Calling random_number ( ) in pure procedure

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You cannot call the random_number intrinsic in a pure procedure, but you could write a pure RNG that is slightly modified from a non-pure one, as discussed in Pure random number generators, and call that.

Alternatively, since a pure procedure can access a module variable but not change it, you could create a wrapper that calls random_number, sets a module variable, and then calls the pure subroutine, as shown below.

module m
implicit none
real :: xran
contains
subroutine wrap_add(x)
real, intent(in out) :: x
call random_number(xran)
print*,"xran =",xran
call add(x)
end subroutine wrap_add
!
pure subroutine add(x)
real, intent(in out) :: x
x = x + xran
end subroutine add
end module m
!
program main
use m
implicit none
real :: x
x = 10.0
call wrap_add(x)
print*,"x =",x
call wrap_add(x)
print*,"x =",x
end program main

sample output:

 xran =  0.349059999    
 x =   10.3490601    
 xran =  0.555532038    
 x =   10.9045925
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