Question

I have two lists, I need to match elements in one against the other and output those elements into a new matrix (output). What is the fastest way to this in Fortran? Brute force so far:

do i = 1,Nlistone
   do j = 1,Nlisttwo
     if A(i).eq.B(j) then
        output(i) = B(j)
     end if
   end do
end do

openmp version:

!$OMP PARALLEL PRIVATE(i,j)
do i = 1,NA
   do j = 1,NB
     if A(i).eq.B(j) then
        filtered(i) = A(j)
     end if
   end do
end do
!$OMP END PARALLEL DO

There are definitely better ways to do this and sorting is not an option here sorry (+ the vector elements are not in any particular order). Is there a boolean argument similar to mask in python?

Answer 1

It might be faster to use the intrinsic ANY function something like this

do i = 1,Nlistone
     if (any(B==A(i))) output(i) = A(i)
end do

but I wouldn’t bet on this improving performance, I’d test both versions. You should be able to safely wrap this inside an !$OMP PARALLEL DO construct since each element of output is only written to by one thread.