2.4 · Control Structures

Goal: master sequence, selection (binary / multi-way) and iteration. Nested loops are NOT in compulsory.

The three structures

Structure	Description
Sequence	Statements execute top to bottom
Selection	Choose between alternatives (binary / multi-way)
Iteration	Repeat statements while/until a condition

Sequence

INPUT a
INPUT b
sum ← a + b
OUTPUT sum

Nothing fancy — just straight-line execution.

Selection · Binary (IF / THEN / ELSE)

IF mark >= 50 THEN
    OUTPUT "Pass"
ELSE
    OUTPUT "Fail"
END IF

Selection · Multi-way (IF / ELSE IF / ELSE)

IF mark >= 80 THEN
    OUTPUT "A"
ELSE IF mark >= 70 THEN
    OUTPUT "B"
ELSE IF mark >= 60 THEN
    OUTPUT "C"
ELSE
    OUTPUT "F"
END IF

Or with a CASE construct (if your textbook uses it):

CASE mark OF
    80..100 : OUTPUT "A"
    70..79  : OUTPUT "B"
    60..69  : OUTPUT "C"
    OTHERWISE: OUTPUT "F"
END CASE

Iteration · FOR (counting loop)

sum ← 0
FOR i ← 1 TO 10
    sum ← sum + i
END FOR
OUTPUT sum     // 55

Iteration · WHILE (pre-test loop)

i ← 1
sum ← 0
WHILE i <= 10
    sum ← sum + i
    i ← i + 1
END WHILE
OUTPUT sum

Iteration · REPEAT … UNTIL (post-test loop)

Used in textbooks based on Pascal. Always executes at least once.

i ← 1
sum ← 0
REPEAT
    sum ← sum + i
    i ← i + 1
UNTIL i > 10
OUTPUT sum

⚠️ Nested loops are NOT in the compulsory part

The C&A Guide says clearly:

"Sequence, selection and iteration (nested loop is not required) constructs to create a program."

Nested loops belong to Elective 2C. In Module D you only need single-layer loops.

Combining structures

Most real algorithms combine all three. Example: count students who passed in a class of N:

INPUT n
pass_count ← 0
FOR i ← 1 TO n
    INPUT mark
    IF mark >= 50 THEN
        pass_count ← pass_count + 1
    END IF
END FOR
OUTPUT pass_count

Common student mistakes

• Forgetting END IF / END FOR / END WHILE.
• Using IF a = b for assignment (assignment uses ←).
• Writing nested loops in compulsory pseudocode (it's a 2C-only construct).
• Infinite loops — forgetting to update the loop control variable.

Exam-style question

Q (5 marks): Write pseudocode that reads a list of 20 numbers and outputs the count of numbers greater than 10.

Sample answer:

count ← 0
FOR i ← 1 TO 20
    INPUT n
    IF n > 10 THEN
        count ← count + 1
    END IF
END FOR
OUTPUT count

Key takeaways

• Three structures: sequence, selection, iteration.
• Selection: binary and multi-way.
• Iteration: FOR (counting), WHILE (pre-test), REPEAT-UNTIL (post-test).
• No nested loops in compulsory part.

➡️ Next: 2.5 Trace Tables