Designing Subroutines

Syntax

A Function receives some input via its formal arguments from outside world and computes and returns one value, the function value, with the function name. In some cases, you do not want to return any value or you may want to return more than one values. Then, Fortran's subroutines are what you need. Functions and subroutines are referred to as subprograms. The syntax of a Fortran subroutine is:

SUBROUTINE  subroutine-name (arg1, arg2, ..., argn)
   IMPLICIT  NONE
   [specification part]
   [execution part]
   [subprogram part]
END SUBROUTINE  subroutine-name

Here are some elaborations of the above syntax:

The first line of a subroutine starts with the keyword SUBROUTINE, followed by that subroutine's name.
Following subroutine-name, there is a pair of parenthesis in which a number of arguments arg1, arg2, ..., argn are separated with commas. These arguments are referred to as formal arguments. Formal arguments must be variable names and cannot be expressions and constants. Here are a examples:
1. The following is a subroutine called Factorial. It has two formal arguments n and Answer.
```
SUBROUTINE  Factorial(n, Answer)
```
2. The following is a subroutine called TestSomething. It takes four formal arguments a, b, c, and Error.
```
SUBROUTINE  TestSomething(a, b, c, Error)
```
A subroutine must end with END SUBROUTINE, followed by its name.
Between SUBROUTINE and END SUBROUTINE, there are IMPLICIT NONE, specification part, execution part and subprogram part. These are exactly identical to that of a PROGRAM.
Subroutines can be internal to a program or a module. Subroutine can also be external and in this case INTERFACE blocks are required.

If a subroutine does not need any formal argument, it can be written as

SUBROUTINE  subroutine-name ()
   IMPLICIT  NONE
   [specification part]
   [execution part]
   [subprogram part]
END SUBROUTINE  subroutine-name

where arg1, arg2, ..., argn are left out.

Unlike functions, the pair of parenthesis can be removed:

SUBROUTINE  subroutine-name
   IMPLICIT  NONE
   [specification part]
   [execution part]
   [subprogram part]
END SUBROUTINE  subroutine-name

Semantics

The meaning of a subroutine is very simple:

A subroutine is a self-contained unit that receives some "input" from the outside world via its formal arguments, does some computations, and then returns the results, if any, with its formal arguments.
Unlike functions, the name of a subroutine is not a special name to which you can save a result. Subroutine's name is simply a name for identification purpose and you cannot use it in any statement except the CALL statement.
A subroutine receives its input values from its formal arguments, does computations, and saves the results in some of its formal arguments. When the control of execution reaches END SUBROUTINE, the values stored in some formal arguments are passed back to their corresponding actual arguments.
Any statements that can be used in a PROGRAM can also be used in a SUBROUTINE.