Chapter: Conditional Statements in Python – if, elif, and else

📘 Introduction

Conditional statements in Python are used to perform different actions based on different conditions. The most commonly used are:

• if (for a single condition)
• elif (for multiple conditions)
• else (for the default action)

These help you make decisions in your code—just like we do in real life!

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🧠 Why Use Conditional Statements?

Imagine you’re writing a program to decide if someone can vote:

• If the person is 18 or older → allow voting
• Otherwise → not allowed to vote

This is exactly where if-else comes into play.

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🔹 Syntax of if Statement

if condition:
    # code block (runs only if condition is True)

✅ Example:

age = 20
if age >= 18:
    print("You are eligible to vote.")

📝 Output:

You are eligible to vote.

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🔸 Adding else for an Alternate Path

if condition:
    # if condition is True
else:
    # if condition is False

✅ Example:

age = 16
if age >= 18:
    print("You are eligible to vote.")
else:
    print("You are not eligible to vote.")

📝 Output:

You are not eligible to vote.

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🔸 Using elif for Multiple Conditions

if condition1:
    # if condition1 is True
elif condition2:
    # if condition2 is True
else:
    # if none of the above are True

✅ Example:

marks = 75

if marks >= 90:
    print("Grade: A")
elif marks >= 75:
    print("Grade: B")
elif marks >= 60:
    print("Grade: C")
else:
    print("Grade: F")

📝 Output:

Grade: B

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💡 Nested if Statements

You can place one if inside another if.

✅ Example:

age = 25
citizen = True

if age >= 18:
    if citizen:
        print("Eligible to vote.")
    else:
        print("Not a citizen, cannot vote.")
else:
    print("Too young to vote.")

📝 Output:

Eligible to vote.

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🧪 Practice Exercises

1. Even or Odd Checker
   • Take an integer input and print whether it’s even or odd.
2. Temperature Message
   • If temp > 30 → “It’s hot”
   • If temp between 20-30 → “Nice weather”
   • Else → “It’s cold”
3. Login System
   • Check if the username and password match and print “Login successful”, otherwise print “Invalid credentials”.

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✅ Quick Tips

• Conditions use comparison operators: ==, !=, <, >, <=, >=
• Indentation is mandatory after if, elif, and else (usually 4 spaces)
• Python evaluates conditions top-down, so order matters in elif chains

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🧾 Summary

Keyword	Purpose
if	Tests a condition
elif	Tests another condition if previous are False
else	Default block if none match

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🧑‍💻 Mini Project: Simple Grading System

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

if score >= 90:
    print("Grade A")
elif score >= 75:
    print("Grade B")
elif score >= 60:
    print("Grade C")
else:
    print("Grade F")

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