2D Lists in Python

In programming, lists are used to store multiple items in a single variable:

grocery_list = ["Eggs", "Milk", "Waffles"]

The type of items that can be stored in a list isn’t limited to primitive data types, such as strings, ints and booleans - lists can also store objects and other data structures! Suppose this grocery shopper wanted to get their groceries from several different locations - with lists, they can create a list of lists, each one indicating a different store that each set of groceries needs to be purchased from:

grocery_list_enhanced = [["Butchers Shop","Meat", "Cheese", "Olives"], ["Bakery", "Bread", "Coffee Beans", "Cinnamon Buns"], ["Pizzeria", "Dough"]]

Each list within the list has its own set of items associated with it, and each list is a single item within the grocery_list_enhanced variable. When lists are stored within a list, it’s generally referred to as a 2D List.

2D Lists have the exact same properties as a regular list. To access an item within the 2D list, we reference the index at which the item is located:

grocery_list_enhanced = [["Butchers Shop","Meat", "Cheese", "Olives"], ["Bakery", "Bread", "Coffee Beans", "Cinnamon Buns"], ["Pizzeria", "Dough"]]
butcher = grocery_list_enhanced[0]
print(butcher)                 #prints [`Butchers Shop`,`Meat`, `Cheese`, `Olives`]

To access an item within one of the list items, we have to reference its position within the list item itself:

grocery_list_enhanced = [["Butchers Shop","Meat", "Cheese", "Olives"], ["Bakery", "Bread", "Coffee Beans", "Cinnamon Buns"], ["Pizzeria", "Dough"]]
butcher = grocery_list_enhanced[0]
cheese = butcher[2]
print(cheese)                 #prints 'Cheese'

Rather than store the list item and then the list item’s item in separate variables, we can combine the two index references in one call:

grocery_list_enhanced = [["Butchers Shop","Meat", "Cheese", "Olives"], ["Bakery", "Bread", "Coffee Beans", "Cinnamon Buns"], ["Pizzeria", "Dough"]]
cheese = grocery_list_enhanced[0][2]
print(cheese)                 #prints 'Cheese'

As demonstrated in the example above, the first index reference (0) accesses the list item, and the second index reference (2) accesses the item within the list. The general format for accessing items in a 2D list follows that same format:

2d_list[list][list item]

Practice

A bookstore stores its books in a 2D list, where each book’s name and author is stored in a list item, in that order. They’re having trouble recovering some of the information - help them access the book_list and print out the author of the book The Autobiography of Malcolm X, and print the book written by the author Dan Brown.

These types of lists are referred to as 2D lists because they can be used to emulate two dimensional structures in the real world. We can reimagine our 2D lists as grids, rather than as linear data structures:

tic_tac_toe = [
   ["X","O", "X"], 
   ["-", "-", "-"], 
   ["O", "X", "X"]
]

The grid representation allows us to think about and access each list as a row, and each item within the list as a column:

To get a value from the tic tac toe board, we can do so by finding its row, and then its column:

tic_tac_toe[row][column] 

Iterating and Searching a 2D List

To find or print a value from a 2D list requires the use of nested iteration. Finding a value requires one for loop to iterate through all of the lists in a 2D list, and a second loop to iterate through all values within a list:

for list in tic_tac_toe:
   for item in list:
       print(item)

This same nested structure can be used to find a specific value in a 2D list.

Practice

The bookstore you’ve been working with wants to create a program that lets the cashier know if they have a certain book in stock. The program will print “true” if the book that they’ve entered is in stock. Help them create this program using a nested for loop.