while loop - sentinel value

Objective

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

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

Introductory Problem

Using a while loop, ask the user for the minutes of their bus/car ride 3 times. Print the sum of these numbers

intro.py

x = 3
sum = 0
while x > 0:
    ride = int(input("How long? "))
    sum = sum + ride
print("Sum:",sum)

while loop (sentinel)

Previously, you learned how to write loops that read and process a sequence of values. Today you will learn about while loops with sentinel values. A sentinel value denotes the end of a data set, but it is not part of the data. A loop that uses a sentinel value is called a sentinel-controlled loop.

1 <variable = input>	Ask the user for a value
2 while <condition>:	while loop executes while condition is True
3 <do something>	do something inside the while loop
4 <variable = input>	Ask the user for a value again
5 <do something>	(optional) do something outside the while loop

Use a variable named data to store the input value.

Don't use a float for equality checking in the condition because the values are approximated;.

Examples

Example 1

Using a while loop, ask the user how long their bus ride was this month until they enter 0. Print the sum of these numbers.

Steps:

• Initialize a variable sum to 0.

• Ask "How long? (enter 0 to end): “

• Create a condition that will execute until the user enters 0.

• Sum the inputs

• Print the sum after the user enters 0.

example1.py

sum = 0
data = int(input("How long? (enter 0 to end) "))
while data != 0:
    sum = sum + data
    data = int(input("How long? (enter 0 to end) "))
print("Sum:",sum)

The input value 0 is the sentinel value for these loops.

Example 2

Using a while loop, ask the user for the length of their bus/car ride until the user enters 0. Print the average of these numbers.

Initialize a counter variable to 0. Every time a number is entered, increment the variable.

example2.py

sum = 0
x = 0
data = int(input("How long? (enter 0 to end) "))
while data != 0:
    sum = sum + data
    x = x + 1
    data = int(input("How long? (enter 0 to end) "))
average = sum / x
print("Average:",average)

Example 3

Using a while loop, ask the user for the length of their bus/car ride until the user enters 0. Print the maximum of these numbers. Initialize max to the value of the first input.

example3.py

sum = 0
x = 0
data = int(input("How long? (enter 0 to end) "))
while data != 0:
    if x == 0:
        max = data
    else:
        if data > max:
            max = data
    x = x + 1
    data = int(input("How long? (enter 0 to end) "))
print("Max:",max)

Example 4

Using a while loop, ask the user to enter numbers until they enter 0. Print the total number of even numbers.

example4.py

evens = 0
data = int(input("Give me a number (enter 0 to end): "))
while data != 0
    if data % 2 == 0:
        evens = evens + 1
    data = int(input("Give me a number (enter 0 to end): "))
print("Evens:",evens)

Conclusion

• What is an example of when you would use a while loop that does not have a fixed number of inputs?

• Explain the role of the sentinel variable.