2.6. If-Elif-Else Statements

To add more conditions to an if-else statement, we can use elif.

The elif statement must be used between the if and the else statements. You can also have as many elif statements as you want!

Here is how we use the elif statement:

if condition_1:
    # code to execute if condition 1 is true
elif condition_2:
    # code to execute if condition 2 is true
elif condition_3:
    # code to execute if condition 3 is true
...
else:
    # code to execute if none of the conditions are true

Take note of the following:

• elif is a keyword

• Each condition must evaluate to either True or False

• : is placed at the end of each condition

• The first condition that evaluates to True controls which section of code executes

• The code inside the elif statement must be indented. Note that only the code blocks are indented. The lines beginning with if, elif or else are not indented.

Here is an example of an if-elif-else statement.

x = 1

if x < 0:
    print("x is a negative number")
elif x > 0:
    print("x is a positive number")
else:
    print("x is 0!")

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

Question 1

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

light = 'green'

if light == 'green':
    print('GO!')
elif light == 'yellow':
    print('Slow down!')
elif light == 'red':
    print('Stop!')
else:
    print("That can't be right. Traffic lights can't be {}.".format(light))

Solution

In this example the condition in the first if statement evaluates to True so the program prints 'GO!'.

Question 2

Consider the two programs below. They are similar but not quite the same. If you set the variable light to 'yellow', both programs will print Slow down!.

Program 1

light = 'yellow'

if light == 'green':
    print('GO!')
if light == 'yellow':
    print('Slow down!')
if light == 'red':
    print('Stop!')

Program 2

light = 'yellow'

if light == 'green':
    print('GO!')
elif light == 'yellow':
    print('Slow down!')
elif light == 'red':
    print('Stop!')
else:
    print("That can't be right. Traffic lights can't be {}.".format(light))

Suppose we changed the value stored in the variable light. For which of the following values of light will program 1 and program 2 result in different outputs? Select all that apply.

1. light = 'green'
   

2. light = 'yellow'
   

3. light = 'red'
   

4. light = 'blue'
   

5. light = 'purple'
   

Solution

Solution is locked

Code challenge: Red team blue

Implement the algorithm illustrated in the diagram below in Python. Pay close attention to the indentation.

Solution

Solution is locked