D. Haugli, Lecturer Aer E 160 Aerospace Engineering
11/5/2004 Subroutine Subprograms Iowa State University
Subroutine Subprograms
Use. A subroutine is a subprogram that, like a function, can receive multiple input arguments,
but that unlike a function, can return more than one result. To access a subroutine, use a CALL
statement, such as
CALL <insert subroutine name> (argument1, argument2, etc.)
(Note: functions may not be accessed with a CALL statement; subroutines must be accessed with
a call statement and may not be placed in an equation, write statement or condition.) Subroutines
may be called by other subroutines, by functions and by main programs.
Arguments must be of the same type and in order by type. The total number of arguments must
be the same in the CALL and SUBROUTINE statements.
Example 1. Consider three real variables, a, b and c. Let these be the arguments of a
subroutine named fire. If the CALL statement in the main program is,
CALL fire (a, b, c)
The corresponding subroutine statement might look like
SUBROUTINE fire(x)
where x is an array with three spaces,
REAL, INTENT (IN) :: x(3)
The result is that in the subroutine, x(1) receives the value of a, x(2) receives the
value of b and x(3) receives the value of c.
The names of the arguments do not matter, and arrays and variables can be mixed. Programs and
subprograms will share memory when both use the same name for an argument; otherwise, each
program unit (main program or subprogram) has its own memory.
Form. A subroutine has a form similar to any other FORTRAN program unit. The essential
structure is outlined below. Arguments must be included both for variables coming into the
subroutine from the calling program, and for values being returned to the calling program.
SUBROUTINE <insert subroutine name> (arg1, arg2, etc.)
USE <Insert any modules to use here.>
IMPLICIT NONE
<Insert declarations (CHARACTER, INTEGER, REAL, etc.)>
<Insert executable statement – this is the part of the code that
tells the subroutine what to compute and how to compute it.>
RETURN
END SUBROUTINE
