# random.3erl - Man Page

Pseudo-random number generation.

## Description

This module provides a random number generator. The method is attributed to B.A. Wichmann and I.D. Hill in 'An efficient and portable pseudo-random number generator', Journal of Applied Statistics. AS183. 1982. Also Byte March 1987.

The algorithm is a modification of the version attributed to Richard A. O'Keefe in the standard Prolog library.

Every time a random number is requested, a state is used to calculate it, and a new state is produced. The state can either be implicit (kept in the process dictionary) or be an explicit argument and return value. In this implementation, the state (the type *ran()*) consists of a tuple of three integers.

**Note:**

This random number generator is not cryptographically strong. If a strong cryptographic random number generator is needed, use one of functions in the *crypto* module, for example, *crypto:strong_rand_bytes/1*.

**Note:**

The improved *rand* module is to be used instead of this module.

## Data Types

ran()= {integer(), integer(), integer()}

The state.

## Exports

seed() -> ran()

Seeds random number generation with default (fixed) values in the process dictionary and returns the old state.

seed(SValue) -> undefined | ran()

- Types:
SValue = {A1, A2, A3} | integer()

A1 = A2 = A3 = integer()

*seed({A1, A2, A3})* is equivalent to *seed(A1, A2, A3)*.

seed(A1, A2, A3) -> undefined | ran()

- Types:
A1 = A2 = A3 = integer()

Seeds random number generation with integer values in the process dictionary and returns the old state.

The following is an easy way of obtaining a unique value to seed with:

random:seed(erlang:phash2([node()]), erlang:monotonic_time(), erlang:unique_integer())

For details, see *erlang:phash2/1*, *erlang:node/0*, *erlang:monotonic_time/0*, and *erlang:unique_integer/0*.

seed0() -> ran()

Returns the default state.

uniform() -> float()

Returns a random float uniformly distributed between *0.0* and *1.0*, updating the state in the process dictionary.

uniform(N) -> integer() >= 1

- Types:
N = integer() >= 1

Returns, for a specified integer *N >= 1*, a random integer uniformly distributed between *1* and *N*, updating the state in the process dictionary.

uniform_s(State0) -> {float(), State1}

- Types:
State0 = State1 = ran()

Returns, for a specified state, a random float uniformly distributed between *0.0* and *1.0*, and a new state.

uniform_s(N, State0) -> {integer(), State1}

- Types:
N = integer() >= 1

State0 = State1 = ran()

Returns, for a specified integer *N >= 1* and a state, a random integer uniformly distributed between *1* and *N*, and a new state.

## Note

Some of the functions use the process dictionary variable *random_seed* to remember the current seed.

If a process calls *uniform/0* or *uniform/1* without setting a seed first, *seed/0* is called automatically.

The implementation changed in Erlang/OTP R15. Upgrading to R15 breaks applications that expect a specific output for a specified seed. The output is still deterministic number series, but different compared to releases older than R15. Seed *{0,0,0}* does, for example, no longer produce a flawed series of only zeros.

## Referenced By

arc4random.3bsd(3), drand48(3), drand48_r(3), erl_syntax_lib.3erl(3), guestfs-release-notes-1.36(1), nbdkit-release-notes-1.10(1), rand(3), random_r(3), wipe(1).