Python nested loops and matrices

Visit every cell of a rectangular matrix.

Python

matrix = [[1, 2, 3], [4, 5, 6]]
for row in matrix:
    row_total = 0
    for value in row:
        row_total += value
    print(row_total)

Output

6
15

Explanation

A nested loop completes the inner loop for each outer iteration. A matrix can be represented as a list of rows, each of which is another list.

This example sums rows independently. Initializing row_total inside the outer loop is deliberate: each row needs a fresh total.

Explore the dry run

Guided walkthroughReady

1matrix = [[1, 2, 3], [4, 5, 6]]

2for row in matrix:

3 row_total = 0

4 for value in row:

5 row_total += value

6 print(row_total)

Start to inspect the source, values, and output. These prepared steps explain the published sample; grouped loop steps describe multiple iterations.

View the complete dry-run table

Prepared trace of the published program. Some rows group repeated iterations.
What happens	State after step	Output so far
Create two rows of three values.	`matrix = [[1, 2, 3], [4, 5, 6]]`	—
Begin the first row.	`row = [1, 2, 3], row_total = 0`	—
Add 1, then 2, then 3 in three inner iterations.	`row_total: 1 -> 3 -> 6`	—
Print the first row total.	`row_total: 1 -> 3 -> 6`	6
Reset for the second row.	`row = [4, 5, 6], row_total = 0`	6
Add 4, then 5, then 6 in three inner iterations.	`row_total: 4 -> 9 -> 15`	6
Print the second row total.	`row_total: 4 -> 9 -> 15`	6 15

O(r*c) time for r rows of c columns, O(1) auxiliary space excluding the matrix.

Python's [[0] * 3] * 2 aliases the same inner list; use a comprehension for independent rows.
An empty row prints 0; an empty matrix prints nothing.

Predict the row totals after adding [7, 8, 9].

Reveal answer and reasoning

The output becomes 6, 15, and 24 on separate lines.