Selection

Spec & Goals 3 min

AQA Spec 3.2.2 · Programming constructs — selection

By the end of this lesson you can:

1. Explain what selection is and identify the condition that controls it.
2. Write an IF … THEN … ELSE … ENDIF statement, including multi-way selection with ELSE IF.
3. Trace a program to state which branch runs for a given input.

Warm-Up 5 min

Last lesson you stored values in variables and constants. Now we make programs decide what to do with them.

Quick starter

You buy nasi lemak for RM 5 and pay with RM 10. A till must decide whether to give change. Which of these is the condition the till tests?

• Print the receipt.
• Is the amount paid greater than the price?
• Add the items together.

Key Concept — making decisions with selection 14 min

Selection chooses which block of code runs, based on whether a condition is true or false. It is one of the three basic programming constructs.

The basic IF statement

An IF statement runs one block when the condition is true, and an optional ELSE block when it is false.

AQA pseudo-code

IF age ≥ 18 THEN
  OUTPUT 'Adult'
ELSE
  OUTPUT 'Minor'
ENDIF

Multi-way selection with ELSE IF

When there are more than two paths, add ELSE IF branches. The program tests each condition in order and runs the first one that is true.

IF speed > 110 THEN
  OUTPUT 'Too fast'
ELSE IF speed > 90 THEN
  OUTPUT 'Watch your speed'
ELSE
  OUTPUT 'Speed OK'
ENDIF

Nested IF

A nested IF is an IF statement placed inside another. The inner test only happens when the outer condition is true.

IF loggedIn = True THEN
  IF isAdmin = True THEN
    OUTPUT 'Admin menu'
  ELSE
    OUTPUT 'User menu'
  ENDIF
ENDIF

Worked Example — a grade classifier 12 min

Problem: input an exam mark out of 100 and output the grade: A for 70+, B for 60–69, C for 50–59, otherwise U.

AQA pseudo-code

mark ← USERINPUT
IF mark ≥ 70 THEN
  OUTPUT 'A'
ELSE IF mark ≥ 60 THEN
  OUTPUT 'B'
ELSE IF mark ≥ 50 THEN
  OUTPUT 'C'
ELSE
  OUTPUT 'U'
ENDIF

The same algorithm in Python

mark = int(input("Enter the mark: "))
if mark >= 70:
    print("A")
elif mark >= 60:
    print("B")
elif mark >= 50:
    print("C")
else:
    print("U")

Trace which branch runs when the user enters mark = 64:

The output is B. Once mark ≥ 60 is true, the remaining branches are skipped.

Try It Yourself 12 min

📝 Exam Practice 10 min

Answer the way the examiner expects — the command word and the marks tell you how much to write.

total ← USERINPUT
IF __________ THEN
  OUTPUT total * 0.9
__________
  OUTPUT __________
ENDIF

Recap & Key Terms 3 min

Selection lets a program decide what to do. An IF statement tests a condition and runs one branch. ELSE IF gives multi-way choices, and a nested IF puts one decision inside another.

Selection

A construct that uses a condition to choose which code to run.

Condition

An expression that is either true or false, used to control selection.

IF statement

The structure IF … THEN … ELSE … ENDIF that carries out selection.

Nested IF

An IF statement contained inside another IF statement.

Homework 1 min

Task (≤ 15 min): Write AQA pseudo-code that inputs a year and outputs whether it is a leap year (divisible by 4) using selection.

year ← USERINPUT
IF year MOD 4 = 0 THEN
  OUTPUT 'Leap year'
ELSE
  OUTPUT 'Not a leap year'
ENDIF