Lists

Python lists are mutable and ordered sequence of elements. We use lists when we want to track multiple elements at once and want to preserve their ordering.

Basics

You can instantiate an empty list in two ways:

lst = []
lst = list()

If you would like to initialize a list with a predetermined size and starting values:

default = 0
size = 5

lst = [default for _ in range(size)]
lst = [default] * size

2-dimensional lists are lists of lists. We often use this to represent a matrix or a grid. Here are some ways to instantiate 2-d lists.

# Corresponds to how many lists there are.
rows = 3

# Corresponds to how many elements are in the inner list.
columns = 2

default = 0

matrix = [[default for _ in range(columns)] for _ in range(rows)] 
matrix = [[default] * columns for _ in range(rows)]

Note: 

matrix = [[default] * columns] * rows

The above code will not work due to reference issues. The same list will be duplicated across all rows.

Iteration

We can iterate through a 1d list like so:

my_list = [1, 2, 3, 4]

for i in range(len(my_list)):
    print(my_list[i])

We can iterate through a 2d list in multiple ways (as you know from working on WordSearch).

row-column-traversal

  1. Row-order traversal

    for row in range(len(matrix)):
    for col in range(len(matrix[row])):
        # Prints out each element in the matrix.
        print(matrix[row][col])

  2. Column-order traversal

    for col in range(len(matrix[0])):
    for row in range(len(matrix)):
        # Prints out each element in the matrix.
        print(matrix[row][col])

If you don't care about the index when iterating, you can also use the built-in iterator for lists to go through every element.

for row in matrix:
    for element in row:
        # Prints each element in matrix
        print(element)

Methods

Be familiar with the following list functions:

  • append: adds a new element to the end of the list
  • pop: removes element at the given index
  • insert: inserts an element to the list at the given index
  • sort: sorts the list, can provide a custom lambda for the key
  • extend: adds iterable elemnts to the end of the list

Lists are 0-based indexed, meaning that the first element starts at index 0 instead of 1:

>>> my_list = [1, 2, 3, 4, 5]
[1, 2, 3, 4, 5]

>>> my_list[0]
1

To get the elements near the back of the list, we can use negative indices: -1 corresponds to the last element, -2 the second to last, etc...

>>> my_list[-1]
5

We can slice a list by using the following syntax:

>>> my_list[0:2]
[1, 2]

Notice that the ending index is exclusive (this is very similar to the range function).

For more information, see: documentation.