Use pseudocode to write: an 'IF' statement including the 'ELSE' clause and nested IF statements

Topic 11.2 – Constructs (Cambridge AS & A‑Level Computer Science 9618)

Learning objectives

• Write correct pseudocode for the selection structures required by the syllabus:

  • Simple selection (IF‑ELSE)
  • Multiple selection (CASE / SWITCH)
  • Nested IF statements

• Explain why a particular selection structure is the most appropriate for a given problem (AO2, AO3).

1. Simple selection – IF‑ELSE

1.1 Formal syntax

1.2 Simple example – Pass / Fail

IF mark ≥ 40 THEN
OUTPUT "Pass"
ELSE
OUTPUT "Fail"
END IF

1.3 When to use IF‑ELSE

• Only two possible outcomes (true / false).
• The decision depends on a logical expression, not a single variable value.
• When the “else” part is required to handle the false case.

1.4 Common pitfalls

1. Forgetting END IF – the compiler cannot determine where the block finishes.
2. Using the assignment operator (=) instead of a comparison operator (= =, ≠, <, >).
3. Inconsistent indentation – makes nested structures hard to read.
4. Placing ELSE without a preceding IF or writing two ELSE clauses.

2. Nested IF statements

2.1 General pattern

IF outer_condition THEN
IF inner_condition THEN
… statements …
ELSE
… statements …
END IF
ELSE
… statements …
END IF

2.2 Example – Grading with two criteria

IF score ≥ 70 THEN
OUTPUT "Distinction"
ELSE
IF score ≥ 50 THEN
OUTPUT "Merit"
ELSE
OUTPUT "Pass"
END IF
END IF

2.3 Tips for readable nesting

• Indent each level by a consistent amount (e.g., 4 spaces or one tab).
• Keep nesting to a maximum of three levels; deeper logic is often clearer as a CASE statement or a separate function.
• Comment the purpose of each test, especially when the conditions are complex.
• Use meaningful variable names (e.g., is_premium rather than m).

3. Multiple‑selection – CASE / SWITCH (brief overview)

3.1 When to prefer CASE

• The decision is based on a single expression that can take several discrete values.
• Often clearer and less error‑prone than a long chain of IF‑ELSE statements.

3.2 Formal syntax

3.3 Example – Grade classification

CASE mark OF
90 TO 100: OUTPUT "A+"
80 TO 89 : OUTPUT "A"
70 TO 79 : OUTPUT "B"
60 TO 69 : OUTPUT "C"
OTHER    : OUTPUT "Fail"
END CASE

4. Linking selection structures to the assessment objectives

• AO1 – Knowledge: Recognise the four families of control structures and their formal syntax.
• AO2 – Analysis: Choose the most suitable structure for a problem description (e.g., IF‑ELSE for two outcomes, CASE for many discrete values, nesting when decisions depend on several independent conditions).
• AO3 – Design & Development: Write clear, correctly indented pseudocode that follows the Cambridge conventions.

5. Practice exercise – Shipping‑cost calculator

Problem statement

• If the order total is at least £100, shipping is free.
• Otherwise, if the customer is a “Premium” member, shipping costs £5.
• Otherwise, shipping costs £10.

5.1 Solution using a single IF‑ELSE chain

IF order_total ≥ 100 THEN
shipping_cost ← 0
ELSE
IF membership = "Premium" THEN
shipping_cost ← 5
ELSE
shipping_cost ← 10
END IF
END IF
OUTPUT "Shipping cost: £", shipping_cost

5.2 Same logic written as a nested IF (different layout)

IF order_total ≥ 100 THEN
shipping_cost ← 0
ELSE
IF membership = "Premium" THEN
shipping_cost ← 5
ELSE
shipping_cost ← 10
END IF
END IF
OUTPUT "Shipping cost: £", shipping_cost

5.3 Alternative using CASE (when the first test fails)

IF order_total ≥ 100 THEN
shipping_cost ← 0
ELSE
CASE membership OF
"Premium": shipping_cost ← 5
OTHER   : shipping_cost ← 10
END CASE
END IF
OUTPUT "Shipping cost: £", shipping_cost

5.4 Checklist for the exam

• Correct use of IF … THEN, ELSE and END IF.
• Proper indentation to show nesting.
• Use of comparison operators (≥, =) rather than assignment.
• All variables are initialised before they are used.

6. Summary – Choosing the right selection structure