# random

## Pseudo random number generation

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 current 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 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.

It should be noted that this random number generator is not cryptographically strong. If a strong cryptographic random number generator is needed for example `crypto:rand_bytes/1` could be used instead.

The state.

### seed() -> ran()

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

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

• `A1 = A2 = A3 = integer()`

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

One way of obtaining a seed is to use the BIF `now/0`:

```          ...
{A1,A2,A3} = now(),
random:seed(A1, A2, A3),
...```

### seed(X1 :: {A1, A2, A3}) -> undefined | ran()

• `A1 = A2 = A3 = integer()`

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

### 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

• `N = integer() >= 1`

Given an integer `N >= 1`, `uniform/1` returns a random integer uniformly distributed between `1` and `N`, updating the state in the process dictionary.

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

• `State0 = State1 = ran()`

Given a state, `uniform_s/1`returns a random float uniformly distributed between `0.0` and `1.0`, and a new state.

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

• `N = integer() >= 1`
• `State0 = State1 = ran()`

Given an integer `N >= 1` and a state, `uniform_s/2` returns 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 R15. Upgrading to R15 will break applications that expect a specific output for a given seed. The output is still deterministic number series, but different compared to releases older than R15. The seed `{0,0,0}` will for example no longer produce a flawed series of only zeros.