Code Golf: Conway's Game of Life

Question

Mathematica - 179 163 154 151 chars

    a = {2, 2, 2};
    s = Export["out.txt", 
       CellularAutomaton[{224, {2, {a, {2, 1, 2}, a}}, {1,1}}, 
                (ReadList[#1, Byte, RecordLists → 2>1] - 46)/ 42, #2]〚#2〛
       /. {0 → ".", 1 → "X"}, "Table"] &

Spaces added for readability

Invoke with

    s["c:\life.txt", 100]

Animation:

You can also get a graph of the mean population over time:

A nice pattern for generating gliders from Wikipedia

AFAIK Mathematica uses a Cellular Automaton to generate random numbers using Rule 30.

MATLAB 7.8.0 (R2009a) - 174 171 161 150 138 131 128 124 characters

Function syntax: (124 characters)

Here's the easier-to-read version (with unnecessary newlines and whitespace added for better formatting):

function l(f,N),
  b=char(importdata(f))>46;
  for c=1:N,
    b=~fix(filter2(ones(3),b)-b/2-3);
  end;
  dlmwrite('out.txt',char(b*42+46),'')

And here's how the program is run from the MATLAB Command Window:

l('in.txt',100)

Command syntax: (130 characters)

After a comment about calling functions with a command syntax, I dug a little deeper and found out that MATLAB functions can in fact be invoked with a command-line format (with some restrictions). You learn something new every day!

function l(f,N),
  b=char(importdata(f))>46;
  for c=1:eval(N),
    b=~fix(filter2(ones(3),b)-b/2-3);
  end;
  dlmwrite('out.txt',char(b*42+46),'')

And here's how the program is run from the MATLAB Command Window:

l in.txt 100

Additional Challenge: Tweetable GIF maker - 136 characters

I thought for fun I'd see if I could dump the output to a GIF file instead of a text file, while still keeping the character count below 140 (i.e. "tweetable"). Here's the nicely-formatted code:

function l(f,N),
  b=char(importdata(f))>46;
  k=ones(3);
  for c=1:N+1,
    a(:,:,:,c)=kron(b,k);
    b=~fix(filter2(k,b)-b/2-3);
  end;
  imwrite(~a,'out.gif')

Although IMWRITE is supposed to create a GIF that loops infinitely by default, my GIF is only looping once. Perhaps this is a bug that has been fixed in newer versions of MATLAB. So, to make the animation last longer and make the evolution steps easier to see, I left the frame delay at the default value (which seems to be around half a second). Here's the GIF output using the Gosper Glider Gun pattern:

---

Improvements

• Update 1: Changed the matrix b from a logical (i.e. "boolean") type to a numerical one to get rid of a few conversions.
• Update 2: Shortened the code for loading the file and used the function MAGIC as a trick to create the convolution kernel in fewer characters.
• Update 3: Simplified the indexing logic, replaced ~~b+0 with b/42, and replaced 'same' with 's' as an argument to CONV2 (and it surprisingly still worked!).
• Update 4: I guess I should have searched online first, since Loren from The MathWorks blogged about golfing and the Game of Life earlier this year. I incorporated some of the techniques discussed there, which required me to change b back to a logical matrix.
• Update 5: A comment from Aslak Grinsted on the above mentioned blog post suggests an even shorter algorithm for both the logic and performing the convolution (using the function FILTER2), so I "incorporated" (read "copied") his suggestions. ;)
• Update 6: Trimmed two characters from the initialization of b and reworked the logic in the loop to save 1 additional character.
• Update 7: Eric Sampson pointed out in an e-mail that I could replace cell2mat with char, saving 4 characters. Thanks Eric!

Ruby 1.9 - 189 178 159 155 153 chars

f,n=$*
c=IO.read f
n.to_i.times{i=0;c=c.chars.map{|v|i+=1
v<?.?v:('...X'+v)[[83,2,-79].map{|j|c[i-j,3]}.to_s.count ?X]||?.}*''}
File.new('out.txt',?w)<<c

Edit: Handles newlines with 4 chars less.
Can remove 7 more (v<?.?v:) if you allow it to clobber newlines when the live cells reach the edges.

perl, 127 129 135 chars

Managed to strip off a couple more characters...

$/=pop;@b=split'',<>;map{$n=-1;@b=map{++$n;/
/?$_:($t=grep/X/,@b[map{$n+$_,$n-$_}1,80..82])==3|$t+/X/==3?X:'.'}@b}1..$/;print@b

Python - 282 chars

might as well get the ball rolling...

import sys
_,I,N=sys.argv;R=range(3e3);B=open(I).read();B=set(k for k in R if'A'<B[k])
for k in R*int(N):
 if k<1:b,B=B,set()
 c=sum(len(set((k+o,k-o))&b)for o in(1,80,81,82))
 if(c==3)+(c==2)*(k in b):B.add(k)
open('out.txt','w').write(''.join('.X\n'[(k in B)-(k%81<1)]for k in R))