Pseudocode basics (Cambridge (CIE) AS Computer Science): Revision Note

Variables & Constants

What is a variable?

• A variable is an identifier that can change in the lifetime of a program

• Identifiers should be:

  • In mixed case (Pascal case)

  • Can only contain letters (A-Z, a-z), digits (0-9) and the underscore character (_)

  • Start with a capital letter and not a digit

• A variable can be associated a datatype when it is declared

• When a variable is declared, memory is allocated based on the data type indicated

• To declare a variable, use the DECLARE keyword followed by the name and data type:

DECLARE Age : INTEGER
DECLARE Name : STRING
DECLARE IsLoggedIn : BOOLEAN
DECLARE Temperature : REAL
DECLARE DOB : DATE

• You can then assign a value using the assignment operator ← :

Age ← 18
Name ← "Alice"
IsLoggedIn ← TRUE

What is a constant?

• A constant is an identifier set once in the lifetime of a program

• Constants are generally named in all uppercase characters

• Constants aid the readability and maintainability

• To declare a constant, use the CONSTANT keyword:

CONSTANT Pi = 3.14159
CONSTANT MaxScore = 100
CONSTANT SchoolName = "Meridian Academy"

• Constants are not reassigned during execution – they are fixed values used throughout the algorithm

Example

• You are writing a program to calculate the area of a circle using the formula:

Area ← π × radius²

• π is a constant (it never changes)

• radius and area are variables (they change depending on the input)

CONSTANT Pi = 3.14159

DECLARE Radius : REAL
DECLARE Area : REAL

OUTPUT "Enter the radius of the circle:"
INPUT Radius

Area ← Pi * Radius * Radius

OUTPUT "The area of the circle is: ", Area

Variables and constants in different languages

Arithmetic & logical operators

Arithmetic operators

• Arithmetic operators are used to perform basic maths operations in a program

• These include adding, subtracting, multiplying and dividing values

Common arithmetic operators

Operator precedence (BODMAS / BIDMAS)

• Arithmetic operators follow operator precedence

• Multiplication and division happen before addition and subtraction unless brackets are used

result ← 2 + 3 * 4          // gives 14
result ← (2 + 3) * 4        // gives 20

Relational and logic operators

• Relational operators (also called comparison operators) are used to compare two values

• They return either TRUE or FALSE and are commonly used in conditions and loops

• The logic operators are AND, OR and NOT; they combine or reverse BOOLEAN values, e.g. X < Y AND Y < 20

Common relational operators

Example code

DECLARE X : INTEGER
DECLARE Y : INTEGER

X ← 5
Y ← 10

OUTPUT X = Y          // FALSE
OUTPUT X <> Y         // TRUE
OUTPUT X < Y          // TRUE
OUTPUT X > Y          // FALSE
OUTPUT X <= Y         // TRUE
OUTPUT X >= Y         // FALSE