Procedures

Intent

In, Out, Inout

intent declares how an argument is used. in reads only, out writes only, inout both.

Program

Play the program to compute sum and difference through one subroutine.

intents.f90
Replay: real traced execution (multi-file project)
program intents_demo
    implicit none
    integer :: a, b, sum_, diff
    a = 10
    b = 3
    call summarize(a, b, sum_, diff)
    print '(I0, A, I0)', sum_, " ", diff
contains
    subroutine summarize(x, y, s, d)
        integer, intent(in) :: x, y
        integer, intent(out) :: s, d
        s = x + y
        d = x - y
    end subroutine summarize
end program intents_demo

  1. a ← 10

    3integer :: a, b, sum_, diff4a = 105b = 3
    values this step10a

  2. b ← 3

    4a = 105b = 36call summarize(a, b, sum_, diff)
    values this step3b

  3. call ← summarize(a, b, sum_, diff)

    5b = 36call summarize(a, b, sum_, diff)7print '(I0, A, I0)', sum_, " ", diff
    values this stepsummarize(a, b, sum_, diff)call10a3b

  4. s ← 13

    11integer, intent(out) :: s, d12s = x + y13d = x - y
    values this step13s10x3y

  5. d ← 7

    12    s = x + y13    d = x - y14end subroutine summarize
    values this step7d10x3y

  6. print '(I0, A, I0)', sum_, " ", diff

    6    call summarize(a, b, sum_, diff)7    print '(I0, A, I0)', sum_, " ", diff8contains
    output13 7
    values this step13sum_7diff

Follow the Outputs

  1. a starts at 10.
  2. b starts at 3.
  3. call summarize(a, b, sum_, diff) sends both values into the subroutine.
  4. The subroutine sets sum_ to 10 + 3, which is 13.
  5. It sets diff to 10 - 3, which is 7, and the program prints 13 7. | argument or output | value | | --- | --- | | a | 10 | | b | 3 | | sum_ | 13 | | diff | 7 |
intent(in) Read-only argument; the caller's value is unchanged.
intent(out) Output-only argument; the callee assigns it.
multiple outputs Use `intent(out)` arguments to return more than one value from a subroutine.

Exercise: intents.f90

Reproduce the output 13 7, then change b to 4 and predict the new sum and difference before running it.