Why Programs Need Named Storage

Programs process data that changes as instructions are executed. A variable provides a name for one item of data so later statements can read its current value or replace it. Without variables, a program could not remember input, intermediate results, counters or final answers.

The value belongs to the variable at a particular moment. If Total is assigned 20 and later assigned Total + 5, the new value becomes 25. The identifier remains the same, but the stored value changes. This distinction is essential when tracing code.

Variables should be declared before use. A declaration tells the reader and translator what kind of value the variable is intended to hold. In Cambridge pseudocode, the form is DECLARE identifier : data type.

Variables And Constants

A constant represents a value that should remain fixed, such as the number of months in a year or a tax rate provided by the question. Giving the value a name makes the program easier to read and reduces repeated unexplained numbers.

Cambridge pseudocode uses a constant declaration such as CONSTANT MaximumMark = 100. A constant should not appear on the left side of an assignment later in the program. If a value may change, it must be a variable instead.

Constants also improve reliability. If a fixed value must be changed during development, one declaration is edited rather than every occurrence of a literal value.

Meaningful Identifiers

A meaningful identifier describes the role of the item. StudentCount communicates more than C, and MaximumTemperature communicates more than X. Cambridge explicitly expects meaningful identifiers for variables, constants, arrays, procedures and functions.

An identifier should be concise but unambiguous. Names such as Total, AverageMark and IsValid are usually better than long sentences. Avoid choosing a name that suggests the wrong meaning, such as using Average for a total.

Consistent naming helps readers distinguish different kinds of data. A Boolean name often reads like a question or condition, for example IsFound or ContinueEntry. An array name is commonly plural, such as Marks or StudentNames.

Declaration, Initialisation And Assignment

Initialisation gives variables known starting values. Totals and counters normally start at zero. A maximum or minimum requires special care because an unsuitable starting value may produce a wrong answer.

The assignment arrow points from the expression to the variable. The right side is evaluated first, then its result replaces the old value on the left. Therefore Count <- Count + 1 is meaningful even though it is not a mathematical equation.

A variable must not be read before it has a known value. For example, Total <- Total + Price is unsafe unless Total was initialised earlier.

DECLARE StudentCount : INTEGER
DECLARE TotalCost : REAL
DECLARE IsComplete : BOOLEAN
CONSTANT MonthsInYear = 12

StudentCount <- 0
TotalCost <- 0.0
IsComplete <- FALSE

Tracing Changes In Values

Worked Examples

Examination Guidance

• Use the assignment arrow rather than an equals sign when writing Cambridge pseudocode.
• Declare each variable with an appropriate basic data type.
• State that a constant cannot change during program execution.
• Use identifiers that show purpose, not merely the type of the value.
• In trace questions, use the old value on the right side before replacing it.

Knowledge Check

1. What is a variable?

2. What is a constant?

3. Why use meaningful identifiers?

4. What does initialisation do?

5. What happens in Count <- Count + 1?