Programming Concepts

Every program manipulates data: numbers, text, true/false values. To work with data, the program stores values in named variables (which can change) and constants (which cannot).

Variables

A variable is a named storage location whose value can change while the program is running.

In CIE pseudocode, a variable is declared with DECLARE and a data type:

DECLARE Age : INTEGER
DECLARE Price : REAL
DECLARE Name : STRING
DECLARE FirstInitial : CHAR
DECLARE GameOver : BOOLEAN

After declaration, a value is assigned with the arrow operator ←:

Age ← 17
Price ← 9.99
Name ← "Alice"
FirstInitial ← 'A'
GameOver ← FALSE

Constants

A constant is a named storage location whose value is set once and cannot change for the rest of the program.

CONSTANT PI ← 3.142
CONSTANT VAT ← 0.20
CONSTANT MAX_LIVES ← 3

Constants are usually written in UPPERCASE so they stand out from variables, and they make programs easier to maintain: if VAT changes, only one line of the program has to be edited.

Naming conventions

Identifiers (the names of variables and constants) should:

• Use letters and digits only, no spaces.
• Start with a letter, not a digit.
• Be in PascalCase (each word capitalised, e.g. TotalScore) or camelCase.
• Be descriptive: StudentScore is far better than s or x.
• For constants, prefer ALL_CAPS with underscores.

Data types

Cambridge IGCSE recognises five basic data types. Choosing the right one for each variable makes the program more accurate and uses memory efficiently.

Notes on the difference between char and string:

• A CHAR is exactly one character, written in single quotes ('A').
• A STRING is zero or more characters, written in double quotes ("Apple").

Casting

Sometimes a value needs to be converted from one type to another. This is called casting. For example, a number typed by the user arrives as a STRING and may need to be converted to an INTEGER before arithmetic can be done on it.

Common slip: trying to add "5" to "3" as strings gives "53" (concatenation), not 8. Cast to integer first.

Picking the right data type

Example — Choose the most appropriate data type for each piece of data, using a different type for each.