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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:

```my @alpha = split //, "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
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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