15.7: Subroutines

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A subroutine is a Fortran subprogram that can accept some kind of input information and based on that information, return a result or series of results.

The general form of a subroutine is a follows:

subroutine <name> ( <arguments> )
<declarations>

    <body of subroutine>

    return
end subroutine <name>

The information, in the form of arguments, is passed from the calling routine to the subroutine. Each of the passed arguments must be declared and the declaration must include the type and the intent. The arguments in the calling routine and the subroutine must match and are matched up by position.

For example, given the following simple program to find the sum and average of three numbers.

program subExample

implicit none
real :: x1=4.0, y1=5.0, z1=6.0, sum1, ave1
real :: x2=4.0, y2=5.0, z2=6.0, sum2, ave2

    call sumAve(x1, y1, z1, sum1, ave1)
    write (*,'(a,f5.1,3x,a,f5.1)') "Sum=", sum1,    &
                            "Average=", ave1
    call sumAve(x2, y2, z2, sum2, ave2)
    write (*,'(a,f5.1,3x,a,f5.1)') "Sum=", sum2,    &
                            "Average=", ave2
contains

subroutine sumAve (a, b, c, sm, av)
real, intent(in) :: a, b, c
real, intent(out) :: sm, av

    sm = a + b + c
    av = sm / 3.0

    return
end subroutine sumAve

end program subExample

The arguments in the first call (x1, y1, z1, sum1, and ave1) are matched up to the arguments in the subroutine (a, b, c, sm, and av) by position. That is, the x1 from the call is matched with the a in the subroutine. The arguments in the second call (x2, y2, z2, sum2, and ave2) are again matched up to the arguments in the subroutine (a, b, c, sm, and av) by position. While the names of the variables do not need to match, the data types must match. Variables declared in a function or subroutine are not the same as variables in the calling routine. This is true, even if they are the same name!