4.2 If-else statements

Learning objectives

By the end of this section you should be able to

• Identify which operations are performed when a program with if and if-else statements is run.
• Identify the components of an if and if-else statement and the necessary formatting.
• Create an if-else statement to perform an operation when a condition is true and another operation otherwise.

if statement

If the weather is rainy, grab an umbrella! People make decisions based on conditions like if the weather is rainy, and programs perform operations based on conditions like a variable's value. Ex: A program adds two numbers. If the result is negative, the program prints an error.

A condition is an expression that evaluates to true or false. An if statement is a decision-making structure that contains a condition and a body of statements. If the condition is true, the body is executed. If the condition is false, the body is not executed.

The if statement's body must be grouped together and have one level of indentation. The PEP 8 style guide recommends four spaces per indentation level. The Python interpreter will produce an error if the body is empty.

if age < 12:
  print("Discount for children available")

if num < 0:
  num = 25
if num < 100:
  num = num + 50

positive_num = int(input("Enter a positive number:"))
if positive_num < 0:
  print("Negative input set to 0")
positive_num = 0

if-else statement

An if statement defines actions to be performed when a condition is true. What if an action needs to be performed only when the condition is false? Ex: If the restaurant is less than a mile away, we'll walk. Else, we'll drive.

An else statement is used with an if statement and contains a body of statements that is executed when the if statement's condition is false. When an if-else statement is executed, one and only one of the branches is taken. That is, the body of the if or the body of the else is executed. Note: The else statement is at the same level of indentation as the if statement, and the body is indented.

if-else statement template:

if x >= 15:
  # Do something
else:
  # Do something else

if x > 30:
  y = x - 10
else:
  y = x + 10

if x < 50:
  y = y / 2
else:
  y = y * 2
y = y + 5