Conditionals

CS 65: Introduction to Computer Science I

Conditional execution of code

Sometimes, you want your program to do something different based on different conditions. These are called conditional statements.

For example, consider the pay calculator. If this employer offers 50% extra pay for overtime (i.e., worked > 40 hours), how could we account for that?

wage = float(input("Enter your hourly wage: "))
hours = float(input("Enter your number of hours worked: "))

#option 1: only correct if they didn't work overtime
pay = wage*hours

#option 2: only correct if they worked overtime
#overtime_hours = hours-40
#pay = (wage*40) + (wage*1.5*overtime_hours)

print("Total pay: ",pay)

Enter your hourly wage: 10.00
Enter your number of hours worked: 45
Total pay:  450.0

Syntax for if statements

• keyword if
• a condition
• colon :
• indented block of code

total_bill = 35.63
age = float(input("What is your age? "))

if age > 65:
    print("Applying 10% senior discount")
    total_bill = total_bill * 0.9
    #any indented code is only runs
    #when the condition is True
    
#un-indented code runs no matter what
print("Your total bill is", total_bill)

What is your age? 67
Applying 10% senior discount
Your total bill is 32.067

Syntax for if-else statements

• keyword if
• a condition
• colon :
• indented block of code
• keyword else
• colon :
• indented block of code

height = int(input("Enter your height in inches: "))

if height < 60:
    print("You are not tall enough for this ride.")
else:
    print("You are tall enough for this ride.")

Enter your height in inches: 58
You are not tall enough for this ride.

Group Exercise

How could we make this work using an if-else?

wage = float(input("Enter your hourly wage: "))
hours = float(input("Enter your number of hours worked: "))

#option 1: only correct if they didn't work overtime
pay = wage*hours

#option 2: only correct if they worked overtime
#overtime_hours = hours-40
#pay = (wage*40) + (wage*1.5*overtime_hours)

print("Total pay: ",pay)

Boolean type

The Boolean type has two possible values: True and False

bool_variable = True

if bool_variable:
    print("The condition it True!")

The condition it True!

The condition for an if/if-else statement can be anything that results in a Boolean. An expression, like height < 60 that results in a Boolean is called a Boolean expression.

Some Boolean operators

• < less than
• > greater than
• <= less than or equal
  • note there is no $\leq$ button on your keyboard
• >= greater than or equal
• == equal (comparison)
  • note that = is used for assignment, they're different operators, don't confuse them!
• != not equal

secret_number = 7
guess = int(input("Guess a number: "))

if guess == secret_number:
    print("That was right, good guess!")
else:
    print("Wrong!")

Guess a number: 7
That was right, good guess!

total_bill = 121.50
party_size = int(input("Enter number of people at the table: "))
non_profit = input("Was this a business lunch for a non-profit organization? ")
 
if non_profit != "yes":
    print("Calculating 6% sales tax.")
    total_bill = total_bill * 1.06
    
if party_size >= 8:
    print("Automatically adding 15% gratuity.")
    total_bill = total_bill * 1.15
    
print("Your total is",total_bill)

Enter number of people at the table: 6
Was this a business lunch for a non-profit organization? no
Calculating 6% sales tax.
Your total is 128.79000000000002