2.4. If Statements

The if statement lets us execute a section of code if a specified condition is True.

The structure of an if statement is:

if condition:
    # code you execute if condition is true

Take note of the following:

• if is a keyword

• The condition must evaluate to either True or False

• The code inside the if statement only executes if the condition is True

• : is placed at the end of the condition

• The code inside the if statement must be indented. The indentation defines the code block. This allows you to put multiple lines inside the if statement.

• The indentation can be done using tab or spaces, as long as you’re consistent!

Here is an example of a simple if statement.

x = -2

if x < 0:
    print('x is a negative number')

x is a negative number

In this example the condition is True so the print statement runs.

This is how we can represent this code diagrammatically. The green line indicates the ‘path’ the code takes.

Let’s look at another example.

x = 1

if x < 0:
    print('x is a negative number')

In this example the condition is False so the print statement does not run.

Question 1

What do you think the output of the following code will be?

weather = 'rainy'

if weather == 'rainy':
    print('Take an umbrella!')

if weather == 'sunny':
    print('Take a hat!')

Solution

In this example we have two if statements, one after the after. If you look at the condition in the first if statement, this evaluates to True so the program prints

Take an umbrella!

The second if statement however evaluates to False, so nothing happens.

Question 2

What do you think the output of the following code will be?

status = 'still cooking'

if status == 'finished':
    print('Cake is done.')
    print('Take it out of the oven!')
print('Yum cake!')

Solution

Solution is locked

Code challenge: Big Number

Write a program that reads in a number from the user. If that number is greater than 100 your program should output:

That's a big number!

Example 1

Enter a number: 107
That's a big number!

Example 2

Enter a number: 2

Solution

Solution is locked

Code challenge: 3 is Lucky!

Write a program that asks the user for a number. If that number is 3, the program should say 3 is lucky! If the user enters any other number, the program should say nothing.

Example 1

Enter a number: 3
3 is lucky!

Example 2

Enter a number: 5

Solution

Solution is locked

Code challenge: Favourite Colour

Write a program that asks for the user’s favourite colour. If their favourite colour is red, the program should output:

That's my favourite colour!

Regardless of what colour the user says, the program should also say:

What a nice colour

Here are some examples of how your code should run.

Example 1

What is your favourite colour? red
That's my favourite colour!
What a nice colour

Example 2

What is your favourite colour? blue
What a nice colour

Hint

Don’t forget to take note of the spelling!

Solution

Solution is locked

Code challenge: Temperature

Write a program that reads in today’s temperature (in degrees Celsius) from the user.

If the temperature is less than 15 degrees the program should output:

Take a jumper!

If the temperature is greater than 25 degrees the program should output:

Take a fan.

You can assume all temperatures are given to the nearest degree.

Example 1

What's the temperature like today? 5
Take a jumper!

Example 2

What's the temperature like today? 30
Take a fan.

Example 3

What's the temperature like today? 20

Solution

Solution is locked