Listing

how to store lists of data in Python

Lists

We might call these arrays in other languages but in Python we call them lists:

temperatures_this_week = [22, 23, 21, 24, 29, 29, 33]
precipitation_this_week = [0.5, 0.7, 1.2, 0.1, 0.0, 0.0, 0.0]

Basically, the square brackets encapsulate the list and the commas separate the items!

Items

Lists can contain items of one or a mix of two or more types:

random_data_list = [10, 2.0, "abc", false]

Lists can also contain zero items:

empty_list = []

Adding to lists

When we already have a list with items, we can still add additional items with the append method:

random_data_list = [10, 2.0, "abc", false] 
random_data_list.append(3)

We call methods like append "built-in" methods because the programming language supplies them without us having to write them!

Removing from lists

Just as easily as adding an item to a list, we can easily remove an item from a list using remove:

random_data_list = [10, 20, "abc", false, 3]
random_data_list.remove(5)

Combining lists

We can combine, or concatenate, lists using the familiar + operator:

small_list1 = [10, 20]
small_list2 = ["a", "b"]
big_list = small_list1 + small_list2 
# [10, 20, "a", "b"]

Note that to add a single item using that syntax, we must put the single item to a one-item list:

small_list = [10, 20]
new_list = [10, 20] + [30] 

# not:
# new_list = [10, 20] + 30

Accessing items

To "get" items in a list, we use a syntax similar to arrays in other languages:

seasons = ["Spring", "Summer", "Fall", "Winter"]
print(seasons[0]) 
> "Spring"

For this unfamiliar with zero-indexing, the first item of a list gets assigned an index of 0, while the second item an index of 1, and so on...

We can also use negative numbers to access a list from its end!

seasons = ["Spring", "Summer", "Fall", "Winter"]
print(seasons[-1]) 
> "Winter"
print(seasons[-2])
> "Fall"

Modifying items

To change items in a list, we use an "access and assign" notation:

seasons[2] = "Autumn"

Removing items

To delete items from a list, we use the remove built-in method by passing the value of the item(s) that we wish to remove:

wait_list = ["Joe", "John", "Jose", "Jack", "Jeff"]
wait_list.remove("Joe")
# ["John", "Jose", "Jack", "Jeff"]

Please note that if we have two or more identical values, using remove will only delete the first instance:

wait_list = ["Joe", "John", "Jose", "Jack", "Jeff", "Joe"]
wait_list.remove("Joe")
# ["John", "Jose", "Jack", "Jeff", "Joe"]

As a side note, if we try to use remove on an item that does not exist in the list, we will get an error!

Two-dimensional lists

Two-dimensional lists contain items that consists of lists (i.e. lists-in-a-list)!

people_bios = [
    ["Joe", "Smith", 38], 
    ["John", "Brown", 39], 
    ["Jose", "Chavez", 40], 
    ["Jack", "London", 41]]

Note that each item follows a pattern for the first item and for the second item!

Accessing two-dimensional lists

A very intuitive notation, the first set of square brackets refers to the larger list while the second set refers to smaller "list-in-a-list":

people_ages[1][0] # "John" 
people_ages[1][1] # "Brown"
people_ages[1][2] # 39 
people_ages[3][1] # "London" (Jack's last name)

Modifying two-dimensional lists

Similar with one-dimensional lists, changing a two-dimensional list involves the "access and assign" notation, e.g. to modify John's age to 40:

people_ages[1][2] = 40