}

EDIT: Down one char:

p(char*a){char*b=a,q=0;while(*++b);while(*a)q|=*a++%*--b;return!q;}

EDIT2: 65 chars:

p(char*a){char*b=a;while(*b)b++;while(*a&&*a++==*--b);return!*a;}

Answer 32

JavaScript: 55 chars

p=function(x){return (x==x.split('').reverse().join())}

The above won't work because you need to call join with ""

JavaScript: 55 chars

function p(s){return s==s.split("").reverse().join("")}

Answer 33

Java, without using reverse:

boolean p(String s){int i=0,l=s.length();while(i<l){if(s.charAt(i++)!=s.charAt(--l))l=-1;}return l>=0;

Answer 34

Standard ML (34 characters and boring):

fun p s=s=implode(rev(explode(s)))

Answer 35

C, 68 characters, no libs:

p(char *s){char *e=s;while(*++e);for(;*s==*--e&&s++<e;);return s>e;}

Answer 36

Not the shortest, and very after-the-fact, but I couldn't help giving it a try in MATLAB:

R=@(s)all(s==fliplr(s));

24 chars.

Answer 37

may was well give a c++ example which uses the standard library:

bool p(const std::string &s){std::string s2(s);std::reverse(s2);return s==s2;}

Thanks to Jon For pointing out that this could be shorter if we make some unnecessary copies. Totalling 67 chars.

bool p(std::string s){std::string x=s;std::reverse(x);return s==x;}

Answer 38

Haskell, 36 characters including its own reverse function. (Actually, if you use semicolons to make it a one-liner, and ignore the newline at the end, it'd be 35...)

r[]y=y
r(a:x)y=r x$a:y
p s=s==r s[]

As with other implementations, "p" is the palindrome predicate.

Answer 39

58 characters in Python, without reversing the string:

r="y"
for i in range(len(s)):
 if(s[i]!=s[-i-1]):
  r="n"

Maybe the for loop could be optimized? Python is new to me...

Answer 40

18 character perl regex

/^(.?|(.)(?1)\2)$/

Answer 41

52 characters in C, with the caveat that up to half the string will be overwritten:

p(char*s){return!*s||!(s[strlen(s)-1]-=*s)&&p(++s);}

Without library calls it's 64 characters:

p(char*s){char*e=s;while(*e)++e;return!*s||!(*--e-=*s)&&p(++s);}

Answer 42

Perl (21 characters):

sub{"@_"eq reverse@_}

Hey, the question didn't specify a named subroutine!

Answer 43

javascript recursive version (no reverse junk)

function p(s){l=s.length;return l<2||(s[0]==s[l-1]&&p(s.substr(1,l-2)))}

(72 chars)

or implement reverse inside:

p=function(s,y){return y?(s==p(s)):s[1]?(p(s.substr(1))+s[0]):s[0]}

p("hannah",1);

(67 chars)

or, using no built in functions at all...

p=function(s,y,i){
return i?s[i]?s[i]+p(s,0,i+1):'':y?(s==p(s)):s[1]?(p(p(s,0,1))+s[0]):s[0]
}

p("hannah",1);

(92 chars)

shortest I could come up with: (iterative)

function p(s,l){for(c in s){if(s[c]!=s[l-1-c])s=0}return s}

p("hannah",6);// (is this cheating?)

(59 chars)

looking forward to seeing you do it better in javascript!

(preferably without using any built in functions, especially reverse)

(not really very impressed by the 'return s==s.reverse()' type answers)

Answer 44

Golfscript, 5 char

.-1%=

$ echo -n abacaba | ruby golfscript.rb palindrome.gs
1

$ echo -n deadbeef | ruby golfscript.rb palindrome.gs
0

Answer 45

F#: 29 chars

let p(s:string)=s=s.Reverse()

(assuming System.Linq is imported)

Answer 46

C# using a recursive lambda function and not using the Reverse function (115 chars):

Func<string,bool>p=null;p=w=>{if(w.Length<2)return true;return w[0]==w[w.Length-1]&&p(w.Substring(1,w.Length-2));};

Answer 47

Impossible! language, assuming that string is passed by using normal args:

;~=

3 chars