while loop - countdown

Introduction

Introductory Problem

Ask the user what food they would like to eat everyday. Using a while loop, print their favorite food 5 times.

intro.py

x = 0
food = input("What type of food do you like to eat? ")
while x < 5:
    print(food)
    x = x + 1

while loop

A while loop executes an indented block of code, or instructions, repeatedly while a condition is true. Previously, you learned about if statements that executed an indented block of code while a condition was true. You can think of a while loop like an if condition but the indented block of code executes more than once. Hence, a loop.

Variable Roles

Recall that a stepper variable iterates, or loops, a specific number of times.

Programmers usexorias a stepper variable.

while loop Examples

Example 1

Rewrite the Do Now to print the user’s favorite food 5 times by decrementing the variable rather than incrementing.

Steps:

• Initialize the stepper variable x to 5.

• Ask the user what they like to eat everyday.

• Using a while loop, create a condition that will execute 5 times.

• Print variable.

• Decrement the stepper variable.

example1.py

x = 3
food = input("Favorite Food: ")
while x > 0:
    print(food)
    x = x - 1

Example 2

Using a while loop, print even numbers from 10 to 1.

What value can we initialize the stepper variable to?

example2.py

x = 10
while x > 0:
    print(x)
    x = x - 2

Example 3

Using a while loop, ask the user for a number 3 times. Where in the program should we ask the user for the number? Inside the loop, or outside the loop?

Create a temporary variable named squared that squares the number. Create another temporary variable named cubed that cubes the number.

Print the values of squared and cubed.

example3.py

x = 3
while x > 0:
    num = int(input("Number: "))
    squared = num * num
    print("Squared:",squared)
    cubed = num ** 3
    print("Cubed:",cubed)
    x = x - 1

Example 4

You eat a Pepperoni Pizza slice which is 400 calories and now want to burn off these calories for 10 minutes. You burn 11 calories per minute running. Create a chart to represent how many minutes you have left to exercise and how many calories you have left to burn off.

Steps:

• Create a variable named calories and initialize it to 400.

• Before the while loop, add the following lines to create your chart.

  • print("Mins\tCalories")

  • "\t" means tab.

example4.py

print("Mins\tCalories")
calories = 400
x = 10
while x >= 0:
    print(x,"\t",calories)
    calories = calories - 11
    x = x - 1

Conclusion

• In your own words, what is a while loop? What is a real life example when you count down?

• Explain the role of the stepper variable when the while loop counts down.