RE: Cryptogram Generator

A couple of comments:

1) the open || die statement should actually be

open IN, $infile or die "Can't open input file: $!";
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For reasons of precedence.

2) Here are some faster algorithms. One way is to use a hash to record which characters you've already placed into your substitution string. Here's the main loop:

my %set;
my $substit = "";
for (1..26) {
    my $randchar;
    do { $randchar = chr((int rand 26) + 65) }
        while $set{$randchar}++;
    $substit .= $randchar;
}
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This way you don't have to do a search through the string each time.

And here's an even better way of doing it: build a random permutation of the alphabet string. To do this, we'll actually need an array:

my @alpha = split //, "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
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Now we set another array equal to this one, then call the random shuffle algorithm on it:

my @crypt = @alpha;
fisher_yates_shuffle(\@crypt);
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which shuffles the array in place. Now all you need to do is get back the substitution string:

my $substit = join '', @crypt;
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Here's the definition of the fisher_yates_shuffle sub:

sub fisher_yates_shuffle {
    my $array = shift;
    for (my $i = @$array; --$i; ) {
        my $j = int rand ($i+1);
        next if $i == $j;
        @$array[$i, $j] = @$array[$j, $i];
    }
    return join '', @$array;
}
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(Taken directly from perlfaq4.)

I did some benchmarking on these, and here's what I got:

Benchmark: timing 5000 iterations of orig, f_yates, hash...
      orig: 33 secs (23.60 usr  0.00 sys = 23.60 cpu)
   f_yates:  5 secs ( 2.88 usr  0.00 sys =  2.88 cpu)
      hash: 13 secs ( 5.68 usr  0.00 sys =  5.68 cpu)
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where "orig" is the one you posted, "f_yates" is the one using fisher_yates_shuffle, and "hash" is the one using a hash to record seen characters.

If I've messed anything up, let me know.

Comment on RE: Cryptogram Generator Select or Download Code

Replies are listed 'Best First'.
RE: RE: Cryptogram Generator by Elihu (Novice) on Feb 08, 2000 at 07:39 UTC
Wow. Thanks for the new ideas. I've been playing with the different algorithms a bit. I've seen somewhere how you do benchmarking, I'll try my own just to see for myself how it's done and if I get comparable numbers. I was pretty sure there was a better way to do that. :)	[reply]
Re^3: Cryptogram Generator by duggles (Acolyte) on Dec 11, 2007 at 00:20 UTC
I've been working on a cryptogram solver and discovered this post and found the code useful - I snagged the code for the fisher_yates_shuffle to grab one of my solved cryptograms to recreate one to work on solving and I found that in some instances one or two of the letters in $substit remained the same as it was originally. The line that checks for $i == $j does a next BUT $i keeps on counting down and effectively leaves the unchanged letter alone. I fixed it by incrementing $i before doing the next. I realize in some cases the shuffle results are okay if this happens, but in a cryptogram, it won't fly... ;-) Just thought I'd throw this info back out there just in case you or anyone else may have a need for the fix. Here's the modified sub: `## Taken from perlfaq4 (thanks btrott) sub fisher_yates_shuffle { my $array = shift; for (my $i = @$array; --$i; ) { my $j = int rand ($i+1); # next if $i == $j; # original if ($i == $j) { # this means the letter will be the same as it wa +s before $i++; # put $i back where it was and get another one next; } @$array[$i, $j] = @$array[$j, $i]; } return join '', @$array; }` [download] thanks for the original post! I'm learning a little here and there about using perl to solve cryptos... it's gonna take a while, but it's fun!!! Life is short, but it's wide -- Chuck Pyle	[reply] [d/l]


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