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Manual Reference Pages  - random_number (3)

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NAME

random_number(3f) - [FORTRAN:INTRINSIC:MATHEMATICS:RANDOM] Pseudo-random number

SYNTAX

random_number(harvest)

DESCRIPTION

Returns a single pseudorandom number or an array of pseudorandom numbers from the uniform distribution over the range 0 <= x < 1.

ARGUMENTS

HARVEST
	Shall be a scalar or an array of type REAL.

EXAMPLE

Sample program:

   program demo_random_number
   implicit none
   integer, allocatable :: seed(:)
   integer              :: n
   integer              :: first,last
   integer              :: i
   integer              :: rand_int
   integer,allocatable  :: count(:)
   real                 :: 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

Sample output:

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

STANDARD

[[Fortran 95]] and later

CLASS

Subroutine

SEE ALSO

random_seed(3)

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random_number (3)

March 11, 2021

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