Unit 11.1A · Term 1

Boolean Logic & Branching

Programs need to make decisions. Boolean logic determines whether conditions are True or False, and branching structures (if/elif/else) allow the program to take different paths based on those conditions.

Learning Objectives

11.1.1.7 Apply logic operations to Boolean variables
11.1.1.8 Use logical operations AND, OR, NOT in selection structure
11.1.1.9 Implement the branching algorithm according to the flowchart

Lesson Presentation

11.1A-lesson-04-boolean-branching.pdf · Slides for classroom use

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Conceptual Anchor

The Traffic Light Analogy

A traffic light is a branching algorithm: IF the light is green → go; ELIF yellow → slow down; ELSE (red) → stop. The light's colour is a condition that evaluates to True or False for each check. Logical operators combine conditions: you cross AND the light is green AND no pedestrians are crossing.

Rules & Theory

Comparison Operators

Operator	Meaning	Example	Result
`==`	Equal to	`5 == 5`	`True`
`!=`	Not equal to	`5 != 3`	`True`
`>`	Greater than	`5 > 3`	`True`
`<`	Less than	`5 < 3`	`False`
`>=`	Greater or equal	`5 >= 5`	`True`
`<=`	Less or equal	`5 <= 3`	`False`

Logical Operators: AND, OR, NOT

Operator	Meaning	True when…	Example
`and`	Both conditions must be True	A is True AND B is True	`(x > 0) and (x < 100)`
`or`	At least one condition is True	A is True OR B is True (or both)	`(x == 0) or (x == 1)`
`not`	Inverts the value	A is False	`not (x > 10)`

Truth Tables

A	B	A and B	A or B	not A
True	True	True	True	False
True	False	False	True	False
False	True	False	True	True
False	False	False	False	True

if / elif / else Structure

# Pattern: if → elif → else
if condition_1:
    # executes when condition_1 is True
elif condition_2:
    # executes when condition_1 is False AND condition_2 is True
else:
    # executes when ALL above conditions are False

# Example: Grade classifier
score = int(input("Enter score: "))

if score >= 90:
    print("Grade: A")
elif score >= 80:
    print("Grade: B")
elif score >= 70:
    print("Grade: C")
elif score >= 60:
    print("Grade: D")
else:
    print("Grade: F")

Flowchart → Code

A flowchart diamond (decision box) = if condition. The Yes arrow = the indented block. The No arrow = elif or else. Always trace the flowchart path from top to bottom, matching each diamond to a condition in code.

Worked Examples

1 Boolean Variables

# Boolean variables hold True/False
is_raining = True
has_umbrella = False

# Logical expressions
can_go_outside = not is_raining or has_umbrella
print(can_go_outside)   # False

# Explanation:
# not True → False
# False or False → False

2 Compound Conditions with AND/OR

age = int(input("Enter age: "))
has_id = input("Do you have ID? (yes/no): ") == "yes"

# AND — both must be true
if age >= 18 and has_id:
    print("Access granted")
else:
    print("Access denied")

# OR — either one is enough
day = input("Enter day: ")
if day == "Saturday" or day == "Sunday":
    print("Weekend!")
else:
    print("Weekday")

3 Nested if — Cinema Ticket Pricing

age = int(input("Enter age: "))
is_student = input("Student? (yes/no): ") == "yes"

if age < 12:
    price = 500
elif age >= 12 and age < 18:
    if is_student:
        price = 700
    else:
        price = 900
else:
    price = 1200

print("Ticket price:", price, "tenge")

4 Flowchart Implementation — Number Classifier

Flowchart: Input number → Is it positive? → If yes: Is it even? → Print result

num = int(input("Enter a number: "))

if num > 0:
    if num % 2 == 0:
        print(num, "is positive and even")
    else:
        print(num, "is positive and odd")
elif num < 0:
    print(num, "is negative")
else:
    print("Zero")

# Trace for input 7:
# num = 7
# 7 > 0 → True → enter first if
# 7 % 2 == 0 → 1 == 0 → False → enter else
# Output: "7 is positive and odd"

Pitfalls & Common Errors

= vs == Confusion

= is assignment, == is comparison. Writing if x = 5: is a SyntaxError. It must be if x == 5:.

Forgetting the Colon

if x > 5 → SyntaxError. Every if, elif, else must end with :

Indentation Errors

Python uses indentation to define blocks. Inconsistent indentation causes IndentationError. Use 4 spaces consistently.

Wrong Operator Precedence

not has higher precedence than and, which is higher than or. Use parentheses when combining: (a or b) and c is different from a or (b and c).

Pro-Tips for Exams

Exam Strategy

When tracing a flowchart, follow ONLY one path at a time based on the input value
For and: if the first condition is False, the whole expression is False (short-circuit)
For or: if the first condition is True, the whole expression is True (short-circuit)
Remember that elif is checked only if ALL previous conditions were False
Draw a truth table if a complex boolean expression confuses you

Graded Tasks

Remember

Complete the truth table for: A and (B or not C) for all combinations of A, B, C.

Understand

Explain the difference between if/if/if (three separate ifs) and if/elif/else. Give an example where they produce different results.

Apply

Write a program that reads a year and determines if it is a leap year. (Divisible by 4, but not 100, unless also divisible by 400.)

Apply

Implement the following flowchart: Input a temperature → If temp > 30: "Hot" → Else if temp > 20: "Warm" → Else if temp > 10: "Cool" → Else: "Cold".

Analyze

Given x = 15, trace and determine the output of: if x > 10 and x < 20: print("A") elif x > 5 or x == 15: print("B") else: print("C")

Create

Design a quiz program that asks 3 true/false questions, counts the score, and outputs a grade based on the number correct (3 = Excellent, 2 = Good, 1 = Fair, 0 = Fail).

Self-Check Quiz

1. What is the result of True and False?

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2. What is the result of True or False?

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3. What is not True?

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4. What is the difference between = and ==?

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5. Can you have elif without if?

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