Subroutines and intent in Fortran

A causeway verdict via subroutine: intent tells the compiler which way data flows.

program subroutine_intent
    implicit none
    real :: feet = 11.2
    character(len=24) :: advice

    call causeway_call(feet, advice)
    print *, feet, " ft -- ", trim(advice)

contains
    ! intent tells the compiler which way each argument flows.
    subroutine causeway_call(tide_ft, verdict)
        real, intent(in) :: tide_ft
        character(len=*), intent(out) :: verdict
        if (tide_ft > 9.5) then
            verdict = "stay off the causeway"
        else
            verdict = "safe to cross"
        end if
    end subroutine causeway_call
end program subroutine_intent

How it works

1. call causeway_call(feet, advice) invokes; contains holds the definition below.
2. intent(in) makes the tide reading read-only inside; the compiler enforces it.
3. intent(out) marks the verdict as the result path; len=* takes the caller's length.

Keywords and builtins used here

• call
• character
• contains
• else
• end
• if
• implicit
• intent
• len
• none
• print
• program
• real
• subroutine
• then
• trim

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Step 1 of 3 in Procedures, step 13 of 27 in Language basics.

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