Selection (if/else) and logic

Selection statements allow programs to make decisions and execute different code paths based on conditions. This is fundamental to creating intelligent, responsive software.

Key Terms:

Conditional statements (if/else) evaluate Boolean expressions to determine program flow. An if statement executes code only when a condition is true. An else clause provides an alternative path when the condition is false. else if (or elif) allows testing multiple conditions sequentially.

Boolean expressions return true or false values. They use relational operators: == (equal to), != (not equal), > (greater than), < (less than), >= (greater than or equal), <= (less than or equal).

Logical operators combine multiple conditions:

• AND (&& or and): Both conditions must be true
• OR (|| or or): At least one condition must be true
• NOT (! or not): Inverts the Boolean value

Truth Tables (essential for Cambridge exams):

Nested selection places if statements inside others, enabling complex decision-making. The general syntax follows:

if (condition1)
    // code block
else if (condition2)
    // code block
else
    // default code

if (condition1)
    // code block
else if (condition2)
    // code block
else
    // default code

Order of evaluation matters: logical operators follow precedence (NOT > AND > OR). Use parentheses to control evaluation order explicitly.

Cambridge Note: Examiners expect precise use of terminology. "Selection" is preferred over "branching" or "decision-making" in formal answers.

Selection statements mirror everyday decision-making. Think of a traffic light system: IF light is red THEN stop, ELSE IF light is amber THEN prepare to stop, ELSE (green) proceed. This nested logic ensures only one action occurs.

Real-World Application 1: Authentication Systems

When you log into a website, the system uses selection:

if (username correct AND password correct)
    grant access
else if (username correct AND password incorrect)
    display "wrong password"
else
    display "account not found"

if (username correct AND password correct)
    grant access
else if (username correct AND password incorrect)
    display "wrong password"
else
    display "account not found"

The AND operator ensures both credentials validate. Using OR would create a security vulnerability—imagine granting access with just a correct username!

Real-World Application 2: E-commerce Discount Logic

Online stores calculate prices using nested selection:

if (customer is VIP)
    discount = 20%
else if (order total > £100)
    discount = 15%
else if (first purchase)
    discount = 10%
else
    discount = 0%

if (customer is VIP)
    discount = 20%
else if (order total > £100)
    discount = 15%
else if (first purchase)
    discount = 10%
else
    discount = 0%

Notice the order matters: VIP status checks first, preventing customers getting lesser discounts.

Analogy: A Restaurant Menu Decision Tree

Selecting a meal follows logical steps: IF vegetarian THEN show veggie options, ELSE IF allergic to nuts THEN filter nut dishes, ELSE show full menu. Each condition narrows choices systematically.

Short-Circuit Evaluation optimizes performance: In A AND B, if A is false, B isn't checked (result must be false). In A OR B, if A is true, B isn't evaluated. This prevents errors: if (divisor != 0 AND number/divisor > 5) safely checks division by placing the guard condition first.

Memory Aid (MNEMONIC): "AND needs ALL, OR needs ONE" helps remember logical operator requirements.

Example 1: Grade Boundary Calculator (6 marks)

Question: Write pseudocode that inputs a percentage mark and outputs a grade. A (≥70), B (60-69), C (50-59), D (40-49), F (<40).

Solution:

INPUT mark
IF mark >= 70 THEN
    OUTPUT "Grade A"
ELSE IF mark >= 60 THEN
    OUTPUT "Grade B"
ELSE IF mark >= 50 THEN
    OUTPUT "Grade C"
ELSE IF mark >= 40 THEN
    OUTPUT "Grade D"
ELSE
    OUTPUT "Grade F"
ENDIF

INPUT mark
IF mark >= 70 THEN
    OUTPUT "Grade A"
ELSE IF mark >= 60 THEN
    OUTPUT "Grade B"
ELSE IF mark >= 50 THEN
    OUTPUT "Grade C"
ELSE IF mark >= 40 THEN
    OUTPUT "Grade D"
ELSE
    OUTPUT "Grade F"
ENDIF

Examiner Note: Conditions check from highest to lowest—crucial! If reversed, mark 75 would match "≥40" first, incorrectly outputting "D". Award 2 marks for correct structure, 2 for accurate conditions, 2 for proper output.

Example 2: Leap Year Validator (8 marks)

Question: A year is a leap year if divisible by 4, EXCEPT century years must be divisible by 400. Write code for year 2024, 1900, 2000.

Solution:

year = int(input("Enter year: "))
if year % 400 == 0:
    print("Leap year")
elif year % 100 == 0:
    print("Not leap year")
elif year % 4 == 0:
    print("Leap year")
else:
    print("Not leap year")

year = int(input("Enter year: "))
if year % 400 == 0:
    print("Leap year")
elif year % 100 == 0:
    print("Not leap year")
elif year % 4 == 0:
    print("Leap year")
else:
    print("Not leap year")

Step-by-Step Logic:

1. Check 400 divisibility first (handles 2000 ✓)
2. Then 100 divisibility (catches 1900 ✗)
3. Finally 4 divisibility (covers 2024 ✓)
4. Order is critical!

Test Results: 2024→Leap, 1900→Not leap, 2000→Leap

Examiner Note: Award 3 marks for correct logic sequence, 2 for modulo operations, 2 for testing all paths, 1 for appropriate output. Common error: checking divisibility by 4 before exceptions.