Developer Guide and Reference

ID 767251
Date 10/31/2024
Public
Document Table of Contents

RANDOM_NUMBER

Intrinsic Subroutine: Returns one pseudorandom number or an array of such numbers.

CALL RANDOM_NUMBER (harvest)

harvest

(Output) Must be of type real. It can be a scalar or an array variable. It is set to contain pseudorandom numbers from the uniform distribution within the range 0 <= x < 1.

The seed for the pseudorandom number generator used by RANDOM_NUMBER can be set or queried with RANDOM_SEED. If RANDOM_SEED is not used, the processor sets the seed for RANDOM_NUMBER to a processor-dependent value.

The RANDOM_NUMBER generator uses two separate congruential generators together to produce a period of approximately 10**18, and produces real pseudorandom results with a uniform distribution in [0, 1). It accepts two integer seeds, the first of which is reduced to the range [1, 2147483562]. The second seed is reduced to the range [1, 2147483398]. This means that the generator effectively uses two 31-bit seeds.

The RANDOM_NUMBER generator does not produce subnormal numbers.

For more information on the algorithm, see the following:

  • Communications of the ACM vol 31 num 6 June 1988, titled: Efficient and Portable Combined Random Number Generators by Pierre L'ecuyer.

  • Springer-Verlag New York, N. Y. 2nd ed. 1987, titled: A Guide to Simulation by Bratley, P., Fox, B. L., and Schrage, L. E.

NOTE:

This routine is thread safe.

Example

Consider the following:

REAL Y, Z (5, 5) ! Initialize Y with a pseudorandom number CALL RANDOM_NUMBER (HARVEST = Y) CALL RANDOM_NUMBER (Z)

Y and Z contain uniformly distributed random numbers.

The following shows another example:

REAL x, array1 (5, 5) CALL RANDOM_SEED() CALL RANDOM_NUMBER(x) CALL RANDOM_NUMBER(array1)

The following shows another example:

program testrand intrinsic random_seed, random_number integer size integer, allocatable :: seed(:), gseed(:), hiseed(:), zseed(:) real harvest(10) call random_seed(SIZE=size) print *,"size ",size allocate(seed(size),gseed(size),hiseed(size),zseed(size)) hiseed = -1 zseed = 0 seed = 123456789 seed(size) = 987654321 call random_seed(PUT=hiseed(1:size)) call random_seed(GET=gseed(1:size)) print *,"hiseed gseed", hiseed, gseed call random_seed(PUT=zseed(1:size)) call random_seed(GET=gseed(1:size)) print *,"zseed gseed ", zseed, gseed call random_seed(PUT=seed(1:size)) call random_seed(GET=gseed(1:size)) call random_number(HARVEST=harvest) print *, "seed gseed ", seed, gseed print *, "harvest" print *, harvest call random_seed(GET=gseed(1:size)) print *,"gseed after harvest ", gseed end program testrand