A subprogram is a program unit that can be invoked from
another program unit to perform a specific task. These subprograms
are either written by the user or supplied as part of the Fortran
library.
User-written subprograms include the following:
- Statement functions: A computing procedure
defined by a single statement that is similar in form to an
assignment statement. A statement function is invoked by a
function reference in a main program unit or a subprogram unit.
- Function subprograms: Program units,
also called functions, that contain a set of commonly used
computations. A function subprogram's first statement is a
FUNCTION statement, optionally preceded by
an OPTIONS statement. A function subprogram is invoked by
a function reference in a main program unit or a subprogram unit.
- Subroutine subprograms: Program units,
also called subroutines, that contain a set of commonly used
computations. A subroutine subprogram's first statement is a
SUBROUTINE statement, optionally preceded
by an OPTIONS statement. A subroutine subprogram receives
control when it is invoked with a CALL statement and returns
control with a RETURN statement.
Subprograms supplied as part of the Fortran library are called
intrinsic subprograms (or procedures). They perform mathematical,
numeric, bit manipulation, character, and miscellaneous functions.
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