Nesting Lists in Python

In Python, you can create nested lists. But what exactly is a nested list? It's a list contained within another list. This is also known as an embedded list.

An Example

[1, 2, 3, ["a", "b", "c"]]

Searching in a Nested List

When searching in a nested list, the search is limited to the elements at the first level; it doesn't perform a deep search.

Example

This list includes a nested list:

lista1 = [1, 2, 3, ["a", "b", "c"]]

If you search for the element "a" using the index function:

lista1.index("a")

the command won't find it because "a" is inside a nested list (second-level list).

However, if you search for:

lista1.index(3)

The search for the element 3 will be successful:

2

The number 3 is in the first-level list. It is the third element (at position 2).

Creating a Matrix with Nested Lists

Nested lists are very useful for creating matrices in Python.

Practical Example

Using list comprehension in Python, you can create an identity matrix.

n = 5
matrix = [[(1 if x == y else 0)
for x in range(n)] for y in range(n)]

This command creates an identity matrix, which is a matrix with ones on the diagonal and zeros elsewhere.

[[1, 0, 0, 0, 0], [0, 1, 0, 0, 0], [0, 0, 1, 0, 0], [0, 0, 0, 1, 0], [0, 0, 0, 0, 1]]

The elements of the first-level list represent the rows of the matrix.

The elements of the second-level list (nested list) represent the columns of the matrix.

If you print the list, the final result will be:

[[1, 0, 0, 0, 0], [0, 1, 0, 0, 0], [0, 0, 1, 0, 0], [0, 0, 0, 1, 0], [0, 0, 0, 0, 1]]

This is an identity matrix:

[1, 0, 0, 0, 0]
[0, 1, 0, 0, 0]
[0, 0, 1, 0, 0]
[0, 0, 0, 1, 0]
[0, 0, 0, 0, 1]

That's how you create an identity matrix in Python.