rand() not so random in perl

Question

I have a Perl script that gives me a 50 character string of random numbers, letters and some special characters. I am inputting them in a database. Now, given the length of the string and the amount of characters, I wouldn't think that duplicates would be easily created.

Here's the nifty nugget of code that creates the string:

my $random_id='';
my @c = ( "A" .. "Z", "a" .. "z", 0 .. 9, qw(! @ $ % ^ & *) );
$random_id = join '', map $c[rand @c] , 1 .. 50;

It produces strings like:

C1Qt8L7E7QUD%lkxnh9yjZ2njF0iMj!1o^4DmTbVNhQB9%dke@

The problem is it will duplicate an exact string every once and a while among unique ones, and more than once on some strings. And this is out of say 20 strings. It's bizarre. I can work around it and find a solution... but this perplexes me a bit. Would like to know why. Anybody have an idea?

Answer 1

You need to use srand to seed the random number generator otherwise it will generate the same number series.

http://perldoc.perl.org/functions/srand.html

Edit:

According to the doc atthe url, if the perl version is before 5.004 then it won't automatically call that function. So check the perl version you're running under.

Answer 2

you can also see String::Random - for generating random strings based on a pattern.