Write pseudocode statements for: the declaration of variables

11.1 Programming Basics – Declaring Variables

Learning Objective

• Write correct pseudocode DECLARE statements, choose the appropriate Cambridge data type, initialise where required, and apply the correct scope rules.

Why Declare Variables?

In the Cambridge A‑Level Computer Science (9618) exam every algorithm must begin with a clear declaration of every identifier that will be used. Declaring a variable tells the computer:

• Identifier – the name used later in the algorithm.
• Data type – the kind of value it can store (INTEGER, REAL, …).
• Initial value (optional) – the value it holds before any processing.
• Scope – where in the algorithm the identifier is visible.

Cambridge Data Types (Exact Syllabus Names)

Pseudocode Declaration Syntax

The Cambridge specification uses a language‑independent form. The generic pattern is:

DECLARE <identifier> : <TYPE> [← <initial‑value>]

• Square brackets [ ] denote an optional part.
• The arrow ← is the **initialisation operator** – it may be used only in the declaration block. All later assignments also use ←, but the exam marks a loss of marks if a variable is used before it has been initialised.

Language‑Specific Variants (for exam practice)

Scope of Variables

• Global (programme‑wide) scope: declared before the main algorithm block; visible to every sub‑algorithm.
• Local scope: declared inside a sub‑algorithm (function, procedure, loop, or block); visible only within that block.
• Variables needed in more than one sub‑algorithm should be declared globally; otherwise keep them local to avoid accidental modification.

Naming Conventions & Reserved Words

• Identifiers may contain letters, digits and the underscore _. They must start with a letter.
• Cambridge does not enforce a particular case style, but consistency is rewarded. Common choices:
  • camelCase – totalScore
  • snake_case – total_score
• Do not use reserved words as identifiers. Some examples: IF, FOR, WHILE, END, DECLARE, TRUE, FALSE.

Examples of Variable Declarations

Step‑by‑Step Procedure for Declaring Variables

1. Identify the purpose of each identifier in the algorithm.
2. Select the most appropriate TYPE (INTEGER, REAL, …). Consider range, precision, and whether the value is constant.
3. Decide on scope – global if needed by several sub‑algorithms, otherwise local.
4. Write the DECLARE statement using the exact Cambridge syntax.
5. If a sensible starting value exists, initialise it with ←.
6. Place **all** declarations at the very start of the algorithm or in a clearly marked “Variable Declaration” block.

Common Pitfalls (and How to Avoid Them)

• Omitting the TYPE – every DECLARE must specify a Cambridge type (INTEGER, REAL, …).
• Using a reserved word as an identifier – e.g. DECLARE IF : BOOLEAN is illegal.
• Type‑mismatch in initialisation – do not assign a string to an INTEGER: DECLARE count : INTEGER ← "0".
• Scope errors – a variable declared inside a loop cannot be used after the loop ends.

  FOR i ← 1 TO 10
      DECLARE total : INTEGER ← 0   ← ❌ local to the loop
  END FOR
  ← total ← total + i               ← error – total out of scope

• Incorrect Boolean initial value – only TRUE or FALSE are allowed.
• For REAL variables remember the exam assumes **single‑precision** unless otherwise stated.

Exam‑Ready Checklist

Full‑Question Style Example

Exam Prompt (Cambridge style):

Write a pseudocode algorithm called CALCULATE_AVERAGE that:

1. Reads the number of students n (1 ≤ n ≤ 30).
2. For each student, reads the integer mark (0 – 100) and stores it in an array marks.
3. Calculates the average (a REAL value) of the marks.
4. Outputs the average to two decimal places.

Use appropriate declarations, initialise variables, and observe correct scope.

Solution (highlighting declarations):

ALGORITHM CALCULATE_AVERAGE
    /*--- Variable Declaration Block ---*/
    DECLARE n        : INTEGER
    DECLARE i        : INTEGER ← 0
    DECLARE sum      : INTEGER ← 0
    DECLARE average  : REAL    ← 0.0
    DECLARE marks    : ARRAY[1..30] OF INTEGER

    /*--- Input ---*/
    OUTPUT "Enter number of students (1‑30): "
    INPUT n
    /* n is guaranteed to be within range by the question */

    /*--- Process ---*/
    FOR i ← 1 TO n
        OUTPUT "Enter mark for student ", i, ": "
        INPUT marks[i]
        sum ← sum + marks[i]
    END FOR

    average ← sum / n          /* division yields REAL */
    OUTPUT "Average mark = ", average:2   /* 2 decimal places */
END ALGORITHM

Alignment with Assessment Objectives

• AO1 – Knowledge: Understand Cambridge data types, declaration syntax, constants, arrays, records, and scope.
• AO2 – Application: Write correct DECLARE statements in pseudocode and translate them into Java, Python or VB where required.
• AO3 – Design & Evaluation: Choose the most efficient type and scope, justify the use of constants, and evaluate the impact of initial values on algorithm correctness.