Python Course 5 - Conditional Statements

🎯 Learning Objectives

After completing this section, you will be able to:

Use comparison operators in Boolean expressions
Write conditional statements using if, elif, and else
Use simple and nested conditional statements
Combine conditional statements with logical operators

1. Comparison Operators

Programming often involves situations where the program must be able to distinguish between different situations. In these cases we use conditional statements. The simplest conditional statement looks like this:

number = int(input("Please type in a number: "))

if number > 100:
    print("The number was greater than one hundred")
    number = 0

print("Thank you!")

Here the condition is number > 100. The code block following this header line is executed only if the condition is true.

📊 Comparison Operators

Operator	Purpose	Example
`==`	Equal to	`a == b`
`!=`	Not equal to	`a != b`
`>`	Greater than	`a > b`
`>=`	Greater than or equal to	`a >= b`
`<`	Less than	`a < b`
`<=`	Less than or equal to	`a <= b`

2. If Statements

The keyword if starts a conditional statement. It is followed by a condition and a colon. The block of code that should be executed if the condition is true comes after the colon, indented.

temperature = int(input("Please type in a temperature: "))

if temperature > 0:
    print("The temperature is above zero")

print("Done!")

Sample output (with input 15): Please type in a temperature: 15 The temperature is above zero Done!

Sample output (with input -20): Please type in a temperature: -20 Done!

Flow chart showing if statement execution

If statement flow chart

3. Indentation

Python uses indentation to define code blocks. The indented code under an if statement forms a block that is executed only when the condition is true.

⚠️ Important Indentation Rules

Many text editors will automatically indent the following line when the Enter key is pressed after a colon character. When you want to end an indented code block you can use the Backspace key to return to the beginning of the line.

Keyboard showing backspace key highlighted

Use Backspace to unindent code blocks

4. Else Statements

Often we want the program to execute some other code if the condition is false. This can be achieved with an else statement.

temperature = int(input("Please type in a temperature: "))

if temperature > 0:
    print("The temperature is above zero")
else:
    print("The temperature is below zero")

print("Done!")

Sample output (with input 15): Please type in a temperature: 15 The temperature is above zero Done!

Sample output (with input -20): Please type in a temperature: -20 The temperature is below zero Done!

If-else statement flow chart

5. Elif Statements

Often there are more than two options the program should account for. For example, the temperature could be exactly zero. This can be achieved with an elif statement.

temperature = int(input("Please type in a temperature: "))

if temperature > 0:
    print("The temperature is above zero")
elif temperature < 0:
    print("The temperature is below zero")
else:
    print("The temperature is exactly zero")

print("Done!")

Sample output: Please type in a temperature: 0 The temperature is exactly zero Done!

The keyword elif is short for "else if". There can be any number of elif statements in a conditional statement.

grade = int(input("Please type in your grade: "))

if grade == 5:
    print("Excellent!")
elif grade == 4:
    print("Very good")
elif grade == 3:
    print("Good")
elif grade == 2:
    print("Satisfactory")
elif grade == 1:
    print("Pass")
else:
    print("Fail")

6. Combining Conditions

Conditions can be combined with the logical operators and and or.

The and Operator

The keyword and combines two conditions. The combined condition is true only if both of the component conditions are true.

number = int(input("Please type in a number: "))

if number >= 5 and number <= 10:
    print("The number is between 5 and 10")

The or Operator

The keyword or also combines two conditions, but it returns true if either one or both of the conditions are true.

number = int(input("Please type in a number: "))

if number < 5 or number > 10:
    print("The number is not between 5 and 10")

The not Operator

The keyword not negates a condition.

number = int(input("Please type in a number: "))

if not (number >= 5 and number <= 10):
    print("The number is not between 5 and 10")

7. Nested Conditionals

Conditional statements can also be nested within each other.

username = input("Please type in your username: ")

if username == "jerry":
    password = input("Please type in your password: ")
    if password == "swordfish":
        print("Welcome!")
    else:
        print("Incorrect password!")
else:
    print("Incorrect username!")

⚠️ Important: While nested conditionals can be useful, too much nesting can make code difficult to read. Consider reorganizing your code if it becomes too deeply nested.

8. Programming Exercises

Complete the following exercises to practice conditional statements in Python!

📝 Exercise 1: Absolute Value (1 point)

Write a program that asks for an integer number and then prints its absolute value.

Please type in a number: -7 The absolute value is 7

# Write your code here

📝 Exercise 2: Soup or No Soup (1 point)

Write a program that asks for a temperature and then prints "Soup time!" if it's 10 degrees or colder.

Please type in a temperature: 8 Soup time!

# Write your code here

📝 Exercise 3: Orwell (1 point)

Write a program that asks for an integer number. If the number is 1984, print "Orwell".

Please type in a number: 1984 Orwell

# Write your code here

📝 Exercise 4: Greater or Equal (1 point)

Write a program that asks for two integers. Print which is greater, or if they are equal.

Please type in the first number: 5 Please type in the second number: 3 The greater number was: 5

# Write your code here

📝 Exercise 5: The Elder (1 point)

Write a program that asks for the names and ages of two persons and prints the name of the elder.

Person 1: Name: Alan Age: 26 Person 2: Name: Ada Age: 27 The elder is Ada

# Write your code here

📝 Exercise 6: Alphabetically in the Middle (1 point)

Write a program that asks for three words and prints the middle one alphabetically.

Please type in the 1st word: first Please type in the 2nd word: second Please type in the 3rd word: third The word in the middle is second

# Write your code here

📝 Exercise 7: Grade and Points (1 point)

Write a program that translates points to grades: 0-49 points = fail, 50-59 = 1, 60-69 = 2, 70-79 = 3, 80-89 = 4, 90-100 = 5.

Please type in the amount of points [0-100]: 75 Grade: 3

# Write your code here

📝 Exercise 8: FizzBuzz (1 point)

Write a program that asks for a number. If divisible by 3, print "Fizz". If by 5, print "Buzz". If by both, print "FizzBuzz". Otherwise print the number.

Number: 15 FizzBuzz

# Write your code here

🎉 Summary

You've completed Conditional Statements! You now know how to:

Use comparison operators (==, !=, >, <, >=, <=)
Write if, elif, and else statements
Combine conditions with and, or, not
Use nested conditional statements
Apply conditionals to solve real problems
Understand proper indentation in Python

Total points: 8/8

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Course 4: Arithmetic Operations

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Course 6: Loops

🐍 Python Course 5: Conditional Statements

🎯 Learning Objectives

📚 Table of Contents

1. Comparison Operators

📊 Comparison Operators

2. If Statements

3. Indentation

⚠️ Important Indentation Rules

4. Else Statements

5. Elif Statements

6. Combining Conditions

The and Operator

The or Operator

The not Operator

7. Nested Conditionals

8. Programming Exercises

🎉 Summary

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