# random

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Module

random

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Module Summary

Pseudo-random number generation.

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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.

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Data Types

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

The state.

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Exports

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seed() -> ran()

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

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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).

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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.

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seed0() -> ran()

Returns the default state.

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uniform() -> float()

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

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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.

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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.

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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.

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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.