Python - Lists

Noted from Learn Python 3 | Codecademy

What is a List?

In programming, it is common to want to work with collections of data. In Python, a list is one of the many built-in data structures that allows us to work with a collection of data in sequential order.

Suppose we want to make a list of the heights of students in a class:

• Noelle is 61 inches tall
• Ava is 70 inches tall
• Sam is 67 inches tall
• Mia is 64 inches tall

In Python, we can create a variable called heights to store these integers into a list:

heights = [61, 70, 67, 64]

Notice that:

1. A list begins and ends with square brackets ([ and ]).
2. Each item (i.e., 67 or 70) is separated by a comma (,)
3. It’s considered good practice to insert a space (_) after each comma

What can a List contain?

Lists can contain more than just numbers.

Let’s revisit our classroom example with heights:

• Noelle is 61 inches tall
• Ava is 70 inches tall
• Sam is 67 inches tall
• Mia is 64 inches tall

Instead of storing each student’s height, we can make a list that contains their names:

names = ["Noelle", "Ava", "Sam", "Mia"]

We can even combine multiple data types in one list. For example, this list contains both a string and an integer:

mixed_list_string_number = ["Noelle", 61]

Lists can contain any data type in Python! For example, this list contains a string, integer, boolean, and float.

mixed_list_common = ["Mia", 27, False, 0.5]


Empty Lists

A list doesn’t have to contain anything. You can create an empty list like this:

empty_list = []

Why would we create an empty list?

Usually, it’s because we’re planning on filling it up later based on some other input.

List Methods

As we start exploring lists further in the next exercises, we will encounter the concept of a method.

In Python, for any specific data-type ( strings, booleans, lists, etc. ) there is built-in functionality that we can use to create, manipulate, and even delete our data. We call this built-in functionality a method.

For lists, methods will follow the form of list_name.method(). Some methods will require an input value that will go between the parenthesis of the method ( ).

An example of a popular list method is .append(), which allows us to add an element to the end of a list.

append_example = [ 'This', 'is', 'an', 'example']
append_example.append('list')

print(append_example)

Will output:

['This', 'is', 'an', 'example', 'list']

Growing a List: Append

We can add a single element to a list using the .append() Python method.

Suppose we have an empty list called garden:

garden = []

We can add the element "Tomatoes" by using the .append() method:

garden.append("Tomatoes")

print(garden)

Will output:

['Tomatoes']

We see that garden now contains "Tomatoes"!

When we use .append() on a list that already has elements, our new element is added to the end of the list:

Create a list
garden = ["Tomatoes", "Grapes", "Cauliflower"]

Append a new element
garden.append("Green Beans")
print(garden)

Will output:

['Tomatoes', 'Grapes', 'Cauliflower', 'Green Beans']

Growing a List: Plus (+)

When we want to add multiple items to a list, we can use + to combine two lists (this is also known as concatenation).

Below, we have a list of items sold at a bakery called items_sold:

items_sold = ["cake", "cookie", "bread"]

Suppose the bakery wants to start selling "biscuit" and "tart":

items_sold_new = items_sold + ["biscuit", "tart"]
print(items_sold_new)

Would output:

['cake', 'cookie', 'bread', 'biscuit', 'tart']

In this example, we created a new variable, items_sold_new, which contained both the original items sold, and the new items. We can inspect the original items_sold and see that it did not change:

print(items_sold)

Would output:

['cake', 'cookie', 'bread']

We can only use + with other lists. If we type in this code:

my_list = [1, 2, 3]
my_list + 4

we will get the following error:

TypeError: can only concatenate list (not "int") to list

If we want to add a single element using +, we have to put it into a list with brackets ([]):

my_list + [4]

Accessing List Elements

We are interviewing candidates for a job. We will call each candidate in order, represented by a Python list:

calls = ["Juan", "Zofia", "Amare", "Ezio", "Ananya"]

First, we’ll call "Juan", then "Zofia", etc.

In Python, we call the location of an element in a list its index.

Python lists are zero-indexed. This means that the first element in a list has index 0, rather than 1.

In this example, the element with index 2 is "Amare".

We can select a single element from a list by using square brackets ([]) and the index of the list item. If we wanted to select the third element from the list, we’d use calls[2]:

print(calls[2])

Will output:

Amare

Note: When accessing elements of an array, you must use an int as the index. If you use a float, you will get an error. This can be especially tricky when using division. For example, print(calls[4/2]) will result in an error, because 4/2 gets evaluated to the float 2.0.

To solve this problem, you can force the result of your division to be an int by using the int() function. int() takes a number and cuts off the decimal point. For example, int(5.9) and int(5.0) will both become 5. Therefore, calls[int(4/2)] will result in the same value as calls[2], whereas calls[4/2] will result in an error.

Accessing List Elements: Negative Index

We can use the index -1 to select the last item of a list, even when we don’t know how many elements are in a list.

Consider the following list with 6 elements:

pancake_recipe = ["eggs", "flour", "butter", "milk", "sugar", "love"]

If we select the -1 index, we get the final element, "love".

print(pancake_recipe[-1])

Would output:

love

This is equivalent to selecting the element with index 5:

print(pancake_recipe[5])

Would output:

love

Modifying List Elements

Let’s return to our garden.

garden = ["Tomatoes", "Green Beans", "Cauliflower", "Grapes"]

Unfortunately, we forgot to water our cauliflower and we don’t think it is going to recover.

Thankfully our friend Jiho from Petal Power came to the rescue. Jiho gifted us some strawberry seeds. We will replace the cauliflower with our new seeds.

We will need to modify the list to accommodate the change to our garden list. To change a value in a list, reassign the value using the specific index.

garden[2] = "Strawberries"

print(garden)

Will output:

["Tomatoes", "Green Beans", "Strawberries", "Grapes"]

Negative indices will work as well.

garden[-1] = "Raspberries"

print(garden)

Will output:

["Tomatoes", "Green Beans", "Strawberries", "Raspberries"]

Shrinking a List: Remove

We can remove elements in a list using the .remove() method.

Suppose we have a filled list called shopping_line that represents a line at a grocery store:

shopping_line = ["Cole", "Kip", "Chris", "Sylvana"]

We could remove "Chris" by using the .remove() method:

shopping_line.remove("Chris")

print(shopping_line)

If we examine shopping_line, we can see that it now doesn’t contain "Chris":

["Cole", "Kip", "Sylvana"]

We can also use .remove() on a list that has duplicate elements.

Only the first instance of the matching element is removed:

Create a list
shopping_line = ["Cole", "Kip", "Chris", "Sylvana", "Chris"]

Remove a element
shopping_line.remove("Chris")
print(shopping_line)

Will output:

["Cole", "Kip", "Sylvana", "Chris"]

Two-Dimensional (2D) Lists

We’ve seen that the items in a list can be numbers or strings. Lists can contain other lists! We will commonly refer to these as two-dimensional (2D) lists.

Once more, let’s look at a class height example:

• Noelle is 61 inches tall
• Ava is 70 inches tall
• Sam is 67 inches tall
• Mia is 64 inches tall

Previously, we saw that we could create a list representing both Noelle’s name and height:

noelle = ["Noelle", 61]

We can put several of these lists into one list, such that each entry in the list represents a student and their height:

heights = [["Noelle", 61], ["Ava", 70], ["Sam", 67], ["Mia", 64]]

We will often find that a two-dimensional list is a very good structure for representing grids such as games like tic-tac-toe.

A 2d list with three lists in each of the indices.
tic_tac_toe = [
            [["X"],["O"],["X"]],
            [["O"],["X"],["O"]],
            [["O"],["O"],["X"]]
]

Accessing 2D Lists

Let’s return to our classroom heights example:

heights = [["Noelle", 61], ["Ali", 70], ["Sam", 67]]

Two-dimensional lists can be accessed similar to their one-dimensional counterpart. Instead of providing a single pair of brackets [ ] we will use an additional set for each dimension past the first.

If we wanted to access "Noelle"‘s height:

Access the sublist at index 0, and then access the 1st index of that sublist.
noelles_height = heights[0][1]
print(noelles_height)

Would output:

61

Modifying 2D Lists

Now that we know how to access two-dimensional lists, modifying the elements should come naturally.

Let’s return to a classroom example, but now instead of heights or test scores, our list stores the student’s favorite hobby!

class_name_hobbies = [["Jenny", "Breakdancing"], ["Alexus", "Photography"], ["Grace", "Soccer"]]

"Jenny" changed their mind and is now more interested in "Meditation".

We will need to modify the list to accommodate the change to our class_name_hobbies list. To change a value in a two-dimensional list, reassign the value using the specific index.

The list of Jenny is at index 0. The hobby is at index 1.
class_name_hobbies[0][1] = "Meditation"
print(class_name_hobbies)

Would output:

[["Jenny", "Meditation"], ["Alexus", "Photography"], ["Grace", "Soccer"]]

Negative indices will work as well.

The list of Jenny is at index 0. The hobby is at index 1.
class_name_hobbies[-1][-1] = "Football"
print(class_name_hobbies)

Would output:

[["Jenny", "Meditation"], ["Alexus", "Photography"], ["Grace", "Football"]]

Project: Gradebook

You are a student and you are trying to organize your subjects and grades using Python. Let’s explore what we’ve learned about lists to organize your subjects and scores.