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.

Checkpoint: Example: Quantity check

Using boolean variables

A Boolean variable already has a value of True or False and can be used directly in a condition rather than using the equality operator. Ex: if is_raining == True: can be simplified to if is_raining:.

Concepts in Practice \(\PageIndex{1}\): Using if statements

Given the following, which part is the condition?

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

1. age
2. age < 12
3. print("Discount for children available")

Answer

b. age < 12 is between "if" and ":" and is an expression that evaluates to True or False .

Concepts in Practice \(\PageIndex{2}\): Using if statements

Given the following, which lines execute if the condition is true?

1	print("Have a great day.")
2	if is_raining:
3	print("Don't forget an umbrella!")
4	print("See you soon.")

1. 1, 2, 3
2. 1, 2, 4
3. 1, 2, 3, 4

Answer

c. The program executes the body when the condition is true.

Concepts in Practice \(\PageIndex{3}\): Using if statements

Given the following (same as above), which lines execute if the condition is False?

1	print("Have a great day.")
2	if is_raining:
3	print("Don't forget an umbrella!")
4	print("See you soon.")

1. 1, 2, 3
2. 1, 2, 4
3. 1, 2, 3, 4

Answer

b. The body, line 3, isn't executed because the condition is false.

Concepts in Practice \(\PageIndex{4}\): Using if statements

Given num = -10, what is the final value of num?

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

1. -10
2. 40
3. 75

Answer

c. -10 < 0 is true, so num is assigned with 25 . Then 25 < 100, so num is assigned with 25 + 50, which is 75 .

Concepts in Practice \(\PageIndex{5}\): Using if statements

Given input 10, what is the final value of positive_num?

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

1. 10
2. 0
3. Error

Answer

b. Line 4 is not indented, so positive_num is always assigned with 0 . The positive input value is lost.

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:

1	# Statements before
2
3	if condition:
4	# Body
5	else:
6	# Body
7
8	# Statements after

Checkpoint: Example: Trivia question

Concepts in Practice \(\PageIndex{6}\): Exploring if-else statements

Given the following code, the else branch is taken for which range of x?

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

1. x >= 15
2. x <= 15
3. x < 15

Answer

c. The if branch is taken when x >= 15, so the else branch is taken when x is not greater than or equal to 15 .

Concepts in Practice \(\PageIndex{7}\): Exploring if-else statements

Given x = 40, what is the final value of y?

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

1. 30
2. 40
3. 50

Answer

a. 40 > 30, so the body of the if statement is executed. y = 40 - 10 = 30.

Concepts in Practice \(\PageIndex{8}\): Exploring if-else statements

Given y = 50, which is not a possible final value of y?

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

1. 30
2. 55
3. 105

Answer

b. One of the branches is always taken in an if-else statement. Depending on x's value, y will either have a final value of 30 or 105 .

Try It: Improved division

The following program divides two integers. Division by 0 produces an error. Modify the program to read in a new denominator (with no prompt) if the denominator is 0.

Interactive Code

num = int(input())
den = int(input())

result = num / den
print(result)

Input

10
0
5

Answer

num = int(input())
den = int(input())

if den == 0:
den = int(input())

result = num / den
print(result)

Try It: Converting temperature units

The following program reads in a temperature as a float and the unit as a string: "f" for Fahrenheit or "c" for Celsius.

Calculate new_temp, the result of converting temp from Fahrenheit to Celsius or Celsius to Fahrenheit based on unit. Calculate new_unit: "c" if unit is "f" and "f" if unit is "c".

Conversion formulas:

• Degrees Celsius = (degrees Fahrenheit - 32) * 5/9
• Degrees Fahrenheit = (degrees Celsius * 5/9) + 32

Interactive Code

temp = float(input())
unit = input() # f for Fahrenheit or c for Celsius

print(temp, unit, "is converted to", new_temp, new_unit)

Input

68
f

Answer

temp = float(input())
unit = input() # f for Fahrenheit or c for Celsius

if unit == "f":
new_temp = (temp-32) * 5/9
new_unit = "c"
else:
new_temp = (temp * 9/5) + 32
new_unit = "f"

print(temp, unit, "is converted to", new_temp, new_unit)