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Scattered throughout most programs are certain operations that must be done over and over. To avoid writing the same code over and over, a programmer puts the common code into a single routine that is called whenever the operation is needed. After the common code is executed, the program resumes at the point following the call. Such a routine in XPL0 is called a procedure.
Any block of code can become a procedure simply by giving it a name. The process of naming a procedure is a declaration. Procedure declarations have the general form:
For example, here is a simple procedure:
Number:= Ran(100) + 1;
Once a procedure is declared it can be executed simply by calling its name. For instance, here is a block that calls three procedures:
A block of code does not necessarily need to be called more than once to justify making it into a procedure. An important use of procedures is to make a program more understandable by breaking it down into smaller, simpler pieces. By making a piece of code into a procedure, you can name it according to its use, test it separately, and keep the main body of code uncluttered.