Programming Constructs — Overview

All programs are built using three fundamental constructs. AQA 7517 requires you to use all three correctly in pseudocode and understand how they control program flow.

1. Sequence

Sequence is the default mode of execution — statements execute one after another, in the order they are written, from top to bottom. No branching or repeating occurs.

DECLARE total : INTEGER
total ← 0
total ← total + 10
OUTPUT total      ← outputs 10

Every program has sequence as its base. The other two constructs interrupt or repeat parts of the sequence.

2. Selection

Selection allows a program to make decisions — different code paths execute depending on whether a condition is True or False.

IF … THEN … ELSE … ENDIF

IF score >= 70 THEN
    OUTPUT "Pass"
ELSE
    OUTPUT "Fail"
ENDIF

ELSE IF (Nested Selection)

IF score >= 90 THEN
    grade ← "A"
ELSE IF score >= 70 THEN
    grade ← "B"
ELSE IF score >= 50 THEN
    grade ← "C"
ELSE
    grade ← "F"
ENDIF

CASE … OF … OTHERWISE … ENDCASE (Switch Statement)

Used when a single variable or expression is compared against multiple fixed values — cleaner than many ELSE IFs:

CASE OF day
    1: OUTPUT "Monday"
    2: OUTPUT "Tuesday"
    3: OUTPUT "Wednesday"
    OTHERWISE: OUTPUT "Invalid"
ENDCASE

3. Iteration

Iteration (looping) repeats a block of code. AQA 7517 requires three loop types:

FOR Loop — Count-controlled

Use when the number of repetitions is known in advance.

FOR i ← 1 TO 10
    OUTPUT i
NEXT i

WHILE Loop — Pre-condition (condition tested before each iteration)

Use when the number of iterations is unknown and the condition is checked before the body executes — the loop may not execute at all if the condition is initially False.

WHILE answer ≠ "quit" DO
    INPUT answer
ENDWHILE

REPEAT … UNTIL — Post-condition (condition tested after each iteration)

The loop body always executes at least once — the condition is tested after the body.

REPEAT
    INPUT score
UNTIL score >= 0 AND score <= 100

Nested Loops

Loops can be placed inside other loops. The inner loop completes fully for each iteration of the outer loop. Total iterations = outer iterations × inner iterations.

FOR row ← 1 TO 3
    FOR col ← 1 TO 4
        OUTPUT row * col
    NEXT col
NEXT row
← Outputs 3 × 4 = 12 values