Procedures (Edexcel GCSE Computer Science): Revision Notes

Procedures in programming

What is a procedure?

A procedure is a special type of subprogram that carries out a specific job or task, but unlike functions, it doesn't send any result back to the main programme. Think of it like giving someone instructions to do something - they complete the task, but they don't need to report back with an answer.

Procedures are incredibly useful because they help you organise your code into manageable chunks. Instead of writing the same instructions over and over again, you can create a procedure once and then use it whenever you need it.

Creating a procedure

To create a procedure in Python, you use the def keyword followed by the procedure name and any parameters it needs. Here's the basic structure:

def procedure_name(parameter):
    # code to perform the task
    # more code here

def showMenu(pType):
    if (pType == "N"):
        print("1 - Search")
        print("2 - Delete") 
        print("3 - Add")
    else:
        print("A - Search")
        print("B - Delete")
        print("C - Add")

Notice a few important things about this procedure:

• It starts with def to define the procedure
• pType is a parameter that stores information sent from the main programme
• There's no return statement because procedures don't send values back
• The procedure performs its task (displaying a menu) and then finishes

Calling a procedure

Once you've created a procedure, you need to call it from your main programme to make it work. When you call a procedure, you can pass information to it through arguments.

Here's how the procedure calling process works:

1. The main programme calls the procedure - You write the procedure name followed by any arguments in brackets
2. Arguments get copied to parameters - The values you pass get copied into the procedure's parameter variables
3. The procedure runs - It executes all its code using the parameter values
4. Control returns to main program - Once finished, the programme continues from where it left off

Looking at the example:

typeMenu = ""
typeMenu = input("N for number: ")
typeMenu = typeMenu.upper()
showMenu(typeMenu)

In this code:

• typeMenu is a global variable that gets a value from user input
• When showMenu(typeMenu) is called, the value in typeMenu gets copied into the pType parameter
• The procedure runs and displays the appropriate menu
• Control returns to the main programme after the menu is displayed

Understanding arguments vs parameters

This is a concept that trips up many students, but it's actually quite simple:

Key differences from functions

While procedures and functions are both types of subprograms, they have one crucial difference:

• Functions perform a task AND return a value back to the calling code
• Procedures perform a task but DON'T return any value

Practical applications

Procedures are perfect for tasks like:

• Displaying menus (like in our example)
• Drawing shapes on screen
• Printing reports
• Validating user input
• Clearing the screen
• Playing sounds or animations

The turtle graphics example mentioned shows how you might use a procedure to draw a square - the procedure would contain all the instructions for drawing, and you'd just call it whenever you need a square drawn.