Controlling loops

So far, you have only looped over sequences based on the length of the data you wanted to iterate over. In many cases, this is not necessary and depending on the amount of the data it can also be quite costly. You'll now examine how you can control the flow of the loop and exit out when a specific condition is met. You will also look at control statements such as break, continue and pass.

If else

In many cases, you may need to search for a particular item in a list. Once the item is found, there is no need to continue looping over the other results. Using the same example as before, let's assume you only need to check if the dessert "Churros" is in the list and if it is, print a single statement.

            Starter Code
            favorites = ['Creme Brulee', 'Apple Pie', 'Churros', 'Tiramisú', 'Chocolate Cake']
            for dessert in favorites:
                 if dessert == 'Churros':
                    print('Yes! One of my favorite desserts is', dessert)        

Running the above code will output the following:

Yes! One of my favorite desserts is Churros

But what happens if you look for a dessert and that dessert is not on the list? The code above is currently not set up to handle this. Let's look for the dessert "Pudding" which is not on the list, and also add an else statement to handle the case of when it's not found. If the dessert is not part of the list, you will print a new statement.

                   Starter Code
                   favorites = ['Creme Brulee', 'Apple Pie', 'Churros', 'Tiramisú', 'Chocolate Cake']

                   for dessert in favorites:
                       if dessert == 'Pudding':
                          print('Yes one of my favorite desserts is', dessert) 
                       else:
                           print('No sorry, that dessert is not on my list')

Running the above code will result in the following output: No sorry, that dessert is not on my list

Loop control statements

Break

• The code works as intended but you may notice one flaw. If you change the search term to something that is on the list like "Churros" and run it again you will get the following output:

Yes one of my favorite desserts is Churros No sorry, not a dessert on my list

This is not what you want! The dessert is on the list but it still printed out the else condition. To fix it, you need to use a control statement called break.

Add the following:

                          Starter Code
                          favorites = ['Creme Brulee', 'Apple Pie', 'Churros', 'Tiramisú', 'Chocolate Cake']

                              for dessert in favorites:
                                  if dessert == 'Pudding':
                                     print('Yes one of my favorite desserts is', dessert)
                                     break 
                               else:
                                   print('No sorry, not a dessert on my list')

Running the above code will resolve the issue. The break statement is used to stop the loop, which in turn also stops the else condition. Without the break the loop will continue even after the if condition is satisfied.

Continue

Similar to break, continue can be used to control the iteration of the loop. The key difference is that it can allow you to skip over a section of the loop but then continue on with the rest. If you change your code to this, you will notice the outcome will print everything except the Churros dessert.

               Starter Code
               favorites = ['Creme Brulee', 'Apple Pie', 'Churros', 'Tiramisú', 'Chocolate Cake']

               for dessert in favorites:
                   if dessert == 'Churros':
                   continue
                   print('Other desserts I like are', dessert) 

Pass

The pass statement allows you to run through the loop in its entirety and effectively ignore that the if condition has been satisfied.

                       Starter Code
                       favorites = ['Creme Brulee', 'Apple Pie', 'Churros', 'Tiramisú', 'Chocolate Cake']

                       for dessert in favorites:
                           if dessert == 'Churros':
                               pass
                           print('Other desserts I like are', dessert)