Global and local variables

Global and local variables.

2.2.1 Programming Techniques

Programming Techniques — Note 3 of 6

Candidates need to understand the difference, benefits and drawbacks of global and local variables. Candidates must be able to recognise where each is used and its impact on a program, for example the amount of memory used, and change a program from one to the other.

What you need to know

A variable's scope is the part of the program where it can be used. Global variables can be used anywhere; local variables only exist inside the subroutine where they are declared. Choosing the correct scope makes programs more reliable and easier to maintain.

What to know

Good practice: use local variables and pass data through parameters; keep global variables to a minimum.

Key terms

Global variable: Declared in the main program and accessible from anywhere in the program, including inside subroutines. In OCR pseudocode: global userid = 123.
Global variables: useful for values needed throughout the program (e.g. settings), but they can be changed accidentally anywhere, making bugs hard to find, and they use memory for the whole run.
Local variable: Declared inside a subroutine; it only exists while the subroutine runs and can only be accessed within it.
Local variables: memory is freed when the subroutine ends; the same name can be used in different subroutines without conflict; subroutines are self-contained and easier to reuse and test.
If a local variable has the same name as a global one, the local variable is used inside the subroutine.

Exam technique

Tip:Be able to change a program that uses global variables into one that uses local variables and parameters, and vice versa.