Manual Reference Pages - random_number (3fortran)

NAME

RANDOM_NUMBER(3) - [MATHEMATICS:RANDOM] Pseudo-random number

SYNOPSIS

call random_number(harvest)

         subroutine random_number(harvest)

          real,intent(out) :: harvest(..)

CHARACTERISTICS

o HARVEST and the result are default real variables

DESCRIPTION

RANDOM_NUMBER(3) returns a single pseudorandom number or an array of pseudorandom numbers from the uniform distribution over the range 0 <= x < 1.

OPTIONS

o HARVEST : Shall be a scalar or an array of type real.

EXAMPLES

Sample program:

    program demo_random_number
    use, intrinsic :: iso_fortran_env, only : dp=>real64
    implicit none
    integer, allocatable :: seed(:)
    integer              :: n
    integer              :: first,last
    integer              :: i
    integer              :: rand_int
    integer,allocatable  :: count(:)
    real(kind=dp)        :: rand_val
       call random_seed(size = n)
       allocate(seed(n))
       call random_seed(get=seed)
       first=1
       last=10
       allocate(count(last-first+1))
       ! To have a discrete uniform distribution on the integers
       ! [first, first+1, ..., last-1, last] carve the continuous
       ! distribution up into last+1-first equal sized chunks,
       ! mapping each chunk to an integer.
       !
       ! One way is:
       !   call random_number(rand_val)
       ! choose one from last-first+1 integers
       !   rand_int = first + FLOOR((last+1-first)*rand_val)
          count=0
          ! generate a lot of random integers from 1 to 10 and count them.
          ! with a large number of values you should get about the same
          ! number of each value
          do i=1,100000000
             call random_number(rand_val)
             rand_int=first+floor((last+1-first)*rand_val)
             if(rand_int.ge.first.and.rand_int.le.last)then
                count(rand_int)=count(rand_int)+1
             else
                write(*,*)rand_int,’ is out of range’
             endif
          enddo
          write(*,’(i0,1x,i0)’)(i,count(i),i=1,size(count))
    end program demo_random_number

Results:

     >  1 10003588
     >  2 10000104
     >  3 10000169
     >  4 9997996
     >  5 9995349
     >  6 10001304
     >  7 10001909
     >  8 9999133
     >  9 10000252
     >  10 10000196

STANDARD

Fortran 95

Manual Reference Pages - random_number (3fortran)

NAME

SYNOPSIS

CHARACTERISTICS

DESCRIPTION

OPTIONS

EXAMPLES

STANDARD

SEE ALSO