Nested Lists in Python can be best defined as a ‘list within another list’. A Python list can have elements of different type, when we take a list as an element of the main list, then that list is called a nested list.

Consider two lists, l1, l2

l1 = [1, 2, 3, 4]
l2 = [5, l1]

In the above code, we have added l1 as an element of list l2, hence, l1 is our nested list.

l1 = [1, 2, 3, 4]
l2 = [5, l1]
print(l2)

O/P

[5, [1, 2, 3, 4]]

Hence,
l[0] = 5
l2[1] = [1, 2, 3, 4]

So, l2[3] is our nested list. To print of the elements, we can use for loop, like

for i in l2[1]:
  print(i, end= ' ')

O/P

1 2 3 4

Similarly,

l2[1][0] = 1
l2[1][1] = 2
l2[1][2] = 3
l2[1][3] = 4

Representing Lists as Matrices

A matrix is a two-dimensional data structure where numbers are arranged into rows and columns.

For example, a matrix with two rows and two columns will look like

In Python, we can represent this matrix with Nested Lists. For example, above matrix can be represented as

mat = [[2, 3], [1, 4]]

Similarly, matrix for 3 by 3 will look like :

mat = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

Here, [1, 2, 3] is the first row. Let’s try to access this matrix or Nested Lists.

mat = [[2, 3], [1, 4]] 
for i in mat: # loop row by row 
  print(i) 

O/P

[2, 3]
[1, 4]

Now, lets try to retrieve columns in each row

mat = [[2, 3], [1, 4]] 
for i in mat: # loop row by row 
  for j in i: # loop column
    print(j, end = ' ') 
  print()

O/P

2 3
1 4

Another way of traversing nested list

mat = [[2, 3], [1, 4]] 
for i in range(len(mat)): # loop row by row 
  for j in range(len(mat[i])): # loop column
    print(mat[i][j], end = ' ') 
  print()

O/P

2 3
1 4

That’s all here.