Nested Loops

A loop inside a loop — perfect for grids.

A nested loop is a loop placed inside another loop. The inner loop runs fully for every single pass of the outer loop. They are ideal for working through a 2-D array.

How a nested loop runs

nested.py

for row in range(3):
    for col in range(2):
        print("row", row, "col", col)

Output

row 0 col 0
row 0 col 1
row 1 col 0
row 1 col 1
row 2 col 0
row 2 col 1

How it works

The outer loop runs 3 times (row = 0, 1, 2).
For each of those, the inner loop runs fully (col = 0, 1).
That gives 3 × 2 = 6 lines in total.

Building a pattern

stars.py

for row in range(3):
    for star in range(5):
        print("*", end="")
    print()

Output

*****
*****
*****

How it works

The inner loop prints 5 stars on the same line — end="" stops print() starting a new line.
The empty print() after the inner loop moves down to the next line.
The outer loop repeats this for 3 rows.

Walking through a 2-D array

walk.py

grid = [
    [1, 2, 3],
    [4, 5, 6]
]
for row in grid:
    for value in row:
        print(value, end=" ")
    print()

Output

1 2 3 
4 5 6

How it works

The outer loop takes each row of the grid.
The inner loop takes each value within that row.
This visits every cell, row by row.

Inner finishes firstThe inner loop completes all of its passes for each single pass of the outer loop.

What you have learned

A nested loop is a loop inside another loop.
The inner loop runs in full on every pass of the outer loop.
Total passes = outer count × inner count.
Nested loops are the natural way to process a 2-D array.