Introduction to lists

So far, we have worked with individual pieces of data like the string 'hello'. Then with variables we saw how to give this data a name. Now in this lesson, we'll see how we can group data together with lists.

Creating a list

A list is our first form of a collection. A collection is just a way of grouping data together, and lists certainly accomplish this. For example, let's consider the top cities for travel according to the magazine Travel and Leisure. Here is how we are used to seeing a list of travel locations in a document or on a website.

Travel Locations

1. Solta
2. Greenville
3. Buenos Aires
4. Los Cabos
5. Walla Walla Valley
6. Marakesh
7. Albuquerque
8. Archipelago Sea
9. Iguazu Falls
10. Salina Island
11. Toronto
12. Pyeongchang

Here is what that list looks like as a Python list:

['Solta', 'Greenville', 'Buenos Aires', 'Los Cabos', 'Walla Walla Valley', 'Marakesh', 'Albuquerque', 'Archipelago Sea', 'Iguazu Falls', 'Salina Island', 'Toronto', 'Pyeongchang']

['Solta',
 'Greenville',
 'Buenos Aires',
 'Los Cabos',
 'Walla Walla Valley',
 'Marakesh',
 'Albuquerque',
 'Archipelago Sea',
 'Iguazu Falls',
 'Salina Island',
 'Toronto',
 'Pyeongchang']

We indicate that we are initializing a list with an opening bracket, [, and we end the list with a closing bracket ]. We separate each list item, also called an element, with a comma.

['Croatia', 'USA', 'Argentina', 'Mexico', 'USA', 'Morocco', 'New Mexico', 'Finland', 'Argentina', 'Italy', 'Canada', 'South Korea']

['Croatia',
 'USA',
 'Argentina',
 'Mexico',
 'USA',
 'Morocco',
 'New Mexico',
 'Finland',
 'Argentina',
 'Italy',
 'Canada',
 'South Korea']

We can, of course, declare variables and set them equal to our lists so that we can both name and later retrieve the list.

top_travel_cities = ['Solta', 'Greenville', 'Buenos Aires', 'Los Cabos', 'Walla Walla Valley', 'Marakesh', 'Albuquerque', 'Archipelago Sea', 'Iguazu Falls', 'Salina Island', 'Toronto', 'Pyeongchang']

top_travel_cities

['Solta',
 'Greenville',
 'Buenos Aires',
 'Los Cabos',
 'Walla Walla Valley',
 'Marakesh',
 'Albuquerque',
 'Archipelago Sea',
 'Iguazu Falls',
 'Salina Island',
 'Toronto',
 'Pyeongchang']

countries_of_top_cities = ['Croatia', 'USA', 'Argentina', 'Mexico', 'USA', 'Morocco', 'New Mexico', 'Finland', 'Argentina', 'Italy', 'Canada', 'South Korea']

Accessing Elements of Lists

Now our top_travel_cities list contains multiple elements, and just like we are used to list elements having a rank or number associated with them...

1. Solta
2. Greenville
3. Buenos Aires

...a list in Python also assigns a number to each element.

top_travel_cities

['Solta',
 'Greenville',
 'Buenos Aires',
 'Los Cabos',
 'Walla Walla Valley',
 'Marakesh',
 'Albuquerque',
 'Archipelago Sea',
 'Iguazu Falls',
 'Salina Island',
 'Toronto',
 'Pyeongchang']

top_travel_cities[0]

'Solta'

In the above line we are referencing a list and then using the brackets to access a specific element of our list, the first element. We access elements in a list with the index, and there is a separate index for each element in the list. It begins at the number zero, increases for every element thereafter.

So to access the second element we write top_travel_cities[1], and the third element is top_travel_cities[2].

top_travel_cities[2]

'Buenos Aires'

How would we access the last element? Well, we could count all of the elements in the list, and Pyeongchang would just be one less than that. Or we can ask Python to start from the end and move back one:

top_travel_cities[-1]

'Pyeongchang'

And we can move back as many as we want.

top_travel_cities[-2]

'Toronto'

Each element in our list is a string, so, we can always set an element of our string equal to a variable.

top_canadian_city = top_travel_cities[-2]
top_canadian_city

'Toronto'

type(top_canadian_city)

str

Now we have a variable of top_canadian_city, equal to the string 'Toronto', and a variable of top_travel_cities equal to the list of cities.

top_travel_cities

['Solta',
 'Greenville',
 'Buenos Aires',
 'Los Cabos',
 'Walla Walla Valley',
 'Marakesh',
 'Albuquerque',
 'Archipelago Sea',
 'Iguazu Falls',
 'Salina Island',
 'Toronto',
 'Pyeongchang']

type(top_travel_cities)

list

Accessing Multiple Elements

Now imagine that we don't want to access just one element of a list, but multiple elements at once. Python allows us to do that as well:

top_travel_cities[0:2]

['Solta', 'Greenville']

As we can see from the above example, we can access elements of a list by placing two numbers separated by a colon inside of our brackets. The first number indicates the index of the first element we want returned.

The second number could represent the number of elements we want returned back, or maybe it represents the stopping index of the elements that we are retrieving. Looking at our top_travel_cities it could be either.

top_travel_cities

['Solta',
 'Greenville',
 'Buenos Aires',
 'Los Cabos',
 'Walla Walla Valley',
 'Marakesh',
 'Albuquerque',
 'Archipelago Sea',
 'Iguazu Falls',
 'Salina Island',
 'Toronto',
 'Pyeongchang']

Let's try a different experiment to answer our question.

top_travel_cities[4:5]

['Walla Walla Valley']

Ok, so that second number is not representing the number of elements we want returned. Instead it must be the index at which we stop our selection of elements.

top_travel_cities[4:6]

['Walla Walla Valley', 'Marakesh']

This operation is called slice. So, we can say we are slicing the elements with indices 4 and 5 in the line above. Note that even though we are slicing elements, our list remains intact.

top_travel_cities

['Solta',
 'Greenville',
 'Buenos Aires',
 'Los Cabos',
 'Walla Walla Valley',
 'Marakesh',
 'Albuquerque',
 'Archipelago Sea',
 'Iguazu Falls',
 'Salina Island',
 'Toronto',
 'Pyeongchang']

In programming terms, we would say that slicing elements is non-destructive, because it does not change the underlying data structure. We can do it as many times as we like, and our top_travel_cities array remains unchanged. If we wish to store that slice of elements, we can store it in another variable.

top_two = top_travel_cities[0:2]
top_two

['Solta', 'Greenville']

Now we have another variable called top_two that points to an array which contains an array of elements equal to the first two elements of top_travel_cities.

Changing elements with destructive methods

Now that we can read and select certain elements from lists, let's work on changing these lists. To add a new element to a list, we can use the append method.

top_travel_cities.append('San Antonio')

Now let's take another look at top_travel_cities.

top_travel_cities

['Solta',
 'Greenville',
 'Buenos Aires',
 'Los Cabos',
 'Walla Walla Valley',
 'Marakesh',
 'Albuquerque',
 'Archipelago Sea',
 'Iguazu Falls',
 'Salina Island',
 'Toronto',
 'Pyeongchang',
 'San Antonio']

You will see that 'San Antonio' has been added to the list. Note that unlike slice, append is destructive. That is, it changes our underlying data structure. Every time we execute the append method, another element is added to our list. Now what if we accidentally add 'San Antonio' a second time to our list.

top_travel_cities.append('San Antonio')
top_travel_cities

['Solta',
 'Greenville',
 'Buenos Aires',
 'Los Cabos',
 'Walla Walla Valley',
 'Marakesh',
 'Albuquerque',
 'Archipelago Sea',
 'Iguazu Falls',
 'Salina Island',
 'Toronto',
 'Pyeongchang',
 'San Antonio',
 'San Antonio']

If you press shift+enter on the above line of code, we will have 'San Antonio' as the last two elements of the list. Luckily, we have the pop method to remove one of them. The pop method is available to call on any list and removes the last element from the list. As you can see below, calling pop removed our last element.

top_travel_cities.pop()

'San Antonio'

Now if we want to change an element from the middle of the list, we can access and then reassign that element. For example, let's change 'Walla Walla Valley' to the number 4.

top_travel_cities[4]

'Walla Walla Valley'

top_travel_cities[4] = 4

top_travel_cities

['Solta',
 'Greenville',
 'Buenos Aires',
 'Los Cabos',
 4,
 'Marakesh',
 'Albuquerque',
 'Archipelago Sea',
 'Iguazu Falls',
 'Salina Island',
 'Toronto',
 'Pyeongchang',
 'San Antonio']

Our list is changed, but now it's not as sensible, so let's change it back.

top_travel_cities[4] = 'Walla Walla Valley'

With that, our list is back to the way we like it.

top_travel_cities

['Solta',
 'Greenville',
 'Buenos Aires',
 'Los Cabos',
 'Walla Walla Valley',
 'Marakesh',
 'Albuquerque',
 'Archipelago Sea',
 'Iguazu Falls',
 'Salina Island',
 'Toronto',
 'Pyeongchang',
 'San Antonio']

Finding Unique elements and length of lists

If we are not sure whether there are repeated elements, we can use Python to get a unique list.

top_travel_cities.append('Solta')

For example, now that we have added Solta to the end of our list, Solta appears twice.

Well to see a unique list of the elements, we can call the set function. The set function is non-destructive on our list.

unique_travel_cities = set(top_travel_cities)
unique_travel_cities

{'Albuquerque',
 'Archipelago Sea',
 'Buenos Aires',
 'Greenville',
 'Iguazu Falls',
 'Los Cabos',
 'Marakesh',
 'Pyeongchang',
 'Salina Island',
 'San Antonio',
 'Solta',
 'Toronto',
 'Walla Walla Valley'}

The set function initializes a new set in Python. A set is a different type collection in Python.

type(set())

set

A set is just like a list, except elements do not have order and each element appears just once.

unique_travel_cities[1]

-------------------------------------------------------------------

TypeError                         Traceback (most recent call last)

<ipython-input-26-3d2453cad46e> in <module>()
----> 1 unique_travel_cities[1]


TypeError: 'set' object does not support indexing

So here, when we convert our list into a set, our set just consists of the unique elements. But unfortunately this structure is a set, not a list.

type(unique_travel_cities)

set

So let's convert this set, which has a unique list of our travel cities, into a list.

unique_travel_cities = list(unique_travel_cities)

type(unique_travel_cities)

list

So the array of unique_travel_cities is a unique list.

unique_travel_cities

['Toronto',
 'Archipelago Sea',
 'Iguazu Falls',
 'Pyeongchang',
 'Los Cabos',
 'Buenos Aires',
 'Marakesh',
 'Greenville',
 'Albuquerque',
 'Salina Island',
 'Walla Walla Valley',
 'San Antonio',
 'Solta']

And you can see quickly that it differs from the list of top travel cities by checking the length.

len(unique_travel_cities)

13

len(top_travel_cities)

14

top_travel_cities

['Solta',
 'Greenville',
 'Buenos Aires',
 'Los Cabos',
 'Walla Walla Valley',
 'Marakesh',
 'Albuquerque',
 'Archipelago Sea',
 'Iguazu Falls',
 'Salina Island',
 'Toronto',
 'Pyeongchang',
 'San Antonio',
 'Solta']

Note: For most purposes, Python developers prefer to work with lists as opposed to sets, as lists are generally easier to manipulate, as you will see in future lessons.

Summary

In this section we saw how to associate data together in a collection, called a list. A list is similar to a list in the real world - it implies the data has some connection, and that it has an order to it. We initialize a list with the brackets, [], and separate each element by a comma. To access elements from a list, we use the bracket accessor followed by the index of the element we want to retrieve, and our indices begin at zero and increase from there. To add a new element to the end of the list we use the append method, and to remove an element from the end of a list we use pop. We can change elements anywhere between by first accessing the elements and then reassigning them.