CP104 Notes: 2D Lists

Topics

• 2D Lists
• 2D List Indexes
• Traversing 2D Lists
• Traversing 2D Lists by Column
• Building a 2D List

2D Lists

A 2D list (2 Dimensional list) is just a Python list that contains other Python lists. The internal lists can contain any kind of data (although if they contained lists we would have 3D lists, which we do not cover in this course.)

The simplest way to create a 2D list is to assign lists within square brackets to a variable. In general:

some_list = [[v1, v2, ...], ..., [vn-2, vn-1, ...]]

Some examples:

2D List Indexes

2D list elements can be individually accessed by providing the row and column indexes.

Individual elements of a list can be accessed or changed, as well as an entire internal list. Some examples:

              red_green_blue = [[255, 0, 0], [0, 255, 0], [0, 0, 255]]
green = red_green_blue[1]
print(green)
            

              people = [["David", "Lori"], ["Deborah", "Tasmin", "Tristan"]]
people.append(["Benjamin", "Jake"])
print(people)
            

              people[1][0] = "Deb"
print(people)
            

            a_codes = [[65, "A"], [66, "B"], [67, "C"]]
a_codes[1] = [70, "F"]
print(a_codes)
          

Traversing 2D Lists

Lists can be traversed - 'walked' through from one end to the other - in three ways. The following code shows how the contents of the list:

    
people = [['David', 'Lori'], ['Deb', 'Tasmin', 'Tristan'], ['Benjamin', 'Jake']]

  

can be traversed:

• By index with a while loop:

          
  rows = len(people)
  i = 0
  while i < rows:
      cols = len(people[i])
      j = 0
      while j < cols:
          print(people[i][j])
          j += 1
      i += 1
  
        

• By index with a for loop:

          
  for i in range(len(people)):
      for j in range(len(people[i])):
          print(people[i][j])
  
        

• By value with a for loop:

          
  for row in people:
      for col in row:
          print(col)
  
        

All three versions print the contents of people as:

David
Lori
Deborah
Tasmin
Tristan
Benji
Jake

Note that because the lengths of each internal list of people differ, the internal loops that use an index refer specifically to the length of each internal list, e.g.
len(people[i])
This is not an issue for the for loops that loop through the list values rather than using indexes.

Traversing 2D Lists by Column

We will look at on traversing a matrix, a rectangular 2D list where every row has the same number of columns. We can do this only with indexes - using a for loop with values won't work, because such a loop only works row-by-row.

Traversing by column is as simple as swapping the row and column loops in the nested loops. Thus:

        
rows = len(red_green_blue )
cols = len(red_green_blue [0])  # assumes all rows have same length
j = 0
while j < cols:
    i = 0
    while i < rows:
        print(red_green_blue [i][j])  # [i][j] is unchanged
        i += 1
    j += 1

      

or

        
for j in range(len(red_green_blue[0])):
    for i in range(len(red_green_blue)):
        print(people[i][j])

     

Building a 2D List

A 2D list can be built dynamically. We will look only at building a matrix since that is the simplest example. The following code builds a 3 × 4 matrix containing all zeroes:

      rows = 3
cols = 4
matrix = []

for i in range(rows):
    row = []  # define a new row
    for j in range(cols):
        row.append(0)  # add value to row
    matrix.append(row)  # add new row to matrix

    

The contents of the variable matrix are:

[[0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]]