Weekend Online Coding and Robotics Club in Medan, Sumatra

1. Introduction: What Is a Nested Loop?

A nested loop is a programming structure where one loop (the inner loop) is placed inside the body of another loop (the outer loop). Python allows multiple levels of nesting, but nesting too deeply is discouraged because the code becomes hard to read and debug.

2. How Nested Loops Execute

For every single iteration of the outer loop, the inner loop runs through its entire cycle from start to finish.

Example with both loops using range(3):

for i in range(3):
    for j in range(3):
        print(i, j)

The outer loop runs 3 times. Each time it runs once, the inner loop completes all 3 of its iterations. Total inner-loop executions: 3 × 3 = 9.

3. Building a 4×4 Matrix of Zeros

Nested loops are essential for two-dimensional data. The outer loop walks rows; the inner loop walks columns:

matrix = []
for r in range(4):
    row = []
    for c in range(4):
        row.append(0)
    matrix.append(row)

print(matrix)

Result: a 4×4 grid filled with zeros.

4. Common Use Cases

• Grids, visual patterns, and coordinate pairs
• Pairwise comparisons between list elements
• Matrix construction, population, and manipulation