How to use sub programs (functions and procedures) to produce structured…

The idea

Sub programs break a program into named, self-contained blocks that can be called whenever needed. A procedure performs an action and may display or update data without returning a value; a function performs a calculation and returns a value to the caller. Parameters pass data in, and arguments are the actual values supplied. Using sub programs produces structured code: each block has one clear job, so programs are easier to read, test, debug and reuse. For example, a function areaCircle(radius) can return 3.142 × radius², while a procedure displayMenu() can print options. Calling these blocks avoids duplicating code and lets one change fix every use.

What earns the marks

• Defines a sub program as a named block of code that is defined once and called by name when required.
• Distinguishes a procedure, which carries out a task and need not return a value, from a function, which returns a value to the point of call.
• Explains that parameters are placeholders in the definition and arguments are the actual values passed when the sub program is called.
• Describes how sub programs produce structured code by separating concerns, reducing duplication and making programs easier to read, test, debug and maintain.
• Applies the idea to a concrete example, such as a function that returns the area of a circle from a radius parameter or a procedure that outputs a menu.
• Explains how data flows into and out of a sub program, including local variables and the returned value of a function.

Common mistakes and how to put them right

• Saying a procedure always returns a value.

  Put it right by stating that a procedure performs a task, whereas a function returns a value.

• Confusing parameters with arguments.

  Put it right by saying parameters appear in the definition and arguments are the values supplied at the call.

• Writing one long program with repeated code instead of extracting reusable blocks.

  Put it right by identifying repeated tasks and moving each into its own sub program.

Exam tips

• When asked to compare functions and procedures, state the return-value difference and give one short example of each.
• When writing code, define each sub program with a meaningful name and show clearly how it is called, including any arguments.
• When explaining structured code, link sub programs to at least two benefits such as reduced duplication and easier debugging.