Lesson 16 — Matrix traversal

Matrices: lists of lists

A matrix (grid) is a list of rows, and each row is a list of cells. m[r][c] is the cell in row r, column c. Nested loops visit every cell: the outer loop picks a row, the inner loop a column. Row sums are easy (sum(row)); column sums need the column index.

m = [[1, 2], [3, 4]]
m[1][0]                     # 3
sum(row[0] for row in m)    # 4 — the first column

Why it matters: spreadsheets, game boards and images (pixels) are all grids.