Module III — Flow Control

1. Introduction to Decision Making

Decision-making statements in Python allow a program to choose different paths of execution based on whether a condition is True or False.

In real life, we make decisions like:

• If it rains, I will carry an umbrella.
• If marks ≥ 90, grade = A; else if marks ≥ 75, grade = B; else grade = C.

Python provides the following decision-making statements:

Statement	Purpose
if	Executes a block only if the condition is True
if-else	Executes one block if True, another if False
if-elif-else	Checks multiple conditions in sequence
Nested if	An if inside another if

2. The if Statement

Syntax

if condition:
    statement(s)

How it works

1. The condition is evaluated.
2. If it is True, the indented block is executed.
3. If it is False, the block is skipped.

Conceptual Flowchart

Example 1

Check if a number is positive

# Program to check if a number is positive

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

if num > 0:
    print("The number is positive.")

print("Program ended.")

Explanation: If the user enters 7, the condition num > 0 is True, so the message is printed. The last line print("Program ended.") is outside the if block (no indentation), so it always executes.

Example 2

Check if a person is eligible to vote

# Program to check voting eligibility

age = int(input("Enter your age: "))

if age >= 18:
    print("You are eligible to vote.")

Note: If age is less than 18, nothing is printed because there is no else block.

3. The if-else Statement

Syntax

if condition:
    statement(s)   # executes if condition is True
else:
    statement(s)   # executes if condition is False

How it works: Exactly one of the two blocks will execute — never both, never neither.

Conceptual Flowchart

Example 3

Check whether a number is even or odd

# Program to check even or odd

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

if num % 2 == 0:
    print(num, "is even.")
else:
    print(num, "is odd.")

Explanation: num % 2 == 0 is True for even numbers → if block runs. Otherwise → else block runs.

Example 4

Find the larger of two numbers

# Program to find the larger of two numbers

a = int(input("Enter first number: "))
b = int(input("Enter second number: "))

if a > b:
    print(a, "is larger.")
else:
    print(b, "is larger.")

4. The if-elif-else Statement

Syntax

if condition1:
    statement(s)
elif condition2:
    statement(s)
elif condition3:
    statement(s)
else:
    statement(s)   # optional

How it works:

• Conditions are checked from top to bottom.
• The first condition that is True executes its block.
• The rest are skipped.
• If none are True, the else block runs (if present).

Conceptual Flowchart

Example 5

Grade calculator

# Program to assign grade based on marks

marks = int(input("Enter marks (0-100): "))

if marks >= 90:
    grade = "A+"
elif marks >= 80:
    grade = "A"
elif marks >= 70:
    grade = "B"
elif marks >= 60:
    grade = "C"
elif marks >= 50:
    grade = "D"
else:
    grade = "F"

print("Your grade is:", grade)

Explanation: If marks = 85, marks >= 90 is False, marks >= 80 is True → grade = "A". The remaining elif and else are skipped.

Example 6

Check whether a character is a vowel, consonant, or not a letter

# Program to classify a character

ch = input("Enter a character: ")

if len(ch) != 1:
    print("Please enter exactly one character.")
elif ch.isalpha():
    if ch.lower() in "aeiou":
        print(ch, "is a vowel.")
    else:
        print(ch, "is a consonant.")
else:
    print(ch, "is not a letter.")

Note: This example uses nested if inside an elif — a preview of the next topic.

Example 7

Simple calculator using if-elif-else

# Simple calculator

a = float(input("Enter first number: "))
b = float(input("Enter second number: "))
op = input("Enter operator (+, -, *, /): ")

if op == "+":
    result = a + b
elif op == "-":
    result = a - b
elif op == "*":
    result = a * b
elif op == "/":
    if b != 0:
        result = a / b
    else:
        result = "Error: Division by zero"
else:
    result = "Invalid operator"

print("Result:", result)

5. Nested if Statements

Definition: A nested if is an if (or if-else, or if-elif-else) statement placed inside another if, elif, or else block.

Syntax

if condition1:
    if condition2:
        statement(s)
    else:
        statement(s)
else:
    statement(s)

When to use:

• When a decision depends on multiple related conditions.
• When the second condition should only be checked if the first is True.

Conceptual Flowchart

Example 8

Find the largest of three numbers using nested if

# Program to find the largest of three numbers

a = int(input("Enter first number: "))
b = int(input("Enter second number: "))
c = int(input("Enter third number: "))

if a >= b:
    if a >= c:
        largest = a
    else:
        largest = c
else:
    if b >= c:
        largest = b
    else:
        largest = c

print("The largest number is:", largest)

Explanation: First, compare a and b. If a >= b, then compare a with c. Otherwise, compare b with c.

Example 9

Check whether a year is a leap year

# Program to check leap year

year = int(input("Enter a year: "))

if year % 4 == 0:
    if year % 100 == 0:
        if year % 400 == 0:
            print(year, "is a leap year.")
        else:
            print(year, "is not a leap year.")
    else:
        print(year, "is a leap year.")
else:
    print(year, "is not a leap year.")

Logic: A year is a leap year if:

• It is divisible by 4, AND
• If divisible by 100, it must also be divisible by 400.

Example 10

Check whether a number is positive, negative, or zero — and even/odd

# Nested if: sign and parity

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

if num > 0:
    print("Positive number")
    if num % 2 == 0:
        print("Even")
    else:
        print("Odd")
elif num < 0:
    print("Negative number")
    if num % 2 == 0:
        print("Even")
    else:
        print("Odd")
else:
    print("Zero")

6. Comparison of Decision-Making Statements

Statement	Use Case	Blocks Executed
if	Single condition	0 or 1
if-else	Two-way decision	Exactly 1
if-elif-else	Multiple mutually exclusive conditions	Exactly 1 (or 0 if no else)
Nested if	Hierarchical / dependent conditions	Depends on conditions

7. Common Mistakes to Avoid

Mistake	Correction
Forgetting the colon :	Always end the condition with :
Incorrect indentation	Use consistent 4 spaces for each block
Using = instead of ==	= is assignment; == is comparison
Writing elif as else if	Python uses elif
Overlapping conditions in elif	Order matters — put the most specific first
Empty block	Use pass if a block must be empty

Example of pass

if x > 0:
    pass   # to be implemented later
else:
    print("Non-positive")

8. Summary Table

Concept	Syntax	Example
if	if cond:	if x > 0: print("+")
if-else	if cond: ... else: ...	if x%2==0: ... else: ...
if-elif-else	if c1: ... elif c2: ... else: ...	Grade calculator
Nested if	if c1: if c2: ...	Largest of 3 numbers

9. Practice Exercises

1. Write a program to check whether a number is divisible by both 3 and 5.
2. Write a program to input a character and check whether it is uppercase, lowercase, a digit, or a special character.
3. Write a program to find the smallest of four numbers using nested if.
4. Write a program that takes a student's marks in 5 subjects and prints the grade based on average.
5. Write a program to check whether a triangle is equilateral, isosceles, or scalene based on its three sides.
6. Write a program to determine the roots of a quadratic equation ax² + bx + c = 0 (real or complex).
7. Write a program to simulate a simple ATM menu using if-elif-else.

10. Key Takeaways

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Module III — Flow Control · Topic (a): Decision Making · Detailed Teaching Notes