11: Lists in Python

The Theory:

Store and use lots of data in the same variable
Lists in python: Store and use lots of data in the same variable

A list is a data structure which can store lots of different values in a specific order.

Creating a list

You can create an empty list to_do_list = [] or you can create lists that start with values already stored inside to_do_list = ['learn python', 'eat cake']

In python, a list can store pretty much anything. You can have lists of numbers: prime_numbers = [2, 5, 7, 11]

You can have lists of strings: colours = ["red, "orange", "yellow", "green", "blue", "indigo", "violet"]

And you can have lists which contain a mixture of different data types: my_list = [True, 1, 2, 3, "potato", 2.59]

List Indexing

The position of an item within a list is called the index.

List indexing starts at 0 not 1. This often catches people out, because what we think of as the 1st item in a list python calls the item at index 0

For example:

colours = ["red, "orange", "yellow", "green", "blue", "indigo", "violet"]

This creates a list called colours. There are 7 values stored in the list. The value at index 0 is "red" and the value at index 5 is "indigo"

You can access or change these values using square brackets:

print(colours[0]) would display the first colour red and colours[5] = "purple" would replace "indigo" with "purple"

Rather bizarrely (but very usefully), you can also use negative numbers as an index:

colours[-1] means the last value in the list called colours, which is "violet" and colours[-3] would be "blue" because it’s the third from the end.

Adding and removing values

Once you’ve made a list you can add data to it using append:

to_do_list = []
to_do_list.append("get up)
to_do_list.append("eat breakfast")

is the same as:

to_do_list = ["get up", "eat breakfast"]

Note that the order that you add items to a list is important. Append means add on to the end, so your new values get added after any existing values in the list

You can also remove values from a list:

to_do_list.remove("get up")

Or you can remove the value at a particular index using pop:

to_do_list.pop() will remove the last item in the list or to_do_list.pop(0) removes the item at index 0`

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