# 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.

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

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

{% endcode %}

## 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.

![](/files/-Ld100SxyORCtHFnAUt_)

{% hint style="info" %}
The variable role of done is a One-Way Flag.
{% endhint %}

![](/files/-Ld1eH2L7uzani0hbIM-)

## Examples

### Example 1

Initialize the variable done to False. &#x20;

Create a while loop with the condition not done. &#x20;

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.

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

```python
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")
```

{% endcode %}

### **Example 2**

Create a new menu item "B - Bus Ride"\
Ask the user how many minutes they ride the bu&#x73;**.**\
If the user enters less than 30, print "not long", else print "too long".

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

```python
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")
```

{% endcode %}

### 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.

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

```python
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")
```

{% endcode %}

## 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.

<br>

\ <br>


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://www.pythonclassroom.com/loops/while-loop-menu.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.