Module 5: Conditional Statements

In this module, we will learn how to make decisions in our code. Conditional statements allow us to run certain blocks of code only when specific conditions are met.

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5.1 The if Statement

The code inside an if block runs only if the condition is True.

Syntax:

if condition:
    # code to run if condition is True

:::important Indentation (Spacing) Python uses spaces (indentation) to group code blocks. The code under the if statement must be shifted in by 4 spaces. If you do not indent, Python will throw an IndentationError. :::

Example:

age = 20
if age >= 18:
    print("You are eligible to vote.") # 4 spaces indentation

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5.2 The if else Statement

Use if-else to run one block of code if the condition is True, and a different block if the condition is False.

Example:

age = 15
if age >= 18:
    print("You are eligible to vote.")
else:
    print("You are not eligible to vote yet.")

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5.3 The if elif else Statement

When you have multiple conditions to check, use elif (short for "else if").

Grade Checking Example:

score = 85

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

In this structure, Python checks conditions from top to bottom. It runs the block for the first condition that is True and skips the rest.

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5.4 Nested Conditions

Writing an if statement inside another if statement is called a Nested Condition.

Example:

number = 12

if number > 0:
    print("Positive Number")
    if number % 2 == 0:
        print("And it is Even.")
    else:
        print("And it is Odd.")
else:
    print("Negative Number or Zero")

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5.5 Practical Examples

Here are some common examples useful for practice:

1. Even or Odd Checker:

num = int(input("Enter a number: "))
if num % 2 == 0:
    print(f"{num} is an Even number.")
else:
    print(f"{num} is an Odd number.")

2. Leap Year Checker:

year = int(input("Enter a year: "))
if (year % 4 == 0 and year % 100 != 0) or (year % 400 == 0):
    print(f"{year} is a Leap Year.")
else:
    print(f"{year} is not a Leap Year.")

:::tip Think About It Look at the leap year logic. We combined the logical operators and and or to check multiple conditions in a single line! :::