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Environment variable PERL_RAND_SEED in Perl v5.38

How many of you have generated random numbers in Perl?

I guess almost everyone.

Let me ask another question: What function do you use to generate random numbers in Perl?

Well, no prize for the correct answer.

The answer is rand. You all got it right.

Now for a trick question: How does rand generate a random number every time?

Do you think, there is another player in this game?

Once again, you guessed it right.

You’re a smart audience!

The partner in crime is srand.

So what is srand?

Well it’s a random seed generator.

Wait, what is this seed business now?

In computer science and mathematics, a seed is an initial value used
by a  pseudo-random number generator (PRNG) to produce a sequence of
numbers that appear random but are actually deterministic.

This definition is loaded but makes it very clear what it is.

Remember, computers cannot generate truly random numbers without external entropy.

So, if you use the same seed, the PRNG will produce the exact same sequence of random numbers.

Enough talk, let’s see some action.

As I mentioned earlier, we use rand to generate random numbers. Here’s how it looks:

$ perl -Mv5.14 -e "print rand"
0.449823787975223

$ perl -Mv5.14 -e "print rand"
0.0417291764906658

$ perl -Mv5.14 -e "print rand"
0.0634949901877953

So we’re getting random numbers every time. That means we’re using a different seed each time, right?

I’m curious to find out which seed was used.

$ perl -Mv5.14 -e "print rand; print '---'; print srand;"
0.927663794792355---3368917688

$ perl -Mv5.14 -e "print rand; print '---'; print srand;"
0.894359557963075---3759042874

$ perl -Mv5.14 -e "print rand; print '---'; print srand;"
0.482739364569174---1623001244

Pretty cool, right?

By the way, the ability for srand to return the seed was added in Perl v5.14.

That was quite a while ago, not many of you might remember that, I guess.

In my opinion, it’s one of the best feature added to Perl.

Want to see how?

Check this out: random.pl

use v5.14;

print "Random number : ", rand, "\n";
my $auto_seed = srand;
print "Used Auto Seed: $auto_seed\n";
print "Random number : ", rand, "\n";
# Reseed with the same seed used above
srand($auto_seed);
print "Random number : ", rand, "\n";

Run the code now:

$ perl random.pl
Random number : 0.97473596602595
Used Auto Seed: 3333813029
Random number : 0.483446575076417
Random number : 0.483446575076417

In Perl v5.38, we can set the seed value with the new environment variable: PERL_RAND_SEED

First without the use of environment variable:

$ perl -Mv5.38 -E 'say rand'
0.796259729963825

$ perl -Mv5.38 -E 'say rand'
0.539082965188758

Now with environment variable:

$ PERL_RAND_SEED=1 perl -Mv5.38 -E 'say rand'
0.0416303447718782

$ PERL_RAND_SEED=1 perl -Mv5.38 -E 'say rand'
0.0416303447718782

Now no need to get your hand dirty, you got all the power in your hand.

Happy Hacking!!