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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.
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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.

Problems

Last modified 4yr ago


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