Module Stdlib.Random

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.

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 230.

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 231, then Random.full_int bound yields identical output across systems with varying int sizes.

If bound is less than 230, then Random.full_int bound is equal to Random.int bound.

If bound is at least 230 (on 64-bit systems, or non-standard environments such as JavaScript), then Random.full_int returns a value whereas Random.int raises Stdlib.Invalid_argument.

  • since 4.13
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 -231 <= min and max < 231), then int_in_range yields identical output across systems with varying int sizes.

  • since 5.2
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.

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
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.

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
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.

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
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 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