Local and global variables (Edexcel GCSE Computer Science): Revision Notes

Local and global variables

What is scope?

In programming, scope refers to the area or region of a programme where a variable can be accessed and used. Think of it like different rooms in a house - some things are only available in certain rooms, while others can be accessed from anywhere in the house.

When you create a variable, it exists and can only be used within its specific scope. This is really important because it means you can have different variables with exactly the same name in different parts of your programme, and they won't interfere with each other. They might have the same name, but they're actually completely separate variables with different values.

Local scope and local variables

Local scope describes the area within a single subprogram (like a function or procedure). When you create a variable inside a subprogram, it becomes a local variable.

Here are the key things to remember about local variables:

• They are only accessible within the subprogram where they were created
• Once the subprogram finishes running, the local variable disappears
• Other parts of your programme cannot see or use these variables
• This helps prevent accidental changes to variables from other parts of your code

Global scope and global variables

Global scope covers the entire programme that's running. Global variables are created in this wider scope, which means they can be accessed from anywhere in your programme.

Key points about global variables:

• They can be accessed from the main programme and all subprograms
• They exist for the entire duration of your programme
• Any part of your code can read and modify these variables
• They're useful for storing information that multiple parts of your programme need to share

How variable lookup works

When your programme encounters a variable name, it follows a specific rule to decide which variable to use:

Code example breakdown

Let's look at how this works in practice:

# Global Variables
theNumbers = [1, 2, 3, 4, 5]
total = 154

# Subprogram
def doAvg(pNums):
    avg = 0.0
    total = 0
    for number in pNums:
        total = total + number
    avg = total / len(pNums)
    return (avg)

# Main Programme
print(doAvg(theNumbers))
print(total)

Exam tips

• Always identify the scope first when analysing variables in code
• Look carefully at where variables are declared - inside a function = local, outside = global
• Remember the lookup rule - local scope is checked before global scope
• Watch out for same-named variables - they can exist in different scopes without conflicting
• Practice tracing through code to see which version of a variable is being used at each point

Real-world applications

Understanding scope is crucial for:

• Game development - player stats (global) vs temporary calculations (local)
• Web applications - user session data (global) vs page-specific variables (local)
• Mobile apps - app settings (global) vs screen-specific data (local)