Selection (if, elif, else, switch/case)

Selection allows a program to make decisions and follow different paths based on conditions. This is one of the three fundamental programming constructs (alongside sequence and iteration) and is required for AQA 3.2 / OCR J277 2.2.

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Why Do We Need Selection?

Without selection, programs can only execute instructions in a fixed order (sequence). Selection lets programs respond to different inputs and situations, making them useful in the real world. For example, a login system must check whether a password is correct before granting access.

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IF Statements

The simplest form of selection is the IF statement, which executes a block of code only if a condition is True.

Pseudocode

IF age >= 18 THEN
    OUTPUT "You can vote."
ENDIF

Python

if age >= 18:
    print("You can vote.")

Key Point: In Python, the colon : at the end of the if line and the indentation of the body are essential. In pseudocode, the block ends with ENDIF.

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IF...ELSE

An IF...ELSE provides two paths — one for when the condition is True and one for when it is False.

Pseudocode

IF score >= 50 THEN
    OUTPUT "Pass"
ELSE
    OUTPUT "Fail"
ENDIF

Python

if score >= 50:
    print("Pass")
else:
    print("Fail")

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IF...ELIF...ELSE (Nested / Chained Selection)

When there are more than two possible outcomes, use ELIF (short for "else if") to test additional conditions in sequence.

Pseudocode

IF score >= 90 THEN
    grade ← "A*"
ELSE IF score >= 80 THEN
    grade ← "A"
ELSE IF score >= 70 THEN
    grade ← "B"
ELSE IF score >= 60 THEN
    grade ← "C"
ELSE
    grade ← "U"
ENDIF

Python

if score >= 90:
    grade = "A*"
elif score >= 80:
    grade = "A"
elif score >= 70:
    grade = "B"
elif score >= 60:
    grade = "C"
else:
    grade = "U"

Exam Tip: The order of conditions matters. Place the most restrictive condition first. If you tested score >= 60 before score >= 90, a score of 95 would be graded as "C".

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Nested IF Statements

You can place one IF statement inside another. This is called nesting.

if age >= 18:
    if has_id:
        print("Entry allowed")
    else:
        print("ID required")
else:
    print("Too young")

Nested selection can always be rewritten using logical operators (AND, OR, NOT):

if age >= 18 and has_id:
    print("Entry allowed")
elif age >= 18:
    print("ID required")
else:
    print("Too young")

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Selection Flowchart (IF/ELIF/ELSE)

flowchart TD
    A[Start] --> B{score >= 90?}
    B -->|Yes| C[grade = A*]
    B -->|No| D{score >= 80?}
    D -->|Yes| E[grade = A]
    D -->|No| F{score >= 70?}
    F -->|Yes| G[grade = B]
    F -->|No| H{score >= 60?}
    H -->|Yes| I[grade = C]
    H -->|No| J[grade = U]
    C --> K[End]
    E --> K
    G --> K
    I --> K
    J --> K

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Logical Operators

Operator	Meaning	Example
AND	Both conditions must be True	age >= 18 AND has_ticket == True
OR	At least one condition must be True	day == "Saturday" OR day == "Sunday"
NOT	Reverses the Boolean value	NOT is_raining

In Python these are written in lowercase: and, or, not.

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SWITCH / CASE (Select...Case)

A SWITCH (or CASE) statement selects one of many blocks of code based on the value of a single variable. It is an alternative to long chains of IF...ELIF.

Pseudocode

SWITCH day:
    CASE "Monday":
        OUTPUT "Start of the week"
    CASE "Friday":
        OUTPUT "Nearly the weekend"
    CASE "Saturday":
        OUTPUT "Weekend!"
    CASE "Sunday":
        OUTPUT "Weekend!"
    DEFAULT:
        OUTPUT "Midweek"
ENDSWITCH

Python

Python 3.10+ introduced match...case:

match day:
    case "Monday":
        print("Start of the week")
    case "Friday":
        print("Nearly the weekend")
    case "Saturday" | "Sunday":
        print("Weekend!")
    case _:
        print("Midweek")

Exam Tip: You may be asked to convert between IF...ELIF chains and SWITCH/CASE statements. Make sure you can do both fluently.

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Comparison: IF vs SWITCH

Feature	IF...ELIF...ELSE	SWITCH/CASE
Tests	Any condition	A single variable against fixed values
Range checks	Yes (score >= 50)	No (exact matches only)
Flexibility	More flexible	Cleaner for many exact-match options
Readability	Can become long	Easier to read for menus

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Common Exam Mistakes

1. Missing ENDIF / incorrect indentation — examiners will penalise you.
2. Using = instead of == for comparison in Python.
3. Overlapping conditions — ensure your elif conditions don't accidentally include values already covered.
4. Forgetting the ELSE / DEFAULT — always consider what happens when none of the conditions match.

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Summary

• Selection lets a program choose different paths based on conditions.
• IF...THEN...ENDIF for a single condition.
• IF...THEN...ELSE...ENDIF for two paths.
• IF...ELIF...ELSE for multiple paths.
• SWITCH/CASE for matching a variable against many fixed values.
• Logical operators (AND, OR, NOT) combine conditions.
• Always consider the order of conditions and include a default/else case.

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Extended Worked Examples

Example 1: Cinema ticket price calculator

A cinema charges different prices depending on the customer's age. Children under 5 are free, under 16 pay GBP 5, adults pay GBP 10, and over-65s pay GBP 7.

age = int(input("Customer age: "))

if age < 5:
    price = 0
elif age < 16:
    price = 5
elif age < 65:
    price = 10
else:
    price = 7

print(f"Ticket price: GBP {price}")

Trace for input 14:

Condition	Evaluates to	Action
age < 5	False	skip
age < 16	True	price = 5, exit chain
age < 65	skipped	—
else	skipped	—

Output: Ticket price: GBP 5