Students should know why named constants and variables are used.

The idea

This criterion asks you to explain the purpose of named constants and named variables in programs. A variable is a named store whose value can change while the program runs, for example a score that increases each time a player answers correctly. A constant is a named store whose value is set once and should not change, for example a VAT rate or the maximum size of a list. Naming them makes code readable, because the name describes the data, and makes it maintainable, because changing the value in one place updates every use. Constants also prevent accidental changes to values that must stay fixed. A good answer distinguishes the two by whether the value is intended to change, and gives a reason such as readability, reuse or preventing errors.

What earns the marks

• States that a variable is a named store whose value can change during execution.
• States that a constant is a named value that is set once and should not be changed while the program runs.
• Explains that meaningful names make code easier to read and understand.
• Explains that using a named constant means a fixed value is defined in one place, so it is easier to change and less likely to be altered by accident.
• Gives a suitable example, such as a score variable that changes or a fixed tax rate held as a constant.

Common mistakes and how to put them right

• Confusing constants with variables by saying a constant can be changed when needed.

  Put it right: A constant is intended to stay fixed.

• Thinking names are only for the computer's benefit.

  Put it right: Names mainly help people read and maintain the code.

• Believing a variable must always change.

  Put it right: It is allowed to change, not required to.

Exam tips

• Decide whether the value is meant to change during the program; if not, it is a constant.
• Link each reason to a benefit such as readability, easier maintenance or preventing accidental change.
• Use a concrete example to show you understand the difference between the two.