Defining 2D lists in python

One way to create a 2D list in python is to use a 2D list literal. You've already learned how to create a 1D list literal, and the syntax here is similar. However, instead of a simple list, here we have a list of lists, or nested list.

The python code below stores the contents of this table:

Table of heating bills 
heating_bill = [
    [112, 32, 10, 96],
    [60, 15, 0, 70],
    [196, 65, 15, 180]

Notice that heating_bill is a list with 3 items. Each item is itself a list.

  • The first element heating_bill[0] is itself a list: [112, 32, 10, 96].
  • The second element heating_bill[1] is itself a list: [60, 15, 0, 70].
  • The third element heating_bill[2] is itself a list: [196, 65, 15, 180].

How to access individual elements in a 2D list

Accessing 2D lists using indices

Rather than focusing on 2D lists as a list of lists, another perspective is to think of it as a rectangular grid, each position having a row position and a column position. So to indicate a specific element, you give two index values. The first index gives the row and the second index gives the column.

For example:

  • heating_bill[0][0] refers to row index 0, column index 0, whose value is 112.
  • heating_bill[2][3] refers to row index 2, column index 3, whose value is 180.
  • heating_bill[1][2] refers to row index 1, column index 2, whose value is 0.

Creating 2D lists using for-loops

Consider the following 4x3 2D list, with all values initialized to 0:

temperature = [
    [0, 0, 0],
    [0, 0, 0],
    [0, 0, 0],
    [0, 0, 0]
]

This 2D list could be created more flexibly with the following code:

# Create an empty list
temperature = []
# Four rows:
for row in range(4):
    # Create an empty sub-list
    new_row = [] 
    # Fill the row with three 0's   
    for column in range(3):
        new_row.append(0)
    # Put the newly-created row into the outer list:
    temperature.append(new_row)