2D Collections in Python

by | Sep 15, 2024

Back to: Python Programming

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A 2 Dimensional list is effectively a list made up of other lists.

2dlist = [list1, list2, list3]

These are useful if you need a grid or matrix of data, similar to an Excel spreadsheet.

fruits =     ['apple', 'banana', 'orange', 'coconut']
vegetables = ['celery', 'carrots', 'potatoes', 'beans']
meats =      ['chicken', 'turkey', 'fish']

groceries = [fruits, vegetables, meats]

print(groceries[2][1])

Remember that list indexes always start at 0 the the output here will be the third [2] list, second [1] item.

turkey
fruits =     ['apple', 'banana', 'orange', 'coconut']
vegetables = ['celery', 'carrots', 'potatoes', 'beans']
meats =      ['chicken', 'turkey', 'fish']

groceries = [fruits, vegetables, meats]

print(groceries[0])  # <-- will print the list of ‘fruits’ - the first list
print(groceries[0][2])  # <-- will print the ‘orange’ – it is in row 0, col 2
print(groceries[2][2])  # <-- will print the ‘fish’ – it is in row 2, col 2

['apple', 'banana', 'orange', 'coconut']
orange
fish

Exercise: what references are needed to print potatoes? 

print(groceries[  ][  ]) 

To access an element from a 2D list you need 2 indices.  Using a single index returns the entire row (or list).

print(groceries[2])  # <-- will print the entire ‘meats’ list

Alternatively, we could create the same 2D list without using list names:

groceries = [['apple', 'banana', 'orange', 'coconut'],
             ['celery', 'carrots', 'potatoes', 'beans'],
             ['chicken', 'turkey', 'fish']]

print(groceries[2][1])
turkey

To iterate over the elements in a 2D list, we need to use “nested ‘for’ loops”.  Using a single for loop, we’ll only iterate over the rows.

groceries = [['apple', 'banana', 'orange', 'coconut'],
             ['celery', 'carrots', 'potatoes', 'beans'],
             ['chicken', 'turkey', 'fish']]

for food_groups in groceries:
    print(food_groups)
['apple', 'banana', 'orange', 'coconut']
['celery', 'carrots', 'potatoes', 'beans']
['chicken', 'turkey', 'fish']

To iterate through the elements found in each row as well, we use a nested loop:

groceries = [['apple', 'banana', 'orange', 'coconut'],
             ['celery', 'carrots', 'potatoes', 'beans'],
             ['chicken', 'turkey', 'fish']]

for food_groups in groceries:
    for food in food_groups:
        print(food)
apple
banana
orange
coconut
celery
carrots
potatoes
beans
chicken
turkey
fish

To make this look more organised:

groceries = [['apple', 'banana', 'orange', 'coconut'],
             ['celery', 'carrots', 'potatoes', 'beans'],
             ['chicken', 'turkey', 'fish']]

for food_groups in groceries:
    for food in food_groups:
        print(food, end=" ")
    print()
apple banana orange coconut 
celery carrots potatoes beans 
chicken turkey fish

Exercise

Use a 2D tuple to create a phone number-pad that looks like the output below.

1 2 3 
4 5 6 
7 8 9 
* 0 #

Solution: (using 2D list of lists):

num_pad = [[1, 2, 3],
           [4, 5, 6],
           [7, 8, 9],
           ["*", 0, "#"]]

for row in num_pad:
    for num in row:
        print(num, end=" ")
    print()

Solution: (using 2D list of tuples):

num_pad = [(1, 2, 3),
           (4, 5, 6),
           (7, 8, 9),
           ("*", 0, "#")]

for row in num_pad:
    for num in row:
        print(num, end=" ")
    print()

Solution: (using 2D list of sets):

num_pad = [{1, 2, 3},
           {4, 5, 6},
           {7, 8, 9},
           {"*", 0, "#"}]

for row in num_pad:
    for num in row:
        print(num, end=" ")
    print()

and so on…

# 2D tuple of lists
num_pad = ([1, 2, 3],
        [4, 5, 6],
        [7, 8, 9],
        ["*", 0, "#"])

# 2D tuple of tuples
num_pad = ((1, 2, 3),
           (4, 5, 6),
           (7, 8, 9),
           ("*", 0, "#"))

# 2D tuple of sets
num_pad = ({1, 2, 3},
           {4, 5, 6},
           {7, 8, 9},
           {"*", 0, "#"})

# 2D set of lists (NOT VALID)
num_pad = {[1, 2, 3],
           [4, 5, 6],
           [7, 8, 9],
           ["*", 0, "#"]}

# 2D set of tuples
num_pad = {(1, 2, 3),
           (4, 5, 6),
           (7, 8, 9),
           ("*", 0, "#")}

# 2D set of sets (NOT VALID)
num_pad = {{1, 2, 3},
           {4, 5, 6},
           {7, 8, 9},
           {"*", 0, "#"}}

for row in num_pad:
    for num in row:
        print(num, end=" ")
    print()