finding specific indices with pointer array

Question

I am relatively new to Fortran and break my head about one thing for hours now:

I want to write a subroutine for finding the indexes for specific elements in a real 1D

Answer 1

I think you're on the right track, but you're ignoring some intrinsic Fortran functions, specifically count, and you aren't returning anything!

subroutine comp(arr, min)
   real, intent(in) :: arr(:)
   real, intent(in) :: min
! local variables
   integer, allocatable :: indices(:)
   integer :: i,j, indx


! count counts the number of TRUE elements in the array 
   indx = count(arr > min)
   allocate(indices(indx))

! the variable j here is the counter to increment the index of indices
   j=1
   do i=1,size(arr)
      if(arr(i) > min) then
         indices(j) = i
         j = j+1
      endif
   enddo
end subroutine comp

Then you can use the array indices as

do i=1,size(indices)
   var = arr(indices(i))
enddo

Note that since you are not returning indices, you will lose all the information found once you return--unless you plan on using it in the subroutine, then you're okay. If you're not using it there, you could define it as a global variable in a module and the other subroutines should see it.