Python 101

Week 3:
Lists

Expedia Code Academy

Lists

A list acts as a single variable to store lots of data.

Each of those values is called an element of the list and can be accessed by its position, or index.

Lists

In Python, you can define lists like this: 


            # a list of strings
            expedia_brands = ['Hotels.com', 'EPS', 'Expedia']
            
            # a list of numbers
            exam_grades = [40, 55, 99, 73]
            
            # a list containing mixed types
            a_bit_of_everything = ['hello!', 2, True, 0.535]
            
            # a list of lists
            words = [['hello', 'yes', 'no'], ['hola', 'si', 'no']]

Lists

You can pick out items from a list by using their index which starts from 0, and also ends in -1. 

            # a list of strings
            expedia_brands = ['Hotels.com', 'EPS', 'Expedia', 'Egencia'] 

            print(expedia_brands[0])  # 'Hotels.com'
            print(expedia_brands[2])  # 'Expedia'
            print(expedia_brands[-1])  # 'Egencia'
            print(expedia_brands[-2])  # 'Expedia'

Checkpoint


What error do you see if you try accessing an element that doesn't exist?

(⭐) 3a. How would you check if the first and last elements of a list are the same?

Lists exercise

It is useful to know the number of items in a list too: 

            # Create a list
            random_numbers = [31, 5.36, 0, 25, 535, 42.3]
            
            length = len(sentence)
            print(length) # 6

(⭐⭐️) 3b. Find the average of random_numbers

Hint: add all the numbers together then divide by how many numbers there are

Building lists



              # Start with a list
              sentence = ['Programming', 'is']
              # Add an item to the end
              sentence.append('fun')

              print(sentence)

              # To create an empty list
              empty_list = []

(⭐⭐️️️) 3c. Build a list of 1-100 yourself using append and a while loop

Building lists


We can concatenate two lists together. 

            # Create two lists
            sentence_start = ['Programming', 'is']
            sentence_end = ['really', 'fun']
            
            # Join the two lists together using the + operator
            sentence = sentence_start + sentence_end
            print(sentence)

List Membership

Sometimes it's useful to find if an element exists in a given list. 

            expedia_brands = ['Hotels.com', 'EAN', 'Expedia']

            if 'Booking.com' in expedia_brands:
              print('Something must be wrong!')

            if 'Hotels.com' in expedia_brands:
              print('Yay!')
(⭐️⭐️) 3d. Program Bob (version 2)!
Get Bob to say more! See here for the full conversation, and here for the solution to the original Bob exercise.

Appendix: Sorting & Slicing

We can sort lists: 

            random_numbers = [5, 3, 4, 6, 1, 2]
            sorted_numbers = sorted(random_numbers)
            print(sorted_numbers) # displays [1, 2, 3, 4, 5, 6]

Or take sections (called slices) of a list: 

              letters = ['a', 'b', 'c', 'd', 'e', 'f']
              letters[1:3] # results in ['b', 'c']
              letters[2:3] # results in ['c']

Appendix: Exercises



            numbers = [65, 4, 9, 23, 15, 875, 3.69, 33, 12, 79, 6]

(⭐️⭐️) 3e. Write a program to print the lowest and highest numbers
(⭐️⭐️️️️⭐️️️) 3f. Write a program that splits a list into two halves - if there's an odd number of elements the second list will be longer