Golfing A Weasel Program

Question

Richard Dawkins in his book The Blind Watchmaker, describes a Weasel program. The algorithm can be described as follows:

1. Start with a random string of 28 characters. Valid characters are all upppercase letters, and space.

2. Make 100 copies of that string, with a 5% chance per character of that character being replaced with a random character.

3. Compare each new string with the target "METHINKS IT IS LIKE A WEASEL", and give each a score according to the number of letters in the string which are correct and in the correct position.

4. If any of the new strings has a perfect score (28), halt.

5. Choose the highest-scoring string from step 3. How you work out a tie is up to you, but only one string may be chosen. Take the chosen string and go to step 2.

The winner will be the shortest code snippet to get to the correct answer while printing the highest-scoring string of each generation in the following format:

If people could help by checking other peoples answers would be very helpful!

Answer 1

APL (143)

0{⍵≢T←'METHINKS IT IS LIKE A WEASEL':c∇⍨1+⍺⊣⎕←(⍕⍺),':'c'-- score:',s⊣c←⊃c/⍨G=s←⌈/G←{+/⍵=T}¨c←{⍵{⍵:⍺⋄C[?27]}¨9≠?28/20}¨100⍴⊂⍵}⊃∘(C←27↑⎕A)¨?28/27

Explanation:

• 0{...}⊃∘(C←27↑⎕A)¨?28/27: set C to the first 27 capital letters. There are only 26, so the 27th element will be a space. Select 28 random items from C. This will be the first ⍵. The first ⍺ (generation) will be 0.

• ⍵≢T←'METHINKS IT IS LIKE A WEASEL: set T to the string 'METHINKS IT IS LIKE A WEASEL'. As long as ⍵ is not equal to T:

  • {...}¨100⍴⊂⍵: Make 100 copies of ⍵. For each of these...
    • 9≠?28/20: select 28 random numbers from 1 to 20. Make a bitmask where each 1 means that the random number was not equal to 9. (This means 5% chance of a 0).
    • ⍵{⍵:⍺⋄C[?27]}¨: for each letter in ⍵, if the corresponding bit was 1, keep that letter, otherwise replace it with a randomly chosen element from C.
  • c←: store the 100 mutated strings in c.
  • G←{+/⍵=T}¨c: for each element in c, calculate the score (amount of characters that match T) and store the scores in G.
  • s←⌈/G: find the maximum score and store that in s.
  • c←⊃c/⍨G=s: select the first item from c whose score is equal to s (the maximum), and store it in c again.
  • ⎕←(⍕⍺),':'c'-- score:',s: print the generation in the given format (⍺ is current generation, c is current best string, s is score)
  • c∇⍨1+⍺: Increment the generation and run the mutation again using the current best string (c) as input.

Answer 2

Mathematica - 238 236 225

c:="@"~CharacterRange~"Z"~RandomChoice~28/."@"->" "
For[s=""<>c;i=0,{d,s}=Sort[{#~HammingDistance~"METHINKS IT IS LIKE A WEASEL",#}&@
StringReplace[s,_/;20Random[]<1:>c〚1〛]&~Array~100]〚1〛;
d>0Print[i++,":"s," -- score: ",28-d],]

Example output

0:  CYPMEIHADXRXVTFHERYOZNRVFCSQ  -- score: 0
1:  CYPMEIHADIRXVTFBERYOZNRVFCSQ  -- score: 1
2:  CYPMEIHA IRXVTFBIRYOZNRVFCSQ  -- score: 3
...
50:  METHINKS IT IS LIKE A WEASEL  -- score: 28

Answer 3

Python (273)

from random import choice as c
n=range
a=map(chr,n(65,91)+[32])
s=map(c,[a]*28)
p=x=0
while p<28:
 p,s=max((sum(g==r for g,r in zip(y,'METHINKS IT IS LIKE A WEASEL')),y)for y in ([c(a+[x]*513)for x in s]for _ in n(100)));print '%d: %s -- score: %d' % (x,''.join(s),p);x+=1

Answer 4

Python: 282 characters no semi colons

from random import*
g,r,l,c=0,0,"ABCDEFGHIJKLMNOPQRSTUVWXYZ ",choice
k=map(c,[l]*28)
while(r!=28):
 r,k=max((sum(i==j for i,j in zip(t,"METHINKS IT IS LIKE A WEASEL")),t)for t in[[c(l)if random()<.05 else i for i in k]for i in[1]*100])
 print`g`+":","".join(k),"-- score:",`r`
 g+=1

278 with:

from random import*;g,r,l,c=0,0,"ABCDEFGHIJKLMNOPQRSTUVWXYZ ",choice;k=map(c,[l]*28)
while(r!=28):r,k=max((sum(i==j for i,j in zip(t,"METHINKS IT IS LIKE A WEASEL")),t)for t in[[c(l)if random()<.05 else i for i in k]for i in[1]*100]);print`g`+":","".join(k),"-- score:",`r`;g+=1

Answer 5

K, 173 167

o:"METHINKS IT IS LIKE A WEASEL"
i:0;{~x~o}{-1($i),": ",(r:a@*&b=c)," -- score: ",$c:max@b:+/'o=/:a:{x{if[0~*1?20;x[y]:*1?s];x}/!#x}'100#,x;i+:1;r}/28?s:"c"$32,65+!26;

/

0: FQRZPHACDIBHZOUUCYKKFBJWVNVI -- score: 1
1: FQRZP ACDITHCOUUCYKKFBJWVNVI -- score: 2
2: FQRZP AFDIT COUUCYKKFBJWVNVI -- score: 3
...
51: METHINKS IT IS LIKECA WEASEL -- score: 27
52: METHINKS IT IS LIKECA WEASEL -- score: 27
53: METHINKS IT IS LIKE A WEASEL -- score: 28

Answer 6

R (245 239 238 characters)

t=strsplit("METHINKS IT IS LIKE A WEASEL","")[[1]]
h=sample
s=h(f<-c(LETTERS," "),28,T)
c=0
while(!all(s==t)){for(i in 1:100){z=ifelse(runif(28)<.05,h(f,1),s)
y=sum(s==t)
if(sum(z==t)>y)s=z}
cat(c<-c+1,": ",s," -- score: ",y,"\n",sep="")}

Gives:

1: HSSSIMJM ETJISGBSCIELUYPLSED -- score: 7
2: HSSSIMJM ETJISGBSKIELUYPLSED -- score: 8
3: EETLITLM ETJISTBSKIELUYLLSEL -- score: 11

...

78: METHINKS IT IS LIKEEA WEASEL -- score: 27
79: METHINKS IT IS LIKEEA WEASEL -- score: 27
80: METHINKS IT IS LIKEEA WEASEL -- score: 27
81: METHINKS IT IS LIKE A WEASEL -- score: 28

Answer 7

JavaScript, 277 246

c=m=>" ABCDEFGHIJKLMNOPQRSTUVWXYZ"[0|Math.random()*m];for(s=[k=28];e=k;s[--k]=c(27));for(;alert(k+++": "+s.join("")+" -- score: "+e),e<28;s=t)for(z=100;f=0,z--;f>e&&(t=n,e=f))n=s.map((h,p)=>(h=c(540)||h,f+=h=="METHINKS IT IS LIKE A WEASEL"[p],h))

(requires arrow function support; indentation added only for readability)

// c() returns a random char using `m` as an index max
c=m=>" ABCDEFGHIJKLMNOPQRSTUVWXYZ"[0|Math.random()*m];

// generate base string `s`
for(s=[k=28];e=k;s[--k]=c(27));

// while score `e` is < 28
for(;alert(k+++": "+s.join("")+" -- score: "+e),e<28;s=t)
    for(z=100;f=0,z--;f>e&&(t=n,e=f))            // do 100 mutations; keep best score
        n=s.map((h,p)=>(                         // map `s` to `n` with 5% mutation
            h=c(540)||h,                         // change the char in 5% of cases
            f+=h=="METHINKS IT IS LIKE A WEASEL"[p],  // score++ if char matches
            h                                    // arrow function return character
        ))

Feel free to change alert to console.log if you want a more pleasant execution experience.

There are some nifty golfy bits in here:

• The function c returns a random character from the alphabet string " ABC...". The function takes an argument to use as an upper bound for the random index selection. When generating the base string, we use 27, so the function behaves normally.

  However, we abuse this behavior by asking for a random upper bound of 540 in h = c(540) || h. Only 5% of the time will c actually return a string (because 540 * .05 = 27); the other 95% of the time, the randomly-chosen index falls beyond the length of the string, so the function returns undefined. This falsey value causes a logical-OR cascade in c(540) || h, so the original map value h is used (i.e., no replacement occurs).

• The score-summing operation does f+=h=="METHINKS IT IS LIKE A WEASEL"[p], which says "add true to f if the current map character h matches the p^th character of the WEASEL string". The number-plus-boolean addition coerces the boolean result to either 0 or 1, which means that f is incremented only when there is a match against the target WEASEL string.

Answer 8

C 256

char c[101][29],s,n,z,b,j,i,w;g;main(){for(;w<28;printf("%d: %s -- score: %d\n",g++,c[b=n],w))for(i=w=0;i<101;i++)for(s=j=0;j<28&&!(i==b&&g);j++)(s+=(c[i][j]=g&&rand()%20?c[b][j]:(z=rand()%27)?'A'+z-1:' ')=="METHINKS IT IS LIKE A WEASEL"[j])>w?n=i,w=s:0;}

Simple three loops, initialization, generation of new strings from parent and score calculated by the same statement. It's not very readable even with indentation.

C 252

i,g,n,b,o,s,w,z;char c[2929];main(){for(;(o=i%29)|i|w<28;(i=(i+1)%2929)||printf("%d: %s -- score: %d\n",g++,&c[b=n],w))(s+=o>27?-s:((i-o!=b||!g)&&(c[i]=g&&rand()%20?c[b+o]:(z=rand()%27)?'A'+z-1:' ')=="METHINKS IT IS LIKE A WEASEL"[o]))>w?n=i-o,w=s:0;}

One loop, with one array holding all 101 strings.

This second version breaks the rules because it prints the string from (the equivalent of) step 1, but it was either that or not print the last string. I'm stumped how to fix it without exploding in size. I'm posting it anyway for inspiration.

C 256

struct{char d[29];}p,t,n;i,j=-1,z,s,w,g;main(){for(;w<28;j>1&&printf("%d: %s -- score: %d\n",g++,(p=n).d,w))for(;j++%100;p=j?p:t)for(s=0,i=28;i--;)(s+=(t.d[i]=j&&rand()%20?p.d[i]:(z=rand()%27)?'A'+z-1:' ')=="METHINKS IT IS LIKE A WEASEL"[i])>w?n=t,w=s:0;}

Different approach, instead of making an array to hold 101 strings just regenerate the string 100 times and use struct assignment for easy copying. Initialization is done by starting the "repeat 100 times" counter at -1 and handling it carefully by strategically chosen post-increment. Despite a very different approach it ends up exactly the same as the first attempt - 256 characters.

Answer 9

C# - 436

namespace System.Linq{class W{static void Main(){var r=new Random();
Func<char>c=()=>(char)(r.Next(33,60)%59+32);var s="";
while(s.Length<28)s+=c();var a="METHINKS IT IS LIKE A WEASEL";int b=0;
while (s!=a){int m=-1;var f=s;for(int i=0;i<100;i++){
var l=string.Join("",s.Select(j=>(r.Next(20)!=0?j:c()).ToString()));
int o=Enumerable.Range(0,28).Sum(j=>l[j]==a[j]?1:0);if(o>m){f=l;m=o;}}
Console.WriteLine(b+++": "+(s=f)+" -- score: "+m);}}}}

Answer 10

Scala, 347 341 337 chars:

import util.Random.{nextInt=>r}
val t="METHINKS IT IS LIKE A WEASEL"
def c="ABCDEFGHIJKLMNOPQRSTUVWXYZ "(r(27))
def s(a:String)=t.zip(a).map{x=>if(x._1==x._2) 1 else 0}.sum
def w(a:String,i:Int=0){println(f"$i%2d: $a -- score: ${s(a)}")
if(s(a)!=28){w((0 to 99).map{_=>a.map(o=>if(r(20)<1) c else o)}.sortBy(s).last,i+1)}}
w(t.map(_=>c))

=>

 0: PGSHWAEPALQFTCORUKANPNUTRVXH -- score: 2
 1: PGSHWAEPALQ TCOQUKANPNUTRVXH -- score: 3
...
47: METHINKS WT IS LIKE A WEASEL -- score: 27
48: METHINKS IT IS LIKE A WEASEL -- score: 28

Answer 11

Lua 5.1 (502)

The minimised version:

s,t,b,c,i,q,a,d,f="ABCDFGHJYUEGKSHNCOLPQIEJUSNC","METHINKS IT IS LIKE A WEASEL",1,math.random,table.insert,1,string.sub,100,28 while q~=f do r,p={},{} for x=1,d do i(r,s) i(p,0) e="" for o=1,f do if c(1,20)==1 then if c(1,27)==1 then e=e.." " else e=e..string.char(c(65,90)) end else e=e..a(r[x],o,o) end end r[x]=e for y=1,f do if a(r[x],y,y)==a(t,y,y) then p[x]=p[x]+1 end end if p[x]==f then s=r[x] end end for x=1,d do if p[x]>=q then s,q=r[x],p[x] end end print(b..":",s,"-- score: "..q) b=b+1 end

and the easier to read version (with comments!):

s,t,b,c,i,q,a,d,f="ABCDFGHJYUEGKSHNCOLPQIEJUSNC","METHINKS IT IS LIKE A WEASEL",1,math.random,table.insert,1,string.sub,100,28
--s=random string, t=target, b=counter, c=reference to math.random, i=reference to table.insert, q=top score,a=reference to string.sub, d=constant (100), f=constant (28)
while q~=f do
    r,p={},{}
    for x=1,d do  --add 100 copies to the table of strings
        i(r,s)
        i(p,0)
        e=""
        for o=1,f do  --for each character in string
            if c(1,20)==1 then  -- 5% chance
                if c(1,27)==1 then e=e.." " else e=e..string.char(c(65,90)) end  --set it to an ASCII char between 65 and 90 (A-Z) or a space character
            else
                e=e..a(r[x],o,o)
            end
        end
        r[x]=e  --current string = mutations
        for y=1,f do
            if a(r[x],y,y)==a(t,y,y) then p[x]=p[x]+1 end
        end  --for each char increment score if it is correct
        if p[x]==f then
            s=r[x]
        end  --if 28 then final string is this!
    end
    for x=1,d do
        if p[x]>=q then s,q=r[x],p[x] end  --if this is the highest score so far, then make the string equal to this
    end
    print(b..":",s,"-- score: "..q)  --print it!
    b=b+1  --add one to the counter!
end

To be honest even though this definitely won't win, I was just glad to find and minimise a reasonably short solution for this problem! (emphasis on reasonably) :p

Answer 12

SAS - 374

%macro r;ranuni(7)%mend;%macro s;r=int(%r*27);substr(x,t,1)=byte(ifn(r,64+r,32));%mend;%macro y;char(y,t)=char(x,t)%mend;options nonotes nosource;data x;length x$28;do t=1to 28;%s;end;y="METHINKS IT IS LIKE A WEASEL";do z=1by 1;o=x;do i=1to 100;c=0;x=o;do t=1to 28;if %r<=.05then do;%s;end;c+%y;end;if c>m then do;m=c;v=x;end;end;x=v;put z":" x"-- score:" m;if m<28;end;run;

->

1 :GUUVLNUSILSRZLRBXVVCWXX HXKC -- score:2
2 :MUUVLNUSILSRZLRBXVMCWXX HXKC -- score:3
3 :MUUVLNESILSRILRBXVMCWXX HXKC -- score:4
4 :MEUVLNESILSRIRRBXVMCWXX HXKC -- score:5
....
95 :METHINKS IT IS LIKE A XEASEL -- score:27
96 :METHINKS IT IS LIKE A XEASEL -- score:27
97 :METHINKS IT IS LIKE A XEASEL -- score:27
98 :METHINKS IT IS LIKE A WEASEL -- score:28

With linebreaks/indent/comments:

%macro r;
 ranuni(7)   /* seed 0 will make new each time (seed=clock), otherwise fixed results */
%mend;
%macro s;  /* does the rand char, used both to initialize and replace; */
 r=int(%r*27); 
 substr(x,t,1)=byte(ifn(r,64+r,32)); *r=0 becomes space otherwise upper char;
%mend;
%macro y;  /*compares using new to 9.2 CHAR function which is equivalent to substr(str,start,1) */
 char(y,t)=char(x,t)
%mend;
options nonotes nosource; /*cheapest way to get clean log */
data x;
 length x$28; /*annoyingly necessary*/
 do t=1to 28;%s;end; /*initialize string*/
 y="METHINKS IT IS LIKE A WEASEL"; /*compare string */
 do z=1by 1; /*start iterating */
  o=x; /*save this iteration's string */
  do i=1to 100;
   c=0; /*score for this iteration*/
   x=o; /*string to fudge about start out clean, reusing x so no params to macro*/
   do t=1to 28;
    if %r<=.05then do;%s;end; /*if 5% then change the char out */
    c+%y; /*score that character*/
   end;
   if c>m then do; /*if on better scoring line, save it */
    m=c;
    v=x;
   end;
  end;
  x=v; *for next iter - this is cheaper than other options involving o=v due to first iter;
  put z":" x"-- score:" m;
  if m<28; *quit at 28;
 end;
run;

Answer 13

C 361 331

Not as good as Art's solution, but here's my (newbie) attempt at a C solution. 361 characters if you remove newlines and tabs.

char*w="METHINKS IT IS LIKE A WEASEL";char b[101][29];t,s,n,i,j,x,a;main(){for(;i<28;i++)b[0][i]=w[rand()%28];while(s<28){for(j=1;j<101;j++){x=0;for(i=0;i<28;i++){if(!(rand()%20))b[j][i]=w[rand()%28];else b[j][i]=b[0][i];if(b[j][i]==w[i])x++;}if(x>s){s=x;t=j;}}printf("%d: %s -- score %d\n",n++,b[t],s);for(;i>=0;--i){a=b[0][i];b[0][i]=b[t][i];b[t][i]=a;}t=0;}}

Edit: Got rid of the nested loop and used a 1D array. Was hoping it would make a bigger difference, but it only saved me 30 characters. Here's the code:

char*w="METHINKS IT IS LIKE A WEASEL";char b[2929];t,s,n,i,x;main(){for(;i<28;i++)b[i]=w[rand()%28];while(s<28){for(;i<2929;i++){if((i+1)%29){if(!(i%29))x=0;b[i]=rand()%20?b[i%29]:w[rand()%28]; x+=b[i]==w[i%29];if(x>s){s=x;t=i/29;}}}for(i=0;i<29;i++){x=b[i+t*29];b[i+t*29]=b[i];b[i]=x;}printf("%d: %s -- score %d\n",n++,b,s);t=0;}}

Edit: This is the original, ungolfed code, for those who are interested in knowing how the "golfing" was done. The code produces no warnings when compiled with GCC with -Wall and C99 enabled. Maybe you're a golfing newbie like me, or a C newbie like me, or maybe you're just curious. :) https://gist.github.com/cpx/97edbce4db3cb30c306a

Answer 14

Japt v2.0a0, 112 108 bytes

ª(T=Si26õdI q¹ö28
_¬í¥`Ú0ˆks Š ‰ ¦ke a Øâel`u q)x
(OpW+`: {U} -- sÖ: `+(K=[U]xV¹WÄ
K<28©ßLÆ®p513 iT ö}ÃñV o

Try it online!

-4 bytes thanks to @ETHproductions.

Unpacked & How it works

U||(T=Si26õdI q) ö28  Initialize primary input
U||                   If U is not initialized...
        26õdI           Generate uppercase alphabets
              q         Convert to string
      Si                Add space
   (T=         )        Assign to variable T
                 ö28    Sample 28 random chars from T and form a string
                        Implicitly assign to U

_q í==`Ú0ˆks Š ‰ ¦ke a Øâel`u q)x  Match counting function
_                                  Declare a function...
 q í==                         )     Convert to array of chars and pair with the next,
                                     and map with equality...
      `Ú0ˆks Š ‰ ¦ke a Øâel`u q        "methinks it is like a weasel" to uppercase
                                        split into chars
                                x    Sum (true == 1, false == 0)
                                   Implicitly assign to V

(OpW+`: {U} -- sÖ: `+(K=[U]xV) W+1  Output and increment counter
(Op                           )      Output with newline...
   W+`: {U} -- sÖ: `+                 `{W}: {U} -- score: `
                         [U]xV         Call V on [U] and force cast to number
                      (K=     )        Assign to K
                                W+1  Add 1 to W and implicitly assign to W

K<28&&ßLo@Um_p513 iT ö}} ñV o  Termination check and recursion
K<28&&                         If the match count is less than 28...
      ß                          Recurse the program with...
          Um_                      Map over chars of U...
             p513 iT                 The char repeated 513 times plus T
                     ö}              Sample a char from it
       Lo@             }           Generate array of 100 of the above
                         ñV o      Sort by V, pop the largest, pass it as U

Answer 15

Japt -R, 94 bytes

A different approach to but with a little inspiration from Bubbler's solution.

;C±S ö28
ȶ`Ú0ks   ¦ke a Øâel`gY
@=#dÆ£20ö ?X:CöÃÃñ_¬xVÃo)ʶG}a@Np[X+':Uu '-²`sÖ:`G=U¬xV]¸

Test it (or Try It Online)

---

Explanation

Line 1

Result gets assigned to variable U.

;C±S ö28
;C           :The lower case alphabet
  ±S         :Append a space and reassign result to C
     ö28     :Generate a string of 28 random characters

Line 2

Result gets assigned to variable V.

ȶ`Ú...l`gY
È               :A function that takes 2 arguments; a string (X) and an integer (Y)
  `Ú...l`       :  The compressed string "methinks it is like a weasel"
         gY     :  Get the character at index Y
 ¶              :  Test for equality with X

Line 3

The result of this line is implicitly joined with newlines and output.

@=#dÆ£20ö ?X:CöÃÃñ_¬xVÃo)Ê¥G}a@Np[X+':Uu '-²`sÖ:`G=U¬xV]¸
@                           }a@                            :Repeat until true
                                 [                     ]   :Build an array containing ...
                                  X+':                     :  A colon appended to the number of the current iteration
                                      Uu                   :  The current value of U uppercased
                                         '-²               :  A hyphen repeated twice
                                            `sÖ:`          :  The compressed string "score: "
                                                   U¬      :  Split U to an array of characters
                                                      V    :   Pass each character X at index Y through function V
                                                     x     :   Reduce by addition
                                                 G=        :   Assign the result to variable G
                                                        ¸  :Join with spaces
                               Np                          :Push to N (initially an empty array)
  #d                                                       :100
    Æ                                                      :Generate the range [0,100) and pass each through a function
     £                                                     :  Map over each character X in U
      20ö                                                  :    Generate a random number in the range [0,20), which has a 5% chance of being 0 (falsey)
          ?X                                               :    If thruthy, return X
            :Cö                                            :    Else return a random character from C
               Ã                                           :  End mapping
                Ã                                          :End function
                 ñ_                                        :Sort by passing each through a function
                   ¬                                       :  Split to an array of characters
                     V                                     :  Pass each character X at index Y through function V
                    x                                      :  Reduce by addition
                      Ã                                    :End sorting
                       o                                   :Pop the last element
 =                      )                                  :Reassign to U
                         Ê                                 :Length
                          ¶G                               :Equal to G

Answer 16

PHP 442

<? function r(){$n=rand(65,91);if($n==91) return ' ';else return chr($n);}function s($s){$c=0;$t='METHINKS IT IS LIKE A WEASEL';for($i=0;$i<28;$i++) if($s[$i]==$t[$i]) $c++;return $c;}function m($s){for($i=0;$i<28;$i++) if(rand(0,99)<5) $s[$i]=r();return $s;}$s='';for($i=0;$i<28;$i++) $s.=r();for($i=0;;$i++){$l=s($s);printf("%2d: %s -- score: %d\n",$i,$s,$l);if($l==28) break;$x=$s;for($j=0;$j<100;$j++){$t=m($s);if(s($t)>$l) $x=$t;}$s=$x;}

Readbly:

<?
//random char
function r(){
    $n=rand(65,91);
    if($n==91) return ' ';
    else return chr($n);
}
//score
function s($s){
    $c=0;
    $t='METHINKS IT IS LIKE A WEASEL';
    for($i=0;$i<28;$i++)
        if($s[$i]==$t[$i]) $c++;
    return $c;
}
//mutate
function m($s){
    for($i=0;$i<28;$i++)
    if(rand(0,99)<5) $s[$i]=r();
    return $s;
}
$s='';
for($i=0;$i<28;$i++) $s.=r();
for($i=0;;$i++){
    $l=s($s);
    printf("%2d: %s -- score: %d\n",$i,$s,$l);
    if($l==28) break;
    $x=$s;
    for($j=0;$j<100;$j++){
        $t=m($s);
        if(s($t)>$l) $x=$t;
    }
    $s=$x;
}

Answer 17

Python (330 321)

def b(i,s):print i,':',''.join(s),'-- score:',p(s)
from random import*;a=" ABCDEFGHIJKLMNOPQRSTUVWXYZ";i,s,t=0,choice(a)*28,"METHINKS IT IS LIKE A WEASEL";p=lambda n:sum(n[c]==t[c]for c in range(28))
while p(s)<28:b(i,s);s=sorted([[(c,choice(a))[random()<.05]for c in s]for k in[1]*100],key=lambda n:p(n))[-1];i+=1
b(i,s)

Readable version:

def b(i,s):
    print i,':',''.join(s),'-- score:',p(s)

import random as r
a=" ABCDEFGHIJKLMNOPQRSTUVWXYZ"
i,s,t=0,r.choice(a)*28,"METHINKS IT IS LIKE A WEASEL"
p=lambda n:sum(1for c in range(28)if n[c]==t[c])
while p(s)<28:
    b(i,s)
    s=sorted([[(c,r.choice(a))[r.random()<.05]for c in s]for k in[1]*100],key=lambda n:p(n))[-1];i+=1
b(i,s)

Example output:

0 : SSSSSSSSSSSSSSSSSSSSSSSSSSSS -- score: 3
1 : SSSQSSSSSSSSSSSSISSSSSSSSSSS -- score: 4
2 : SSSQISSSSSSSSSSSISSSSSSSSSSS -- score: 5
3 : SSSQISSSSSSSSSSSIKSSSSSSSSSS -- score: 6
4 : SMSQISSSSSSSISSSIKSSSSGSSSSS -- score: 7
...
53 : METHINKS IT IS UIKE A WEASEL -- score: 27
54 : METHINKS IT IS UIKE A WEASEL -- score: 27
55 : METHINKS IT IS LIKE A WEASEL -- score: 28

edit: removed a few characters based on AMKs and Timtechs answer

Answer 18

Perl 5, 219 bytes

$_="METHINKS IT IS LIKE A WEASEL";sub r{(A..Z,$")[rand 27]};sub t{~~grep/$t[$-++%28]/,pop=~/./g}$}.=r for@t=/./g;printf"%3d: %s -- score: %d
",$i++,(($})=sort{t($b)<=>t$a}map s/./rand>.05?$&:r/ger,($})x100),t$}until/$}/

Try it online!

Answer 19

Ruby - 225 202 203 198 chars

Ruby seems under-represented in this challenge so far so I thought I would give it a try! Improvements welcome.

g=-1
s=[]
puts"#{g+=1}: #{$.,s=(0..99).map{n=(r=0..27).map{|i|x=[' ',*?A..?Z].sample;rand<0.05?x:s[i]||=x};[r.count{|i|n[i]=='METHINKS IT IS LIKE A WEASEL'[i]},n*'']}.max;s} -- score: #$."until$.>27

Answer 20

Java (632)

class C {public static void main(String[] a){String b="AAAAAAAAAAAAAAAAAAAAAAAAAAAA";for(int i=1;;i++){String c=w(b);int s=s(c);if(s==28)break;if(s(b)<s){b=c;System.out.println(i+": "+c+" -- score: "+s);}}}public static String w(String b) {StringBuffer c = new StringBuffer(b);int max = 0;for (int i=0;i<100;i++){for(int j=0;j<28;j++)if(Math.random()<.06){double d=Math.random();c.setCharAt(j,(char)(d==1?32:d*26+65));}String d=c.toString();int s=s(d);if(s>max){max=s;b=d;}}return b;}public static int s(String s){String b="METHINKS IT IS LIKE A WEASEL";int sum=0;for(int j=0;j<28;j++)sum+=s.charAt(j)==b.charAt(j)?1:0;return sum;}}

Java is such a verbose language.. :(

Answer 21

PHP (381 397 323 319 312):

<? function s(&$s,&$i=0){$t='METHINKS IT IS LIKE A WEASEL';$i=0;$c=$s;$f=28;while($f--){$n=rand(0,26);$i+=($s[$f]=($c=='_'||!rand(0,19)?chr($n?$n+64:32):$s[$f]))==$t[$f];}}$s='_';s($s);$x=$y=0;do{$f=100;while($f--){$m=$s;s($m,$i);if($i>$y){$y=$i;$l=$m;}}printf("%2d: %s -- score: $y\n",$x++,$s=$l);}while($y<28);

Readable version:

<?
function s(&$s, &$i = 0) {
    $t = 'METHINKS IT IS LIKE A WEASEL';
    $i = 0;
    $c = $s;
    $f = 28; while ($f--) {
        $n = rand(0, 26);
        $i += ($s[$f] = ($c == '_' || !rand(0, 19) ? chr($n ? $n + 64 : 32) : $s[$f])) == $t[$f];
    }
}

$s = '_';
s($s);
$x = $y = 0;

do {
    $f = 100; while ($f--) {
        $m = $s;
        s($m, $i);

        if ($i > $y) {
            $y = $i;
            $l = $m;
        }
    }

    printf("%2d: %s -- score: $y\n", $x++, $s = $l);
} while ($y < 28);

Optimization credits (319):

• @GigaWatt in this comment
• @Einacio for !rand(0,19) taken from here

Optimization credits (312):

• @Einacio's comments

Answer 22

Ruby, 218

g,s,p,t=-1,'',1;while t!=28;t,b=-1;100.times{|i|m,n='',0
28.times{|j|n+=1if(m[j]=(rand<p ?[*?A..?Z,' '].sample: s[j]))=="METHINKS IT IS LIKE A WEASEL"[j]}
b,t=m,n if n>t};puts"#{g+=1}: #{s=b} -- score: #{t}";p=0.05;end

example run

0: LRAZZMKL IKUOGEHLKPWEVNEAZWX -- score: 6
1: LRAZZMKL IKUIGEALKMWEVNEAZWX -- score: 7
2: HRAGZMKL IKUIGEALKMWEVNEAZWX -- score: 7
3: HVAGZMKL IKUISAALKYWEVNEAZWX -- score: 8
                  ...
48: METHIUKS IT IS LIKEIA WEASEL -- score: 26
49: METHINKS IT IS LIKEIA WEASEL -- score: 27
50: METHINKS IT IS LIKEIA WEASEL -- score: 27
51: METHINKS IT IS LIKE A WEASEL -- score: 28

Answer 23

Ruby, 206 200 199

q,i,*R=*-2..27
puts"#{i+=1}: #{$.,s=(-2..q).map{x=R.map{|j|!s||rand<0.05?[*?A..?Z,' '].sample: s[j]};[R.count{|i|x[i]=='METHINKS IT IS LIKE A WEASEL'[i]},x]}.max;q=97;s.join} -- score: #$."until$.>27

The first line is simply a fancy way to define q=-2, i=-1, and R=(0..27).to_a. All the work is done in the 2nd line:

puts"..."until$.>27 # Prints the string in quotes until we reach the desired score
     ^
     |
 +---+
 |
"#{i+=1}: #{...} -- score: #$."
   ^        ^               ^  
   +--------|---------------|-- Generation counter
 +----------+---------------|-- Current string
 |                          +-- Score of current string (interpolates the `$.` variable)
 |   
 #{$.,s=(-2..q).map{...}.max;q=97;s.join} # Generate the score & string
   ^         ^  ^   ^    ^   ^    ^
   +---------|--|---|----|---|----|------ Store the score; this variable makes
             |  |   |    |   |    |       string interpolation shorter.
             +--|---|----|---+----|------ `q` automatically takes care of generating
                |   |    |        |        the string vs randomizing the string.
                +---|----|--------|------  Make 100 (or 1 the first time) strings,
                    |    |        |        and compute their score.
                    |    +--------|------- Take the string with the max score.
 +------------------+             +------- `s` is stored as an array
 |
 x=R.map{...};[R.count{...},x] # Compute string and its score, store in array
   ^     ^    ^^       ^
   +-----|----|+-------|------ `R` is 0..27, we use the constant to save chars.
         |    +--------|------ `max` orders by first element, then second. We clearly want
         |             |       the highest score, so make the score first.
 +-------+-------------|------ Generate the string, store in `x`.
 |                     +------ Count the number of chars that overlap with 'METHINKS...'
 |                     |
 |                    {|i|x[i]=='METHINKS IT IS LIKE A WEASEL'[i]}
{|j|!s||rand<0.05?[*?A..?Z,' '].sample: s[j]}
    ^   ^         ^             ^       ^
    +---+---------|-------------|-------|---- 5% chance of randomizing, or 100% for
                  |             |       |     first string.
                  +-------------+-------|---- Sample from alphabet + ' '.
                                        +---- Don't alter the string 95% of the time

Answer 24

Ruby - 410

#!/usr/bin/env ruby
C,RC=[32]+(65..90).to_a,->{C[rand(27)].chr}
T,CO,CH,OU,s,sc,a,aa,z,TR="METHINKS IT IS LIKE A WEASEL",->x{a=0;(0...28).each{|z|a+=1 if x[z]==T[z]};a},->{a[aa.rindex(sc)]},->x,y{print x;print " Score: ";puts y},(0...28).map{RC[]}.join,0,[0],[0],0,->{rand(20)==0}
until sc==28
a=[s]*100;aa=[0]*100;(0...100).each{|x|(0...28).each{|y|a[x][y]=RC[] if TR[]};z=CO[a[x]];aa[x]=CO[a[x]];OU[a[x],z]};sc=aa.max;s=CH[] end

Edit* It's currently failing (for some reason a[any] is being set to 0 (type=>fixnum)). However, the actual design is right, I just need to find the bug causing this to happen (it's very mysterious)

Answer 25

PHP 429 426 421 415

<? function t(){$a="ABCDEFGHIJKLMNOPQRSTUVWXYZ ";return $a{rand(0,26)};}$c='';$j=$g=0;$i=28;while($i--)$c.=t();function r($s){$i=28;while($i--)!rand(0,19)&&$s{$i}=t();return $s;}function s($s,&$r){$c="METHINKS IT IS LIKE A WEASEL";$i=28;$r=0;while($i--)$r+=$s{$i}==$c{$i};}while($g<28){$n='';$v=0;$i=100;while($i--){s($t=r($c),$a);($a>$v)&&($v=$a)&($n=$t);}($v>$g)&&($g=$v)&($c=$n);echo $j++.": $c -- score: $g\n";}

pretty print

<?php 
function t(){
    $a="ABCDEFGHIJKLMNOPQRSTUVWXYZ ";
    return $a{rand(0,26)};
}
$c='';
$j=$g=0;
$i=28;
while($i--)
    $c.=t();
function r($s){
    $i=28;
    while($i--)
        !rand(0,19)&&$s{$i}=t();
    return $s;
}
function s($s,&$r){
    $c="METHINKS IT IS LIKE A WEASEL";
    $i=28;
    $r=0;
    while($i--)
        $r+=+($s{$i}==$c{$i});
}
while($g<28){
    $n='';
    $v=0;
    $i=100;
    while($i--){
        s($t=r($c),$a);
        ($a>$v)&&($v=$a)&($n=$t);
    }
    ($v>$g)&&($g=$v)&($c=$n);
    echo $j++.": $c -- score: $g\n";
}

I'll need a less verbose language next time

Answer 26

Python 284

from random import*
C=choice
A=map(chr,range(65,91)+[32])
s=[C(A)for i in[0]*28]
N=x=0
while N!=28:N,s=max((len([i for i,j in zip(X,"METHINKS IT IS LIKE A WEASEL")if i==j]),X)for X in[[c if random()<.95 else C(A)for c in s]for i in[0]*100]);print`x`+":",''.join(s),"-- score:",N;x+=1

Answer 27

GolfScript 207

{91,{64>},32+''+27rand=}:g;'METHINKS IT IS LIKE A WEASEL':s;{`{\.@\=s@==}+s,,\%{+}*}:c;0:k;{k):k\.': '@@c' -- score: '\++++n+}:f;[{g}]28*{~}%{s=!}{{`100,\{\;{100rand 5>{ }{;g}if}%''+}+}~%{c}$)\;{ }%}/{f}/s f

Below is a slightly unpacked version of the above GolfScript with explanation. Since I don't think it could beat the APL or some other answers, I didn't bother to truly minify it. I think that with inlining variable declarations, eliminating unnecessary variables, and other such tricks, this code could achieve approximately 190 characters without really changing the algorithm. I think about about 10-15 characters could be removed if I better sorted out the conversion between arrays of ASCII values and strings.

#g is a function that returns the ASCII value of a random character or space. 
#The ASCII values for capital letters are 65-90, and 32 is space.
{91,{64>},32+''+27rand=}:g; 

#s is the string of interest.
'METHINKS IT IS LIKE A WEASEL':s;

#c is a function that returns the 'score' of a given string
#(the number of correct characters in the correct place).
{`{\.@\=s@==}+s,,\%{+}*}:c;

#t is a function that transforms a given string according to 
#the specification (by replacing characters with a random character 5% of the times).
{{100rand 5>{ }{;g}if}%''+}:t;

#i is the initial random string.
[{g}]28*{~}%:i;

#Use '/' to unfold the initial value until the string is equal to the string of interest.
#Every loop, do the transformation 100 times, then sort by score c, and then take the top scoring string.
#Aggregate the results into an array a.
i{s=!}{{`100,\{\;t}+}~%{c}$)\;{ }%}/:a;

#Instantiate a counter variable k
0:k;

#f is the formatting function, that takes a string and formats it according to the spec.
{k):k\.': '@@c' -- score: '\++++n+}:f;

#Format every string in the array, and then format the string of interest
a{f}/
s f

Answer 28

JavaScript - 312

There is already a shorter JS solution above but it is using experimental pointer functions, so I thought I'd throw in another solution that is running in any JS environment:

for(r=Math.random,R=function(){return'METHINKS CODZAWFLBUGYQRXVJP'[~~(r()*27)]},_=[],_.$=n=0,D=function(s){for(c=[],c.$=i=0;i<28;){c[i]=s&&r()<.95?s[i]:R();_=(c.$+=c[i]=='METHINKS IT IS LIKE A WEASEL'[i++])>_.$?c:_};return c},D();_.$<28;){for(j=0;j++<1e2;)D(_);console.log(n+++': '+_.join('')+' -- score: '+_.$)}

Answer 29

Java: 557 534

enum W{W;public static void main(String[]a){char[]c=new char[28],h,d[];int i,k,e,s=W.s(c);for(i=0;i<28;i++)c[i]=W.r();for(i=0;;){W.p(i++,h=c,s);if(s>27)break;d=new char[100][28];for(char[]b:d){for(k=0;k<28;k++)b[k]=Math.random()<.05?W.r():h[k];if((e=W.s(b))>s){s=e;c=b;}}}}int s(char[]c){int s=0,k;for(k=0;k<28;k++)if(c[k]=="METHINKS IT IS LIKE A WEASEL".charAt(k))s++;return s;}void p(int i,char[]c,int s){System.out.println(i+": "+new String(c)+" -- score: "+s);}char r(){int i=(int)(Math.random()*27);return(char)(i==26?32:i+65);}}

Unwrapped:

enum W {
    W;

    public static void main(String[] a) {
        char[] c = new char[28], h, d[];

        int i, k, e, s = W.s(c);

        for(i = 0; i < 28; i++)
            c[i] = W.r();

        for(i = 0;;) {
            W.p(i++, h = c, s);

            if(s > 27)
                break;

            d = new char[100][28];

            for(char[] b : d) {
                for(k = 0; k < 28; k++)
                    b[k] = Math.random() < .05 ? W.r() : h[k];

                if((e = W.s(b)) > s) {
                    s = e;
                    c = b;
                }
            }
        }
    }

    int s(char[] c) {
        int s = 0, k;
        for(k = 0; k < 28; k++)
            if(c[k] == "METHINKS IT IS LIKE A WEASEL".charAt(k))
                s++;

        return s;
    }

    void p(int i, char[] c, int s) {
        System.out.println(i + ": " + new String(c) + " -- score: " + s);
    }

    char r() {
        int i = (int)(Math.random() * 27);
        return (char)(i == 26 ? 32 : i + 65);
    }
}

Answer 30

Japt, 98 bytes

;ªBiS ö28
=#dÆ£20ö ?X:BiS)öÃÃñL=_¬xȶYg`Ú0ks   ¦ke a Øâel`u
mL o
=o
Op[T°+':X'-²`sÖ`W]¸
W<28©ß

Not a winner, but I really wanted to answer this fun challenge.

Test it

Answer 31

Python 2.7 - 319 bytes

Sure it's not the smallest, but it was fun to program.

from random import*
a=map(chr,range(65,91))+[' ']
c,i,y=choice,0,{}
s=[c(a)for i in[0]*28]
while 1:
 for j in[0]*100:v=[c(a)if .05>random()else x for x in s];y[sum(1if'METHINKS IT IS LIKE A WEASEL'[k]==v[k]else 0for k in range(28))]=v
 v=max(y.keys());s=y[v];print"%d: %s -- score: %d"%(i,''.join(y[v]),v);i+=1
 if v==28:break

Uses a recursing function, so it may reach the maximum recursion depth if there is some kind of weird devolution with the string.

ubuntu@ubuntu-OptiPlex-980:~$ python weasel.py
0: VPBHBSPWFTOG HAXSESCDNFPKWYE -- score: 1
1: VABHBSPWFTOG HAXSESCDNWPKWYE -- score: 2
2: VAWHBSPWFTOGIHAXSESSDNWPKWYE -- score: 3
3: VAWHBSPWFTOGIHAXSEFSGNWPKWYL -- score: 4
4: XAWHBSPYFTOGIHAXSEFSGNWPKWYL -- score: 4
5: XAWHBSKYFTOGIHAXSEFSGNWPKWYL -- score: 5
6: XAWHBSKYFTOGIHAXSEF GNWPKWYL -- score: 6
7: XAWHBSKYFTOGIHALSEF ANWPKWYL -- score: 8
8: XAKHBSKYFTTGIHALSEY ANWPKWYL -- score: 9
9: XAKHISKYFTTGIHALSEE ANWPKWYL -- score: 11
10: XAKHISKSFTTGIHALSEE ANWPKWYL -- score: 12
11: XAKHISKSFTTGIHALSBE ANWPKWKL -- score: 12
12: XAQHISKSFRT IHALSBE ANWPKWKL -- score: 13
13: XAQHISKSFIT IHALSBE ANWPKWKL -- score: 14
14: XAQHISKSFIT IHALSBE ANWPKWKL -- score: 14
15: XAQHISKSFIT IHALSBE ANWPKWKL -- score: 14
16: XALHISKSFIT ISALSBE ANWPKWKL -- score: 15
17: JALHISKSFIT ISALSBE ANWPAWKL -- score: 16
18: JALHISKSFIT ISALIBE ANWPAWKL -- score: 17
19: JALHISKSFIT ISALIBE ANWPAWKL -- score: 17
20: JALHISKSFIT ISALIBE ANWPAWKL -- score: 17
21: JALHISKSFIT ISALIKE ANWPAWYL -- score: 18
22: JALHISKSFIT IS LIKE ANWPAWYL -- score: 19
23: JALHISKSFIT IS LIKE ANWEAWYL -- score: 20
24: JALHISKSFIT IS LIKE ANWEAWYL -- score: 20
25: JALHISKSFIT IS LIKE ANWEAWZL -- score: 20
26: JALHISKS IT IS LIKE ANWEAAZL -- score: 21
27: JACHISKS IT IS LIKE ANWEASZL -- score: 22
28: JACHISKS IT IS LIKE ANWEASZL -- score: 22
29: MACHISKS IT IS LIKE ANWEASZL -- score: 23
30: MACHISKS IT IS LIKE ANWEASZL -- score: 23
31: MACHISKS IT IS LIKE AUWEASZL -- score: 23
32: MACHISKS IT IS LIKE AUWEASZL -- score: 23
33: MACHISKS IT IS LIKE AJWEASZL -- score: 23
34: MACHISKS IT IS LIKE A WEASZL -- score: 24
35: MACHISKS IT IS LIKE A WEASZL -- score: 24
36: MACHINKS IT IS LIKE A WEASZL -- score: 25
37: MACHINKS IT IS LIKE A WEASZL -- score: 25
38: MACHINKS IT IS LIKE A WEASZL -- score: 25
39: MBCHINKS IT IS LIKE A WEASZL -- score: 25
40: MBCHINKS IT IS LIKE A WEASZL -- score: 25
41: MBCHINKS IT IS LIKE A WEASZL -- score: 25
42: MBCHINKS IT IS LIKE A WEASZL -- score: 25
43: MBCHINKS IT IS LIKE A WEASZL -- score: 25
44: MBCHINKS IT IS LIKE A WEASZL -- score: 25
45: MECHINKS IT IS LIKE A WEASCL -- score: 26
46: MECHINKS IT IS LIKE A WEASCL -- score: 26
47: MECHINKS IT IS LIKE A WEASCL -- score: 26
48: MECHINKS IT IS LIKE A WEASCL -- score: 26
49: MECHINKS IT IS LIKE A WEASCL -- score: 26
50: MECHINKS IT IS LIKE A WEASCL -- score: 26
51: MEQHINKS IT IS LIKE A WEASCL -- score: 26
52: MEQHINKS IT IS LIKE A WEASCL -- score: 26
53: MEQHINKS IT IS LIKE A WEASCL -- score: 26
54: MEQHINKS IT IS LIKE A WEASCL -- score: 26
55: MEQHINKS IT IS LIKE A WEASCL -- score: 26
56: MEQHINKS IT IS LIKE A WEASCL -- score: 26
57: METHINKS IT IS LIKE A WEASCL -- score: 27
58: METHINKS IT IS LIKE A WEASCL -- score: 27
59: METHINKS IT IS LIKE A WEASCL -- score: 27
60: METHINKS IT IS LIKE A WEASCL -- score: 27
61: METHINKS IT IS LIKE A WEASCL -- score: 27
62: METHINKS IT IS LIKE A WEASCL -- score: 27
63: METHINKS IT IS LIKE A WEASCL -- score: 27
64: METHINKS IT IS LIKE A WEASCL -- score: 27
65: METHINKS IT IS LIKE A WEASEL -- score: 28

Massive thanks to Sp3000 for golfing help.

Answer 32

Julia, 281 bytes

Golfed:

r=n->['A':'Z',' '][rand(1:27,n)]
s=a->sum(a.=="METHINKS IT IS LIKE A WEASEL".data)
p=(z,a,s)->println(z,": ",join(a)," -- score: ",s)
a = r(28)
b = s(a)
z = 0
p(z,a,b)
while b<28
c=b
d=a
for i=1:100
e=[rand()<.95?i:r(1)[1]for i=a]
f=s(e)
if(f>c)
c=f
d=e
end
end
a=d
b=c
p(z,a,b)
z+=1
end

The algorithm itself isn't very clever, but there are some cool bits in here. Combining a character range with another character, then indexing into it: ['A':'Z',' '][rand(1:27,n)] and taking the sum of an array of booleans (common, but I still love the idea): sum(a.=="METHINKS IT IS LIKE A WEASEL".data). Glad I got under 300!

Ungolfed:

randchar = n -> ['A':'Z',' '][rand(1:27,n)]
score = a -> sum(a.=="METHINKS IT IS LIKE A WEASEL".data)
myprint = (z,a,s) -> println(z,": ",join(a)," -- score: ",s)
currentarray = randchar(28)
currentscore = score(currentarray)
z = 0
myprint(z,currentarray,currentscore)
while currentscore < 28
    bestscore = currentscore
    bestarray = currentarray
    for i = 1:100
        temparray = [rand()<.95?i:randchar(1)[1]for i=currentarray]
        tempscore = score(temparray)
        if(tempscore > bestscore)
            bestscore = tempscore
            bestarray = temparray
        end
    end
    currentarray = bestarray
    currentscore = bestscore
    myprint(z,currentarray,currentscore)
    z+=1
end

Answer 33

JavaScript (Node.js), 328 bytes

t="METHINKS IT IS LIKE A WEASEL";z=0;m=Math.random;r=_=>(n=0|27*m())?String.fromCharCode(64+n):" ";for(s="";28>s[l="length"];s+=r())for(;28>z;){for(a=[s];100>a[l];){for(x="";28>x[l];)x+=.95<=m()?r():s.substr(x[l],1);a.push(x)}for(i=0;100>i;i++){for(j=b=0;28>j;j++)b+=a[i][j]==t[j];b>z&&(alert(a[i]+" -- score: "+b),z=b,s=a[i])}}

Try it online!

This is rather long and can be shortened, just haven't gotten around to that.

---

Explanation :

This is not exactly an explanation but it is better indented and can

t = "METHINKS IT IS LIKE A WEASEL";
z = 0;
m = Math.random;
r = _ => (n = 0 | 27 * m()) ? String.fromCharCode(64 + n) : " ";
for (s = ""; 28 > s[l = "length"]; s += r())
 for (; 28 > z;) {
  for (a = [s]; 100 > a[l];) {
   for (x = ""; 28 > x[l];) x += .95 <= m() ? r() : s.substr(x[l], 1);
   a.push(x)
  }
  for (i = 0; 100 > i; i++) {
   for (j = b = 0; 28 > j; j++) b += a[i][j] == t[j];
   b > z && (alert(a[i] + " -- score: " + b), z = b, s = a[i])
  }
 }

Answer 34

Pyth, 116 bytes

Ls.bqNY"METHINKS IT IS LIKE A WEASEL"bj.e%"%d: %s -- score: %s"[ksbyb).W<yeH28+Z]eoyNmmO.[O+r1G\ k20eZ100]mO+r1G\ 28

Try it online.

Ls.bqNY"..."bj.e%"..."[ksbyb).W<yeH28+Z]eoyNmmO.[O+r1G\ k20eZ100]mO+r1G\ 28   

L                                                                             Define fitness function, y(b):
       "..."                                                                    The target string
  .b        b                                                                   Map over characters of the above and b:
    qNY                                                                           Are they equal? True = 1, False = 0
 s                                                                              Take the sum
                                                                    r1G       The uppercase alphabet
                                                                   +   \      Append a space
                                                                  O           Choose one at random
                                                                ]m       28   Create an array of 28 of the above, wrap in array
                             .W                                               Functional while, with above as initial value
                                                                                Continue condition (current value = H):
                                 eH                                               Last value of H
                                y                                                 Apply fitness function to the above
                               <   28                                             Check the above is less than 28
                                                                                Function body (current value = Z)
                                             m             eZ                     Map each character (as k) in the last value in Z over:
                                                 O+r1G\                             Choose random char/space (as above)
                                               .[       k20                         Pad the above to length 20 using k
                                              O                                     Choose one of the above at random
                                            m                100                  Do the above 100 times
                                         oyN                                      Order the above by the fitness function
                                       ]e                                         Take the last of the above, wrap in array
                                     +Z                                           Append to Z
                                                                              Return value of while is array of all steps taken
              .e                                                              Map each entry with its index over:
                       k                                                        Current index
                        sb                                                      Current entry joined on empty string
                          yb                                                    Fitness of current entry
                      [     )                                                   Wrap the above 3 results in an array
                %"..."                                                          String formatting using the output string
             j                                                                Join on newlines, implicit output

Answer 35

Groovy: 314

def(r,o,g)=[new Random(),{(0..27).sum{x->"METHINKS IT IS LIKE A WEASEL"[x]==it[x]?1:0}},0]
def q={([' ']+('A'..'Z'))[r.nextInt(27)]}
def s=q()*28
for(;;){println"${g++}: $s -- score: ${o(s)}"
if(o(s)==28)break
s=([s]*100).collect{it.collect{5>r.nextInt(100)?q():it}.sum()}.groupBy{o(it)}.max{it.key}.value[0]}

Output:

0:                              -- score: 5
1:          B               S   -- score: 6
2:  E       B               S M -- score: 7
3:  E       B               S M -- score: 7
4:  E       BT      X       S M -- score: 8
...
68: METHINKS IT IS LIKE L WEASEL -- score: 27
69: METHINKS IT IS LIKE L WEASEL -- score: 27
70: METHINKS IT IS LIKE L WEASEL -- score: 27
71: METHINKS IT IS LIKE L WEASEL -- score: 27
72: METHINKS IT IS LIKE A WEASEL -- score: 28

I feel like this is cheating so I'm not counting it as my 'real' answer; but I can save a few chars by passing "METHINKS IT IS LIKE A WEASEL" and " -- score: " as arguments to my script. Although technically it is 'externalizing strings'... which everyone knows is good practice :).

Groovy: 290

def(r,o,g)=[new Random(),{(0..27).sum{x->args[0][x]==it[x]?1:0}},0]
def q={([' ']+('A'..'Z'))[r.nextInt(27)]}
def s=q()*28
for(;;){println"${g++}: $s${args[1]}${o(s)}"
if(o(s)==28)break
s=([s]*100).collect{it.collect{5>r.nextInt(100)?q():it}.sum()}.groupBy{o(it)}.max{it.key}.value[0]}

Run like groovy script.groovy "METHINKS IT IS LIKE A WEASEL" " -- score: "

Answer 36

Golfscript (168 167)

:x;0['METHINKS IT IS LIKE A WEASEL'{}/]:o{;{27rand.!96*+64^}:r~}%{o=!}{100,{;.{20rand{}{;r}if}%}%{{[o\]zip{~=},,}:s~}$)@;\;x 2$+': '+1$+' -- score: '+1$s+n+:x;\)\}/;;x

Sadly, I can't seem to compress it quite to APL's level, but it's afaict currently a shared second place.

This is what it does;

# Grab the empty parameter string as initially empty output string.
:x;
# Start count at row #0
0
# Store the target string as an array in 'o'
['METHINKS IT IS LIKE A WEASEL'{}/]:o
# Create a random starting string, and define 'r' as a function generating a random char.
{;{27rand.!96*+64^}:r~}%
# Unfold as long as the latest string is different from the target string.
{o=!}
{
  // Generate 100 strings, sort by score and keep the highest.
  100,{;.{20rand{}{;r}if}%}%{{[o\]zip{~=},,}:s~}$)@;\;
  // Append the row to the output string, and increase the row number    
  x 2$+': '+1$+' -- score: '+1$s+n+:x;\)\
}/
// Clean some junk on the stack
;; 
// Output string
x

Answer 37

R, 291 characters

Longer than the previously proposed R answer but significantly different to be (I think) of interest:

a=sample;b=a(c<-c(LETTERS,' '),28,T);d=0;h=function(x)sum(x==strsplit("METHINKS IT IS LIKE A WEASEL",'')[[1]]);while(h(b)!=28){if(d){f=replicate(100,{e=runif(28)>.05;S=a(c,28,T);S[e]=b[e];S});g=f[,which.max(apply(f,2,h))];if(h(g)>h(b))b=g};cat(d,': ',b,' -- score: ',h(b),'\n',sep='');d=d+1}

With indentations:

a=sample
b=a(c<-c(LETTERS,' '),28,T)
d=0
h=function(x)sum(x==strsplit("METHINKS IT IS LIKE A WEASEL",'')[[1]])
while(h(b)!=28){
    if(d){
        f=replicate(100,{e=runif(28)>.05;S=a(c,28,T);S[e]=b[e];S})
        g=f[,which.max(apply(f,2,h))]
        if(h(g)>h(b))b=g
        }
    cat(d,': ',b,' -- score: ',h(b),'\n',sep='')
    d=d+1
    }

Outputs:

0: OUZ IT HNGNMSKIHSMCQUGXTOYVZ -- score: 1
1: OUZ IT HNGNMSKYHSMCQUGWXOYVZ -- score: 2
2: OUZ IT HNGNMSKYLSMCQUGWXOYVZ -- score: 3
3: OUZ IT HNGNMSKYLSMC UGWXOYVZ -- score: 4
...
68: METHINKS IT ES LIKE A WEASEL -- score: 27
69: METHINKS IT ES LIKE A WEASEL -- score: 27
70: METHINKS IT IS LIKE A WEASEL -- score: 28

The key here is the line f=replicate(100,{e=runif(28)>.05;S=a(c,28,T);S[e]=b[e];S}):
e=runif(28)>.05 draws randomly 28 numbers between 0 and 1 on a uniform distribution, and gives TRUE if over 5% and FALSE if under 5%. S=a(c,28,T) draws 28 new letters/spaces, while S[e]=b[e] replaces those for which the value was over 5% by the corresponding letter from the previous generation (i .e. those for which the value was less than 5% are therefore new).Then we replicate the operation 100 times. On the following line (g=f[,which.max(apply(f,2,h))]) we compute the score for each of the 100 replicates and keep the one that has the maximum score (in case of equality the first one is kept) to be compared with the previous generation.

Answer 38

C 331

Another C solution that comes out to 331 characters. I think I might be able to improve it if I look a bit closer. Anyway the solution follows:

char l[29],b[29],*a="ABCDEFGHIJKLMNOPQRSTUVWXYZ ",i,j,*t="METHINKS IT IS LIKE A WEASEL",s,m=0;main(){for(i=0;i<28;i++)l[i]=a[rand()%27];while(strcpy(l,b))for(i=0;i<100;i++){for(j=s=0;j<28;j++){if(rand()%20==0)l[j]=a[rand()%27];if(l[j]==t[j])s++;}printf("%s -- score: %d\n",l,s);if(s>m){m=s;strcpy(b,l);}if(!strcmp(b,t))goto e;}e:;}

Answer 39

Haskell - 420 characters

This is more verbose than most; I'd put that down to my inexperience with randomness in haskell and proper use of monads.

import System.Random
r=randomRIO
z=zipWith
c=fmap((' ':['A'..'Z'])!!)$r(0,26)
y=(i.).replicate
i=mapM id
s=sum.z((fromEnum.).(==))"METHINKS IT IS LIKE A WEASEL"
u a n|n==0=c|True=return a
b n a|s n>s a=n|True=a
g=fmap(foldr1 b).y 100.mapM((r(0,20)>>=).u)
e x=do n<-g$x!!0;if(s$x!!0)==28 then return(n:x)else e(n:x)
main=y 28 c>>=(\a->e[a])>>=(i.z(\n x->putStrLn$shows n$": "++x++" -- score: "++(show$s x))[0..]).reverse

Answer 40

Javascript (411 389 376)

Shorter than some, longer than most.. [Methinks it will end.] EDIT: it wasn't ending due to an error. fixed it and ended up dropping the count too -- cool.

t='METHINKS IT IS LIKE A WEASEL';z=0;m=Math.random;r=function(){n=Math.floor(m()*27);return n?String.fromCharCode(64+n):' '};for(s='';s.length<28;s+=r());while(z<28){a=[s];while(a.length<100){x='';while(x.length<28){x+=m()>=.95?r():s.substr(x.length,1)}a.push(x)}for(i=0;i<100;i++){b=0;for(j=0;j<28;j++){b+=a[i][j]==t[j]?1:0;}if(b>z){alert(a[i]+' -- score: '+b);z=b;s=a[i]}}};

expanded:

t = 'METHINKS IT IS LIKE A WEASEL';
z = 0;
m = Math.random;
r = function(){
    n = Math.floor(m()*27);
    return n ? String.fromCharCode(64+n) : ' '
};
for(s='';s.length<28;s+=r());
while( z < 28 ){
    a = [s];
    while( a.length < 100 ){
        x = '';
        while( x.length < 28 ){
            x += m() >= .95 ? r() : s.substr(x.length, 1)
        }
        a.push(x)
    }
    for( i = 0; i < 100; i++ ){
        b = 0;
        for( j = 0; j < 28; j++ ){
            b += a[i][j] == t[j] ? 1 : 0;
        }
        if( b > z ){
            console.log( a[i] + ' -- score: ' + b);
            z = b;
            s = a[i]
        }
    }
};

---

[original:]

t='METHINKS IT IS LIKE A WEASEL';s='';z=0;m=Math.random;r=function(){n=Math.floor(m()*27);return n?String.fromCharCode(64+n):' '};c=function(){a=[s];while(a.length<100){x='';while(x.length<28){x+=m()>=.95?r():s.substr(x.length?x.length-1:0,1)}a.push(x)}for(i=0;i<100;i++){b=0;for(j=0;j<28;j++){b+=a[i][j]==t[j]?1:0;}if(b>z){alert(a[i]+' -- score: '+b);z=b;s=a[i]}i++}};while(s.length<28){s+=r()}while(z<28){c()}

Answer 41

Racket: 426 422 bytes

(let*((r random)(m map)(L" ABCDEFGHIJKLMONPQRSTUVWXYZ")(P(string->list"METHINKS IT IS LIKE A WEASEL"))(c(λ(l p)(m(λ(x)(if(<(r)p)(string-ref L(r 27))x))l)))(s(λ(l P p)(cons(foldl(λ(x y a)(if(eqv? x y)(+ a 1)a))0 l P)l))))(let r((g 0)(o (c P 1)))(let*((o(sort(m(λ(x)(s(c o 0.05)P 0))(range 100))(λ g(apply >(m car g)))))(p(caar o))(c(cdar o)))(printf"~a: ~a -- score: ~a
"g(list->string c)p)(if(= p 28)p(r(+ g 1)c)))))

Ungolfed:

(let* ((r random)
       (m map)
       (L " ABCDEFGHIJKLMONPQRSTUVWXYZ")
       (P (string->list "METHINKS IT IS LIKE A WEASEL"))
       (c (λ (l p) 
            (m (λ (x)
                 (if (< (r) p)
                     (string-ref L (r 27))
                     x))
               l)))
       (s (λ (l P p)
            (cons (foldl (λ (x y a)
                           (if (eqv? x y)
                               (+ a 1)
                               a))
                         0 
                         l 
                         P)
                  l))))
  (let r ((g 0) (o (c P 1)))
    (let* ((o (sort (m (λ (x)
                         (s(c o 0.05)P 0))
                       (range 100))
                    (λ g
                      (apply >(m car g)))))
           (p (caar o))
           (c (cdar o)))
      (printf"~a: ~a -- score: ~a\n" g (list->string c) p)
      (if (= p 28)
          p
          (r (+ g 1) c)))))

Answer 42

C++ 668

Not really short, but the only one in c++. (:

#include "stdafx.h"
using namespace std;int _tmain(){srand(time(NULL));string z="METHINKS IT IS LIKE A WEASEL";string a(28,' ');for(int i=0;i<28;++i){a[i]='A'+(rand()%26);}while(1){vector<std::pair<int,string>>d;for(int i=0;i<100;++i){d.push_back(std::pair<int,string>(0,""));for(int j=0;j<28;++j){int b=rand()%100;if(b<6){if(b<2){d[i].second+=' ';}else{d[i].second+='A'+(rand()%26);}}else {d[i].second+=a[j];}}}for(int i=0;i<100;++i){for(int j=0;j<28;++j){if(d[i].second[j]==z[j]){d[i].first++;}}}int y=0;for(int i=0;i<100;++i){if(d[i].first==28){cout<<d[i].second<<" "<<d[i].first;exit(1);}else if(d[i].first>y){y=d[i].first;a=d[i].second;}}cout<<a<<" "<<y<<endl;}}

more readable version:

#include "stdafx.h"
#include <string>
#include <stdlib.h>
#include <vector>
#include <iostream>
#include <time.h>
using namespace std;


int _tmain()
{
  srand(time(NULL));
  string z = "METHINKS IT IS LIKE A WEASEL";
  string a(28, ' ');
  for (int i = 0; i < 28; ++i)
  {
    a[i] = 'A' + (rand() % 26);
  }
  while(1)
  {
    vector<std::pair<int, string >> d;
    for (int i = 0; i < 100; ++i)
    {
      d.push_back(std::pair<int, string>(0, ""));
      for (int j = 0; j < 28; ++j)
      {
        int b = rand() % 100;
        if (b < 6)
        {
          if (b < 2)
          {
            d[i].second += ' ';
          }
          else {
            d[i].second += 'A' + (rand() % 26);
          }
        }
        else {
          d[i].second += a[j];
        }
      }
    }
    for (int i = 0; i < 100; ++i)
    {
      for (int j = 0; j < 28; ++j)
      {
        if (d[i].second[j] == z[j])
        {
          d[i].first++;
        }
      }
    }
    int y = 0;
    for (int i = 0; i < 100; ++i)
    {
      if (d[i].first == 28)
      {
        cout << d[i].second << " " << d[i].first;
        exit(1);
      }
      else if (d[i].first > y)
      {
        y = d[i].first;
        a = d[i].second;
      }
    }
    cout << a << " " << y << endl;
  }
}

Output:

VEBVTEWX CEWMYLTAJTYPFYYXBDQ 2
 EBVTEWX CEWMYLTAJT PFYYXBDQ 3
 EBVTEKX CEWMYLTAJT PFYYXBDQ 4
 EBVTEKX CEWM  TAJT PFYYXBDQ 5
 EBVTEKX CE M  TAJT P YYX DQ 7
MEPVTEKX CE M  TAJT P YY  D  8
MEPVT KX CE M  TAJT A YY  D  9
MEPVI KX CE M  UAJT A YY  D  10
MEPSI KX CE M  UAJT A YY  D  10
MEPSI K  CE M  UAJE A YY  DE 11
MEPSI K  CE M  UAJE A YY  DL 12
MEPPI K  CE I  UDJE A YY  DL 13
MEPRI K  CE IS UDJE A YY  DL 14
MEPRI K  CE IS UDJE A  Y  DL 14
MEPHI K  CE IS UDJE A  Y  DL 15
MEPHI K  CE IS UDJE A  Y  DL 15
MEPHI K  CE IS RDBE A  E  DL 16
MEPHIQK  CE IS RD E A  E  DL 16
MEPHIQK  CE IS RD E A  E  DL 16
ME HIQK  CE IS RDXE A  E  DL 16
ME HIQK  CE IS W XE A  E  DL 16
ME HIQK  CE IS W XE A  E  EL 17
ME HIQK  CE IS W XE A  E  EL 17
ME HIQK  CE IS   XE A  E  EL 17
MEGHIQK  CE IS   XE A WEO EL 18
MEGHIQK  CE IS   XE A WEO EL 18
MEGHIQK  CE IS   XE A WEO EL 18
MEGHIQK  CE IS   XE A WE  EL 18
MEGHIQK  CE IS   XE A WE SEL 19
MEGHIQK  CE IS L XE A WE SEL 20
MEGHINK  CE IS L XE A WE SEL 21
MEGHINK  CE IS L XE A WE SEL 21
MEGHINK  CN IS L XE A WEASEL 22
MEGHINK   H IS L XE A WEASEL 22
MEGHINK   H IS L XE A WEASEL 22
METHINK   H IS L XE A WEASEL 23
METHINK   H IS L XE A WEASEL 23
METHINK   H IS L XE A WEASEL 23
METHINK   H IS L XE A WEASEL 23
METHINK   H IS L XE A WEASEL 23
METHINK   H IS L XE A WEASEL 23
METHINK  ZH IS L XE A WEASEL 23
METHINK  ZH IS LIXE A WEASEL 24
METHINK  ZH IS LIKE A WEASEL 25
METHINKD ZH IS LIKE A WEASEL 25
METHINKD ZH IS LIKE A WEASEL 25
METHINKD ZH IS LIKE A WEASEL 25
METHINKD ZH IS LIKE A WEASEL 25
METHINKD ZT IS LIKE A WEASEL 26
METHINKD ZT IS LIKE A WEASEL 26
METHINKD ZT IS LIKE A WEASEL 26
METHINKS ZT IS LIKE A WEASEL 27
METHINKS IT IS LIKE A WEASEL 28