Argument Intent

In order to increase robustness to side-effects, and to facilitate efficient compilation, dummy arguments in the procedure declaration can be tagged with an intent attribute:

• INTENT(IN) Input only arguments; they cannot be written inside the procedure (or the compiler will complain);
• INTENT(OUT) Output only arguments; they cannot be read until they are written inside the procedure (assume they come in with garbage); also, if they are not assigned at all in the procedure body, a compilation error will result;
• INTENT(INOUT) Both input and output; assumed to hold valid data at entry, but this value can be modified by the procedure and passed back to the calling program.

For example, in our norm2s subroutine we may declare

real, intent(in)  :: x,y,z
real, intent(out) :: r

Also, the norm2 function declarations may be

real, intent(in) :: x,y,z