Subroutines and functions are routines made up of a sequence of instructions that can receive data, process it, and return a value. The routines can be:
In many aspects, subroutines and functions are the same. However, they are different in a few major aspects, such as how to call them and the way they return values.
To call a subroutine, use the CALL instruction followed by the subroutine name (label or program member name). You can optionally follow this with up to 20 arguments separated by commas. The subroutine call is an entire instruction.
CALL subroutine_name argument1, argument2,...
To call a function, use the function name (label or program member name) immediately followed by parentheses that can contain arguments. There can be no space between the function name and the left parentheses. The function call is part of an instruction, for example, an assignment instruction.
z = function(argument1, argument2,...)
A subroutine does not have to return a value, but when it does, it sends back the value with the RETURN instruction.
RETURN value
The calling program receives the value in the REXX special variable named RESULT.
SAY 'The answer is' RESULT
A function must return a value. When the function is a REXX program, the value is returned with either the RETURN or EXIT instruction.
RETURN value
The calling program receives the value at the function call. The value replaces the function call, so that in the following example, z = value.
z = function(argument1, argument2,...)
The actual instructions that make up a subroutine or a function can be identical. It is the way you want to use them in a program that turns them into either a subroutine or a function. For example, you can call the built-in function SUBSTR as either a function or a subroutine. This is how to call SUBSTR as a function to shorten a word to its first eight characters:
a = SUBSTR('verylongword',1,8) /* a is set to 'verylong' */
You get the same results if you call SUBSTR as a subroutine.
CALL SUBSTR 'verylongword', 1, 8
a = RESULT /* a is set to 'verylong' */
When deciding whether to write a subroutine or a function, ask yourself the following questions:
The rest of this chapter describes how to write subroutines and functions and finally summarizes the differences and similarities between the two.