Do loop inside a where block in Fortran - fortran

Question

Even though I do not exactly know why, it seems that Fortran (90) does not allow do loops inside where blocks. A code structured as follows does not compile with gfortran (Unexpected DO statement in WHERE block at (1)):
real :: a(30), b(30,10,5)
integer :: i, j
do i=1,10
where(a(:) > 0.)
! do lots of calculations
! modify a
do j=1,5
b(:,i,j)=...
enddo
endwhere
enddo
The only workaround that I can think of would be
real :: a2(30)
do i=1,10
a2(:)=a(:)
where(a(:) > 0.)
! do lots of calculations
! modify a
endwhere
do j=1,5
where(a2(:) > 0.)
b(:,i,j)=...
endwhere
enddo
enddo
I suppose that there are more elegant solutions? Especially if the where condition is less straightforward, this will look messy pretty soon... Thanks!

Answer 1

If your arrays are all 1-indexed, you can replace where constructs by explicitly storing and using array masks, for example
program test
implicit none
real :: a(30), b(30,10,5)
integer :: i, j
integer, allocatable :: mask(:)
do i=1,10
mask = filter(a(:)>0.)
! do lots of calculations
! modify a
do j=1,5
b(mask,i,j)=...
enddo
enddo
contains
! Returns the array of indices `i` for which `input(i)` is `true`.
function filter(input) result(output)
logical, intent(in) :: input(:)
integer, allocatable :: output(:)
integer :: i
output = pack([(i,i=1,size(input))], mask=input)
end function
end program