Nested Lists and Nested Loops

In programming, nested refers to one structure being contained within another structure.

For example, when a list contains another list or when a loop contains another loop, it is considered nested.

Copy
# A nested list containing the list [2, 3]
nested = [1, [2, 3], 4]

Nested lists are often used to represent matrices, tables, and multidimensional data structures.

Copy
# 3x3 matrix
matrix = [
  [1, 2, 3],
  [4, 5, 6],
  [7, 8, 9]
]

Nested loops refer to a loop within another loop.

When a for loop contains another for loop, it is commonly called a nested for loop.

In the nested for loop example below, the outer loop for row in matrix: iterates over the rows of the list, while the inner loop for item in row: iterates over the elements in each row.

Copy
# Print elements in a 3x3 matrix
matrix = [
  [1, 2, 3],
  [4, 5, 6],
  [7, 8, 9]
]

# Iterate over rows
for row in matrix:
    # Iterate over elements in the row
    for item in row:
        print(item)
# Outputs 1, 2, 3, 4, 5, 6, 7, 8, 9 on separate lines

Nested loops are useful for iterating through data structures with two or more dimensions, such as multidimensional arrays.

Additionally, loops can be nested multiple times to form 3-level, 4-level nested loops, and so on.