User menus

When we develop console-based applications (those that run in text interfaces), we usually want a nice, reliable way for users to interact with our application.

That user interface is what I call a "menu". For example, this might be an interface for a console-based application:

Welcome to the countries app.

You have visited 0 countries so far.

Enter 'a' to add a country you've visited, or 'q' to quit: 

Imagine the user enters a, then we might show something like this:

...
Enter 'a' to add a country you've visited, or 'q' to quit: a

Country name: 
Date visited: 

You have added (country name) to your list. You have visited 1 country.

Enter 'a' to add another country, 'l' to list the countries you've visited, or 'q' to quit: 

That's an example of an interface. We'll be building this interface for the final project in this section!

Note the way the interface ends: the user can keep typing a over an over again to add more and more countries. This is the perfect place for a loop!

For vs. while

Should you use a for loop, or a while loop in this situation?

tip

For loops are best used when you want to go over a list of elements, so you must know what the list is before the loop starts.

While loops are best to run until a condition is False.

In this case I think a while loop is the better choice, because we don't know how many times the user will want to run our menu. Instead, we want to run until the user types q.

This means we want the loop to run as long as the user's input is not q.

We could define the main structure of our menu in this way:

print("Welcome to the countries app.")
print("You have visited 0 countries so far.")

user_input = input("Enter 'a' to add a country you've visited, or 'q' to quit: ")

while user_input != "q":
    # Do stuff here

    # At the end of the loop, ask the user whether they want to go again
    user_input = input("Enter 'a' to add another country, 'l' to list the countries you've visited, or 'q' to quit: ")

The first time we ask the user for their input we don't tell them they can type l, because at the start of the program they won't have any countries in their list!

I think you're ready to tackle this section's project. Bring it on!