while loop - sentinel menu

Objective

After working through this lesson, you’ll be able to

• Write Python code using a while loop with a sentinel value to create a menu.

Introductory Problem

Using a while loop, ask the user how many snaps they’ve sent each day for the past 5 days. Print the sum of these numbers.

intro.py

sum = 0
x = 0
while x < 5:
    snaps = int(input("How many snaps did you send" ))
    sum = sum + snaps
print(sum)

while loop - sentinel menu

Previously, you learned how to write loops that read and processed a sequence of values until it reached a sentinel value. Recall that a sentinel value marks the end of a data set, but it is not part of the data set. A loop that uses a sentinel value in this way is called a sentinel-controlled loop. Today, you'll use a while loop with a sentinel value to create a menu system. This is also known as a game loop.

The variable role of done is a One-Way Flag.

Examples

Example 1

Initialize the variable done to False.

Create a while loop with the condition not done.

Print these lines:

print("Menu")

print("S - Start")

print("Q - Quit")

If the user enters S, print "Starting Game".

Else if the user enters Q, print "Exiting Game", and assign done to True.

example1.py

done = False
while not done:
    print("Menu")
    print("S - Start")
    print("Q - Quit")
    choice = input(":: ")
    if choice == "S":
        print("Starting Game!")
    elif choice == "Q":
        print("Exiting Game!")
        done = True
    else:
        print("Invalid Choice")

Example 2

Create a new menu item "B - Bus Ride" Ask the user how many minutes they ride the bus. If the user enters less than 30, print "not long", else print "too long".

example2.py

done = False
while not done:
    print("Menu")
    print("S - Start")
    print("B - Bus Ride")
    print("Q - Quit")
    choice = input(":: ")
    if choice == "S":
        print("Starting Game!")
    elif choice == "Q":
        print("Exiting Game!")
        done = True
    elif choice == "B":
        bus = int(input("How long is your bus ride? "))
        if bus < 30:
            print("not long")
        else:
            print("too long")
    else:
        print("Invalid Choice")

Example 3

Create a new menu item "N - Number"

Ask the user for a number. If they enter an even number, print even. Else if they print an odd number, print odd.

example3.py

done = False
while not done:
    print("Menu")
    print("S - Start")
    print("B - Bus Ride")
    print("N - Number")
    print("Q - Quit")
    choice = input(":: ")
    if choice == "S":
        print("Starting Game!")
    elif choice == "Q":
        print("Exiting Game!")
        done = True
    elif choice == "B":
        bus = int(input("How long is your bus ride? "))
        if bus < 30:
            print("not long")
        else:
            print("too long")
    elif choice == "N":
        num = int(input("Number: "))
        if num % 2 == 0:
            print("even")
        else:
            print("odd")
    else:
        print("Invalid Choice")

Conclusion

• In your own words, what is a sentinel? What are real life examples of sentinels?

• Explain the role of the One-Way Flag variable when using a while loop with a sentinel value to create a menu.