Cambridge IGCSE and O Level Programming Book
4.01 Subroutines A subroutine is a sequence of program code that performs a specific task but does not represent the entire system. All subroutines in Python require a name and the keyword def which is short for define. When a subroutine is activated (this is referred to as ‘called’), the calling program is halted and control is transferred to the subroutine. After the subroutine has completed execution, control is passed back to the calling program. This modularised approach to programming brings with it advantages over a simple sequenced program. Consider a GUI program that maintains its running status while waiting for various subroutines to be called by activation of event triggers. The subroutines execute their code and pass control back to the main program. This allows the programmer to generate the complete program from a series of individual subroutines. Some code is executed when the script is loaded, other elements when certain buttons are clicked and possibly further elements of code are activated when text is changed in a text box. Imagine the complexity of the program code if only a single sequence of code was available to the programmer. syllabus check
Programming concepts: use predefined procedures or functions. 24
Advantages of Using Subroutines The ability to call subroutines from the main code offers a number of advantages: • The subroutine can be called when needed: A single block of code can be used many times in the entire program, avoiding the need for repeating identical code sequences throughout. This improves the modularity of the code, makes it easier to understand and helps in the identification of errors. • There is only one section of code to debug: If an error is located in a subroutine, only the individual subroutine needs to be debugged. Had the code been repeated throughout the main program, each occurrence would need to be altered. • There is only one section of code to update: Improvements and extensions of the code are available everywhere the subroutine is called.
Types of Subroutine Two main types of subroutine exist: • Procedures are small sections of code that can be reused. They do not return a value. In pseudocode, a procedure is named and takes the form:
PROCEDURE . . . ENDPROCEDURE. They are called by using the CALL statement.