Return to the Index

Calling procedures

Using more than one procedure

If you wanted a single procedure to perform a complex task, the procedure would become long and complicated. It is more convenient to have a module containing a number of procedures, each of which you can write and edit separately.

Many OPL modules are in fact a set of procedures linked up each procedure doing just one job (such as a certain calculation) and then passing its results on to other procedures so they can do other operations: (picture)

OPL is designed to encourage programs written in this way, since:


Modules containing more than one procedure

You can have as many procedures as you like in a module. Each must begin with PROC and end with ENDP.

When you run a translated module it is always the first procedure, at the top of the module, which is actually run. When this finishes, the module stops; any other procedures in the file are only run if and when they are called.

Although you can use any name you want, it's common to give the first procedure a name like "start".

Procedures which run on their own should be written and translated as separate modules, otherwise you won't be able to run them.

Calling procedures

To run another procedure, simply give the name of the procedure (with the colon). For example, this module contains two procedures:

Running this module would run procedure "one:", with this effect: "Start" is displayed; after a PAUSE it calls "two:", which displays "Doing..."; after another PAUSE "two:" returns to the "one:" procedure; "one:" displays "Finished"; and after a final PAUSE, "one:" finishes.

Uses of calling procedures

Calling procedures can be used to:

For example, when your program asks you "Do this or do that?", make two procedure calls either "this:" or "that:" procedure depending on what you reply, for example:

To make full use of procedure calls, you must be able to communicate values between one procedure and another. There are two ways of doing this: global variables and parameters.


Values can be passed from one procedure to another by using parameters. They look, and act, very much like arguments to functions.

In the example below, the procedure "price:" calls the procedure "tax:". At the same time as it calls it, it passes a value (in this case, the value which INPUT gave to the variable "x") to the parameter "p" named in the first line of "tax:". The parameter "p" is rather like a new local variable inside "tax:", and it has the value passed when "tax:" is called. (The "tax:" procedure is not changing the variable "x".)

The "tax:" procedure displays the value of "x" plus 17.5% tax.

The parameter type is specified as with variables for example "p" for a floating-point parameter, "p%" for an integer, "p&" for a long integer, "p$" for a string. You can't have array parameters.

The values passed as parameters may be the values of variables, strings in quotes, or constants. So a call might be "calc:(a$,x%,15.8)" and the first line of the called procedure "PROC calc:(name$,age%,salary)"

In the called procedure, you cannot assign values to parameters for example, if "p" is a parameter, you cannot use a statement like "p=10".

You will see a `Type mismatch' error displayed if you try to pass the wrong type of value to a parameter for example, 45 to (a$).

Multiple parameters

In the following example, the second procedure "tax2:" has two parameters:

"tax2:" displays the price plus tax at the rate specified.

This uses the "%" symbol as an operator "p1+p2 %" means "p1" plus "p2" percent of "p1". Note the space before the "%"; without it, "p2%" would be taken as representing an integer variable.

Appendix B has more about the "%" operator.

Returning values

In the following example, the RETURN command is used to return the value of "x" plus tax at "r" percent to be displayed in "price3:". This is very similar to the way functions return a value.

The "tax3:" procedure calculates, but doesn't display the result. This means it can be called by other procedures which need to perform this calculation but do not necessarily need to display it.

Only one value may be returned by the RETURN command.

The name of a procedure which returns a value must end with the correct identifier "$" for string, "%" for integer, or "&" for long integer. To return a floating-point number, it should end with none of these symbols. For example, "PROC abcd$:" can return a string, while "PROC counter%:" can return an integer. In this example, "ref$:" returns a string:

If you don't use the RETURN command, a string procedure returns the null string (""""). Other (numeric) types of procedure return zero.

GLOBAL variables

You can only return one value with the RETURN command. If you need to pass back more than one value, use GLOBAL variables.

Instead of declaring "LOCAL x%,name$(5)" declare "GLOBAL x%,name$(5)". The difference is that:

So this module would run OK:

When you run this, the value 0 is displayed first, and then the value 2.

You would see an `Undefined externals' error displayed if you used LOCAL instead of GLOBAL to declare "a%", since the procedure "two:" wouldn't recognise the variable "a%". In general, though, it is good practice to use the LOCAL command unless you really need to use GLOBAL.

A local declaration overrides a global declaration in that procedure. So if "GLOBAL a%" was declared in a procedure, which called another procedure in which "LOCAL a%" was declared, any modifications to the value of "a%" in this procedure would not effect the value of the global variable "a%".

Passing back values

You can effectively pass as many values as you like back from one procedure to another by using global variables. Any modifications to the value of a variable in a called procedure are automatically registered in the calling procedure.

For example:

This would display "5 8"

`Undefined externals' error

If, perhaps because of a typing error, you use a name which is not one of your variables, no error occurs when you translate the module. This is because it could be the name of a global variable, declared in a different procedure, which might be available when the procedure in question was called. If no such global variable is available, an `Undefined externals' error is shown. This also displays the variable name which caused the error, together with the module and procedure names, in this format: `Error in MODULE\PROCEDURE,VARIABLE'.


Call a procedure by stating its name, including the colon.

Pass parameters to a procedure by following the procedure call with the values for the parameters, eg "calc2:(4.5,32)". In the called procedure, follow the procedure name with the parameter names, eg "PROC calc2:(mod,div%)".

To make variables declared in one procedure accessible to called procedures, declare the variables with GLOBAL instead of LOCAL.

<== Return to the Index