# 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

{% code title="intro.py" %}

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

{% endcode %}

## 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 |

{% hint style="info" %}
Use a variable named data to store the input value.
{% endhint %}

{% hint style="warning" %}
Don't use a float for equality checking in the condition because the values are approximated;.  &#x20;
{% endhint %}

## 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 &#x30;**.**

{% code title="example1.py" %}

```python
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)
```

{% endcode %}

{% hint style="info" %}
The input value 0 is the sentinel value for these loops.
{% endhint %}

### 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 number&#x73;**.**

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

{% code title="example2.py" %}

```python
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)
```

{% endcode %}

### 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 inpu&#x74;**.**

{% code title="example3.py" %}

```python
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)
```

{% endcode %}

### Example 4

Using a while loop, ask the user to enter numbers until they enter 0.  Print the total number of even number&#x73;**.**

{% code title="example4.py" %}

```python
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)
```

{% endcode %}

## 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.

## <br>

<br>


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