Pearson Edexcel · GCSE · Computer Science
Course version: 1CP2 · Version from Sept 2020
Understand the difference between and be able to write programs that make…
Full specification wording
Subprograms: 6.6.3 understand the difference between and be able to write programs that make appropriate use of global and local variables
A global variable is declared in the main program and can be read or changed by any subprogram.
Checked against the Pearson Edexcel 1CP2 specification (opens in a new tab) on .
- Point 3 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A global variable is declared in the main program and can be read or changed by any subprogram. A local variable is created inside a subprogram and exists only while that subprogram runs, so code outside cannot see it. Local variables keep subprograms self-contained: two subprograms can each use a variable named count without clashing. Globals are convenient for values many parts of the program share, such as a score, but overusing them makes code hard to trace because any subprogram might change them. A good approach is to pass needed data in as parameters, keep working values local, and return results, using globals only for genuinely shared state.
What earns the marks
- A global variable is declared outside subprograms and is accessible throughout the program.
- A local variable is declared inside a subprogram and is accessible only within that subprogram.
- Local variables are created when the subprogram is called and cease to exist when it ends.
- Two subprograms may each have a local variable with the same name without conflict.
- Changing a global inside a subprogram affects the value seen elsewhere in the program.
- Appropriate use means passing data as parameters and returning results where possible, reserving globals for values genuinely shared across the program.
Common mistakes and how to put them right
Assuming a local variable can be read in the main program after the subprogram finishes.
Put it right: It no longer exists outside its subprogram.
Believing a local variable in one subprogram is visible in another.
Put it right: Each subprogram has its own scope.
Thinking a global variable must be used whenever two subprograms need the same data.
Put it right: Passing parameters and returning values is often clearer.
Confusing a parameter with a global.
Put it right: A parameter is local to the subprogram that receives it.
Exam tips
- State the scope of each variable in a code snippet and explain where it can be accessed.
- When tracing, track global values across calls and treat each local variable as fresh on every call.
- Justify a design choice by naming the scope that keeps the data available only where it is needed.
Try the lesson
The start of the Subprograms: 6.6.3 understand the difference between and be able to write programs that make appropriate use of global and local variables lesson, made from checked teaching notes for this specification point.
Have a go first
A program has a subprogram CalculateTax that begins with DECLARE rate : REAL and then rate ← 0.2. No other part of the program uses a variable called rate. Which statement correctly describes rate?
Listen
A global variable is declared in the main program and can be read or changed by any subprogram. A local variable is created inside a subprogram and exists only while that subprogram runs, so code outside cannot see it. Local variables keep subprograms self-contained: two subprograms can each use a variable named count without clashing. Globals are convenient for values many parts of the program share, such as a score, but overusing them makes code hard to trace because any subprogram might change them. A good approach is to pass needed data in as parameters, keep working values local, and return results, using globals only for genuinely shared state. A strong answer earns marks for these points. A global variable is declared outside subprograms and is accessible throughout the program. A local variable is declared inside a subprogram and is accessible only within that subprogram. Local variables are created when the subprogram is called and cease to exist when it ends. Two subprograms may each have a local variable with the same name without conflict. Exam tip: State the scope of each variable in a code snippet and explain where it can be accessed.
Key words
Say what each word means before you turn the card.
Scope of variables
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Global versus local access
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Lifetime of local variables
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Appropriate design using parameters and return values
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See every definition
- Scope of variables
- A global variable can be read or changed by any subprogram, while a local variable is accessible only within the subprogram it is declared in.
- Global versus local access
- A global is declared in the main program and accessible throughout, whereas a local is created inside a subprogram and code outside cannot see it.
- Lifetime of local variables
- Created when the subprogram is called and ceasing to exist when it ends, so it no longer exists outside its subprogram.
- Appropriate design using parameters and return values
- Passing needed data in as parameters, keeping working values local and returning results, using globals only for genuinely shared state.
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