# Random.3o - Man Page

Pseudo-random number generators (PRNG).

## Module

Module Random

## Documentation

Module **Random**

: **sig end**

Pseudo-random number generators (PRNG).

With multiple domains, each domain has its own generator that evolves independently of the generators of other domains. When a domain is created, its generator is initialized by splitting the state of the generator associated with the parent domain.

In contrast, all threads within a domain share the same domain-local generator. Independent generators can be created with the **Random.split** function and used with the functions from the **Random.State** module.

**Before5.0** Random value generation used a different algorithm. This affects all the functions in this module which return random values.

### Basic functions

*val init* : **int -> unit**

Initialize the domain-local generator, using the argument as a seed. The same seed will always yield the same sequence of numbers.

*val full_init* : **int array -> unit**

Same as **Random.init** but takes more data as seed.

*val self_init* : **unit -> unit**

Initialize the domain-local generator with a random seed chosen in a system-dependent way. If **/dev/urandom** is available on the host machine, it is used to provide a highly random initial seed. Otherwise, a less random seed is computed from system parameters (current time, process IDs, domain-local state).

*val bits* : **unit -> int**

Return 30 random bits in a nonnegative integer.

*val int* : **int -> int**

**Random.int bound** returns a random integer between 0 (inclusive) and **bound** (exclusive). **bound** must be greater than 0 and less than 2^30.

**Raises Invalid_argument** if **bound** <= 0 or **bound** >= 2^30.

*val full_int* : **int -> int**

**Random.full_int bound** returns a random integer between 0 (inclusive) and **bound** (exclusive). **bound** may be any positive integer.

If **bound** is less than 2^31, then **Random.full_int bound** yields identical output across systems with varying **int** sizes.

If **bound** is less than 2^30, then **Random.full_int bound** is equal to **Random.int bound** .

If **bound** is at least 2^30 (on 64-bit systems, or non-standard environments such as JavaScript), then **Random.full_int** returns a value whereas **Random.int** raises **Invalid_argument** .

**Since** 4.13

**Raises Invalid_argument** if **bound** <= 0.

*val int_in_range* : **min:int -> max:int -> int**

**Random.int_in_range ~min ~max** returns a random integer between **min** (inclusive) and **max** (inclusive). Both **min** and **max** are allowed to be negative; **min** must be less than or equal to **max** .

If both bounds fit in 32-bit signed integers (that is, if -2^31 <= **min** and **max** < 2^31), then **int_in_range** yields identical output across systems with varying **int** sizes.

**Since** 5.2

**Raises Invalid_argument** if **min > max** .

*val int32* : **Int32.t -> Int32.t**

**Random.int32 bound** returns a random integer between 0 (inclusive) and **bound** (exclusive). **bound** must be greater than 0.

**Raises Invalid_argument** if **bound** <= 0.

*val int32_in_range* : **min:int32 -> max:int32 -> int32**

**Random.int32_in_range ~min ~max** returns a random integer between **min** (inclusive) and **max** (inclusive). Both **min** and **max** are allowed to be negative; **min** must be less than or equal to **max** .

**Since** 5.2

**Raises Invalid_argument** if **min > max** .

*val nativeint* : **Nativeint.t -> Nativeint.t**

**Random.nativeint bound** returns a random integer between 0 (inclusive) and **bound** (exclusive). **bound** must be greater than 0.

**Raises Invalid_argument** if **bound** <= 0.

*val nativeint_in_range* : **min:nativeint -> max:nativeint -> nativeint**

**Random.nativeint_in_range ~min ~max** returns a random integer between **min** (inclusive) and **max** (inclusive). Both **min** and **max** are allowed to be negative; **min** must be less than or equal to **max** .

**Since** 5.2

**Raises Invalid_argument** if **min > max** .

*val int64* : **Int64.t -> Int64.t**

**Random.int64 bound** returns a random integer between 0 (inclusive) and **bound** (exclusive). **bound** must be greater than 0.

**Raises Invalid_argument** if **bound** <= 0.

*val int64_in_range* : **min:int64 -> max:int64 -> int64**

**Random.int64_in_range ~min ~max** returns a random integer between **min** (inclusive) and **max** (inclusive). Both **min** and **max** are allowed to be negative; **min** must be less than or equal to **max** .

**Since** 5.2

**Raises Invalid_argument** if **min > max** .

*val float* : **float -> float**

**Random.float bound** returns a random floating-point number between 0 and **bound** (inclusive). If **bound** is negative, the result is negative or zero. If **bound** is 0, the result is 0.

*val bool* : **unit -> bool**

**Random.bool ()** returns **true** or **false** with probability 0.5 each.

*val bits32* : **unit -> Int32.t**

**Random.bits32 ()** returns 32 random bits as an integer between **Int32.min_int** and **Int32.max_int** .

**Since** 4.14

*val bits64* : **unit -> Int64.t**

**Random.bits64 ()** returns 64 random bits as an integer between **Int64.min_int** and **Int64.max_int** .

**Since** 4.14

*val nativebits* : **unit -> Nativeint.t**

**Random.nativebits ()** returns 32 or 64 random bits (depending on the bit width of the platform) as an integer between **Nativeint.min_int** and **Nativeint.max_int** .

**Since** 4.14

### Advanced functions

The functions from module **Random.State** manipulate the current state of the random generator explicitly. This allows using one or several deterministic PRNGs, even in a multi-threaded program, without interference from other parts of the program.

*module State :* **sig end**

*val get_state* : **unit -> State.t**

**get_state()** returns a fresh copy of the current state of the domain-local generator (which is used by the basic functions).

*val set_state* : **State.t -> unit**

**set_state s** updates the current state of the domain-local generator (which is used by the basic functions) by copying the state **s** into it.

*val split* : **unit -> State.t**

Draw a fresh PRNG state from the current state of the domain-local generator used by the default functions. (The state of the domain-local generator is modified.) See **Random.State.split** .

**Since** 5.0