Fortran intent(inout) versus omitting intent

Question

Good practice dictates that subroutine arguments in Fortran should each have a specified intent (i.e. intent(in), intent(out) or intent(inout

Answer 1

To understand the role of intent in/out, you need to know that internally, Fortran effectively passes variables by reference. This isn't always the same as actually passing by reference.

If you pass an internal subsection of an 2-D array to a subroutine, ie: data(i1:i2, j1:j2), then Fortran copies that data into a contiguous section of memory, and passes the new address to the routine. Upon returning, the data is copied back into its original location.

By specifying INTENT, the compiler can know to skip one of the copying operations.

It not only acts as a failsafe for modifying data that you want to remain unchanged, but also can speed up your code when dealing with large datasets.