# Generate random numbers in Perl with the rand() function

The built-in rand() function of perl returns a random fractional number between 0 and 1. 0 included but 1 not. Mathematicians would probably represent the range of possible numbers with something like this: [0, 1) though, because the numbers in Perl are limited to some 15 digits after the decimal point the actual numbers rand() will generated don't cover the whole range.

rand(N) will generated a random number between 0 and N (not including N): [0, N).

## Simple use of rand()

print rand(), "\n"; # 0.242888881432922 print rand(10), "\n"; # 5.320874671411310

In general this is enough for you to know, but if you are interested, you can read a bit deeper explanation. If not, scroll down a bit for more use-cases.

If you run the the above script several times you'll see that the number changes. If this is your first encounter with random numbers in computers this might not surprise you, after all we are talking about random numbers. On the other hand if you come from some other languages this might come to a surprise.

The thing is that in many programming languages, including Perl, the built-in random number generator do not generate truly random numbers. Instead of that they generate pseudo-random numbers with something like the Linear congruential generator.

The best to imagine is probably that there is a giant list of pre-defined numbers and every time we call rand() it will just return the next number. In some languages the built-in random generator starts from the beginning of this list every time a program starts. In order to let the list start from another place you can call the srand() function. So in other languages you might first need to call srand() once and then you can call rand() as many times as you want.

Perl however will call srand() for you every time a new script starts so normally you'll see different numbers every time a script starts.

Just to mess with your mind a bit, if you want to get **repeatable random numbers**,
you can call srand() yourself in your script with some number.

For example, this script, generates 0.744525000061007 on every run. Try it.

srand(42); print rand(), "\n";

One of the advantages of this is that you can have the same series of random numbers on every run which will make tests repeatable.

Anyway, let's get back to the daily use of rand()

As we saw we can just call rand() and it will return a floating-point number between 0 and 1. (not including 1)

print rand(), "\n"; # 0.242888881432922

If we would like to imitate throwing the dice we can use the following code:

**examples/rand_6.pl**

use strict; use warnings; use 5.010; say 1 + int rand(6);

This will generate a random whole number between 1 and 6 (both inclusive). In other words, it will pick one of the following numbers randomly: 1, 2, 3, 4, 5, 6.

Why?

rand(6) will generate a floating point number between 0 and 6 (6 excluded).

int rand(6) will return the integer part of it which can be 0, 1, 2, 3, 4, 5 (but cannot be 6).

1 + int rand(6) will return one more. One of the following numbers: 1, 2, 3, 4, 5, 6

## Not cryptographically secure

The official documentation of rand() in Perl
warns that
**"rand()" is not cryptographically secure. You should not rely
on it in security-sensitive situations.**
It also points to a number
of third-party alternatives from CPAN. Let's have a look at those.

## Crypt::Random

Crypt::Random calls itself **Cryptographically Secure, True Random Number Generator.**
it uses /dev/random or similar device supplied by the operating system.

The basic usage looks like this:

**examples/crypt_random.pl**

use strict; use warnings; use 5.010; use Crypt::Random qw( makerandom ); my $r = makerandom ( Size => 3, Strength => 1, Uniform => 1 ); say $r;

makerandom will return a whole number between 0 and 2**N (including 0 and excluding 2**N) where
N is set using the Size parameter. The **Strength** selects between /dev/random and /dev/urandom.
By default the highest bit of the returned number is always set to 1, which might be useful in some cases,
but I setting Uniform => 1 will return a value that will have the same randomness on every bit.

In other words if Uniform => 1 then Size is the number of random bits generated.

Size Range # possibilities 1 0-1 2 2 0-3 4 3 0-7 8 ... 10 0-1023 1024 ...

If Uniform => 0 which is the default then:

Size Range # possibilities 1 1 1 2 2-3 2 3 4-7 4 ... 10 512-1023 512 ...

Throwing a (6 sided) dice then would look like this:

int (6 * $r / (2**$N))

The full script is this:

**examples/crypt_random_6.pl**

use strict; use warnings; use 5.010; use Crypt::Random qw( makerandom ); my $N = 10; my $r = makerandom ( Size => $N, Strength => 1, Uniform => 1 ); my $dice = int (6 * $r / (2**$N)); say $dice;

An even better solution would be to use the makerandom_itv function that will return whole numbers in a given range: makerandom_itv(Lower => 1, Upper => 7); (Lower inclusive, Upper exclusive)

**examples/crypt_random_range.pl**

use strict; use warnings; use 5.010; use Crypt::Random qw( makerandom_itv ); my $N = 10; my $r = makerandom_itv ( Size => $N, Strength => 1, Uniform => 1, Lower => 1, Upper => 7 ); say $r;

## Data::Entropy

Data::Entropy for entropy (randomness) management. Looks way too complex for generating random numbers.

## Math::Random::Secure

Math::Random::Secure is a Cryptographically-secure, cross-platform replacement for rand().

After loading the module and importing the rand() function it works just like the built-in rand() function.

**examples/math_random_secure.pl**

use strict; use warnings; use 5.010; use Math::Random::Secure qw(rand); say 1 + int rand(6);

## Math::TrulyRandom

Math::TrulyRandom might be still working well, but it has not been released since 1996 so I'd avoid it.

Published on 2015-11-25