Scope of variables: local and global

Overview

Scope of a variable, local variable and global variable.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is meant by the scope of a variable?

   Model answer

   The scope of a variable is the region of the program in which that identifier can be used. Outside that region the name has no meaning.

2. 2

   Define a local variable.

   Model answer

   A local variable is declared inside a subprogram and can only be used within that subprogram. Other parts of the program cannot see it.

3. 3

   Define a global variable.

   Model answer

   A global variable is declared outside every subprogram and can be used anywhere in the program. Any subprogram may read or change it.

4. 4

   Give two advantages of using local variables.

   Model answer

   A local variable cannot be altered by an unrelated part of the program, so faults stay contained. The same identifier can also be reused safely in another subprogram.

5. 5

   State two problems caused by heavy use of global variables.

   Model answer

   Any subprogram can change a global variable, so tracing an unexpected value is difficult. Subprograms also become dependent on each other and harder to reuse.

6. 6

   A variable Total is declared inside procedure AddUp. What happens if the main program tries to print Total?

   Model answer

   The main program cannot see Total because the identifier is local to AddUp. The translator reports an undeclared identifier.

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