3.6. Using a while Loop with Lists and Dictionaries

A for loop is effective for looping through a list, but you shouldn't modify a list inside a for loop because Python will have trouble keeping track of the items in the list. To modify a list as you work through it, use a while loop. We'll look at some examples.

Moving Items from One List to Another

Consider a list of newly registered but unverified users of a website. One way to move them to a separate list is using while loop. We begin with a list of unconfirmed users and an empty list. Here's what that code might look like:


> The while loop will run as long as unconfirmed_users is not empty. The pop() function will remove one user at a time from the end of unconfirmed_users. When the list of unconfirmed users is empty, the loop stops and the list of confirmed users is printed.

Removing All Instances of Specific Values from a List

We learned that remove() can be used to remove a specific value from a list. That command worked because the value we want to remove only appear once. If the value appear more than once, we can put remove() inside a while loop as follows:


> After printing the list, Python enter the while loop because cat is in pets. Once inside the loop, Python removes cat one by one until cat in pets is False.

Exercise 3.6

  1. Deli
    -     Make a list called sandwich_orders and fill it with at least five names of various sandwiches. You can choose from this list:
    cheese sandwich, club sandwich, Dagwood, French dip, hamburger, Monte Cristo, muffuletta, pastrami, submarine.
    - Then, make an empty list called finished_sandwiches.
    - Loop through the list of sandwich orders and print a message for each order:
    I made your {sandwich name}.
    - After all the sandwiches have been made, print a message listing each sandwich that was made:
    The sandwiches in your order are:
    - cheese sandwich
    - club sandwich
    and so on.

  2. No Pastrami
    -     Using the list from no.1, make sure the sandwich 'pastrami' appears in the list at least three times.
    - Print a message saying
    The deli has run out of pastrami.
    Then, use a while loop to remove all occurrences of 'pastrami' from sandwich_orders.
    - Make sure no pastrami sandwiches end up in finished_sandwiches. Then print the remaining sandwiches like this:
    The remaining sandwiches in your order are:
    - cheese sandwich
    - club sandwich
    and so on.