module Random


Random provides an interface for random values generation, using a pseudo random number generator (PRNG).

Random.new_seed # => 112705036
Random.rand     # => 0.167595
Random.rand(5)  # => 2

The above methods delegate to a Random instance.

r =
r.rand      # => 0.0372991
r.next_bool # => true
r.next_int  # => 2223112

This module also defines a global method #rand, which Array#sample and Array#shuffle delegates.

rand     # => 0.293829
rand(10) # => 8

An instance of each class that includes Random is a random number generator with its own state. Usually such RNGs can be initialized with a seed, which defines their initial state. It is guaranteed that after initializing two different instances with the same seed, and then executing the same calls on both of them, you will get the same results. This allows exactly reproducing the same seemingly random events by just keeping the seed.

It is possible to make a custom RNG by including Random and implementing #next_u to return an unsigned number of a pre-determined type (usually UInt32). The numbers generated by your RNG must be uniformly distributed in the whole range of possible values for that type (e.g. 0u32..UInt32::MAX). This allows all other methods of Random to be based on this and still produce uniformly distributed results. Your Random class should also have a way to initialize it. For pseudo-random number generators that will usually be an integer seed, but there are no rigid requirements for the initialization.

Direct including types

Defined in:


Constant Summary


Class Method Summary

Instance Method Summary

Class Method Detail

def = new_seed) #

Initializes an instance with the given seed. (Default: #new_seed)

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def self.new_seed : UInt32 #

Returns a UInt32 read from a counter value generated by the cycle counter register, or the current time on ARM processors.

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def self.rand(x) #

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def self.rand : Float64 #

see #rand

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Instance Method Detail

def next_bool : Bool #

Generates a random Bool. # => true

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def next_float : Float64 #

see #rand

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def next_int : Int32 #

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abstract def next_u : UInt #

Generates a random unsigned integer.

The integers must be uniformly distributed between 0 and the maximal value for the chosen type.

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def rand(max : Int) : Int #

Generates a random integer which is greater than or equal to 0 and less than max.

The return type always matches the supplied argument.   # => 5 # => 2243

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def rand(max : Float) : Float64 #

Returns a random Float64 which is greater than or equal to 0 and less than max.    # => 2.88938 # => 7.70147

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def rand(range : Range(Int, Int)) : Int #

Returns a random integer in the given range.

The return type always matches the supplied argument.                 # => 14 # => -5297435808626736140

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def rand(range : Range(Float, Float)) : Float64 #

Returns a random Float64 in the given range. # => 15.2989

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def rand : Float64 #

Generates a random Float64 between 0 and 1.

r =
r.rand # => 0.167595
r.rand # => 0.0372991

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