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RE: Cryptogram Generator

by btrott (Parson)
on Feb 07, 2000 at 23:30 UTC ( #3006=note: print w/replies, xml ) Need Help??

in reply to Cryptogram Generator

A couple of comments:

1) the open || die statement should actually be

open IN, $infile or die "Can't open input file: $!";
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; }
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:

Now we set another array equal to this one, then call the random shuffle algorithm on it:
my @crypt = @alpha; fisher_yates_shuffle(\@crypt);
which shuffles the array in place. Now all you need to do is get back the substitution string:
my $substit = join '', @crypt;
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; }
(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)
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.

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. :)

      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; }

      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

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