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.

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# 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.

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# 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.

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# 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 very useful for iterating through data structures with two or more dimensions, such as processing multidimensional arrays.

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