Programming concepts: constant declaration

The idea

A constant is a named store whose value is fixed once set and cannot be changed while the program runs. Declaring a constant gives it a name and a value at the point of definition, and the translator then prevents any later reassignment. For example, a program calculating the area of a circle might declare a constant for pi and use it throughout, so the value is written once and cannot be altered accidentally. Using constants improves readability and makes maintenance easier, because changing the value in one place updates every use. Attempting to reassign a constant should raise an error, which helps prevent bugs caused by unintended changes.

What earns the marks

• A constant is a named store whose value is fixed at declaration and cannot be changed during execution.
• A constant is given its value when it is declared, unlike a variable that may be assigned later.
• The translator prevents reassignment of a constant, so attempting to change it causes an error.
• Using constants improves readability and maintenance because the value is defined in one place.
• Constants suit fixed values such as pi or a maximum array size that must not change.

Common mistakes and how to put them right

• Treating a constant as a variable that simply happens not to change.

  Put it right by stressing that the language enforces the fixed value.

• Thinking a constant can be declared without a value and set later.

  Put it right: Explain that a constant must be given its value at declaration.

• Believing constants are only for numbers.

  Put it right: Note that a constant can hold any fixed value, including text.

Exam tips

• Eliminate options that allow the value to be reassigned, since that describes a variable.
• Look for the idea that the value is set once at declaration and enforced by the translator.
• Link the answer to benefits such as readability and easier maintenance.