Write values into, and read values from, an array using iteration

Programming – Arrays and Iteration

Learning Objective (AO1)

Write values into, and read values from, an array using iteration (count‑controlled FOR loops and pre‑condition WHILE loops). Include both one‑dimensional and two‑dimensional arrays, demonstrate correct handling of array bounds and show how to validate user‑supplied indices.

Cambridge IGCSE 0478 Syllabus Links

Quick Reference Box (Keywords & Operators)

Keyword → Syllabus Mapping (Topic 8.1)

• DECLARE – variables & data types
• CONSTANT – constants
• INPUT / OUTPUT – I/O
• IF … THEN … ELSE – selection
• FOR, WHILE, REPEAT … UNTIL – iteration (count‑controlled, pre‑condition, post‑condition)
• Arithmetic & relational operators – operators

Sequence Example (straight‑line code)

DECLARE a ← 5
DECLARE b ← 12
DECLARE sum ← a + b
OUTPUT "The sum is ", sum

This demonstrates the “sequence” part of Topic 8.1 – code executed from top to bottom without any control structures.

Lower‑Bound Conventions

Iteration Structures – Comparison

1‑Dimensional Arrays

• Declaration – size must be a constant known at compile time.

CONSTANT SIZE ← 10
DECLARE numbers[SIZE] AS INTEGER

Writing values with a count‑controlled FOR loop

FOR i ← 0 TO SIZE‑1 DO
    numbers[i] ← (i + 1) * 3      // store the first SIZE multiples of 3
END FOR

Reading values with a count‑controlled FOR loop

FOR i ← 0 TO SIZE‑1 DO
    OUTPUT numbers[i]
END FOR

Pre‑condition loop example – reading a fixed number of marks

CONSTANT CLASS_SIZE ← 5
DECLARE marks[CLASS_SIZE] AS INTEGER
DECLARE i ← 0

WHILE i < CLASS_SIZE DO
    OUTPUT "Enter mark for student ", i+1, ": "
    INPUT marks[i]
    i ← i + 1
END WHILE

Post‑condition loop example – sentinel input (optional)

DECLARE total ← 0
DECLARE count ← 0
DECLARE n ← -1

REPEAT
    OUTPUT "Enter a positive number (0 to stop): "
    INPUT n
    total ← total + n
    count ← count + 1
UNTIL n = 0

AO2 – Simple index validation (single test)

DECLARE index ← 7
IF index ≥ 0 AND index < SIZE THEN
    OUTPUT "Element = ", numbers[index]
ELSE
    OUTPUT "Error: index out of bounds"
END IF

AO2 – Robust validation for a user‑supplied index (repeat until valid)

DECLARE idx ← -1
REPEAT
    OUTPUT "Enter an index between 0 and ", SIZE-1, ": "
    INPUT idx
    IF idx < 0 OR idx ≥ SIZE THEN
        OUTPUT "Invalid index – try again."
    END IF
UNTIL idx ≥ 0 AND idx < SIZE

OUTPUT "Element at index ", idx, " = ", numbers[idx]

2‑Dimensional Arrays

• Declared as a matrix of rows × columns. Both dimensions must be constant.

CONSTANT ROWS ← 3
CONSTANT COLS ← 4
DECLARE matrix[ROWS][COLS] AS INTEGER

Writing values with nested FOR loops

FOR r ← 0 TO ROWS‑1 DO
    FOR c ← 0 TO COLS‑1 DO
        matrix[r][c] ← (r+1) * (c+1)      // simple multiplication table
    END FOR
END FOR

Reading values with nested FOR loops

FOR r ← 0 TO ROWS‑1 DO
    FOR c ← 0 TO COLS‑1 DO
        OUTPUT "matrix[", r, "][", c, "] = ", matrix[r][c]
    END FOR
END FOR

Step‑by‑Step Procedure (Algorithm Design)

Common Pitfalls (AO2)

• Accessing an index outside the declared bounds (e.g. array[SIZE] when the highest valid index is SIZE‑1).
• Mixing lower‑bound conventions – loop limits must match the chosen base (0 or 1).
• Forgetting to reset the loop counter before a second loop, resulting in zero iterations.
• Omitting validation when the index is supplied by the user; this can cause runtime errors.

Example: Storing and Displaying Student Marks (1‑D Array)

Class size = 5. The program asks for each mark, stores it, then prints the marks.

CONSTANT CLASS_SIZE ← 5
DECLARE marks[CLASS_SIZE] AS INTEGER
DECLARE i ← 0

// ---- Write phase (FOR loop) ----
FOR i ← 0 TO CLASS_SIZE‑1 DO
    OUTPUT "Enter mark for student ", i+1, ": "
    INPUT marks[i]
END FOR

// ---- Read phase (FOR loop) ----
OUTPUT "Marks entered:"
FOR i ← 0 TO CLASS_SIZE‑1 DO
    OUTPUT "Student ", i+1, ": ", marks[i]
END FOR

Trace Table – Write Phase

Students should produce a similar table for the read phase, showing the values displayed on each iteration.

Suggested Diagram