Module 3 · Lists in advanced applications · Lesson 2 of 3

Two-dimensional arrays

Put lists inside a list and you get a two-dimensional array — a grid. Think of the outer list as the rows, and each inner list as the cells across one row. A chessboard, a spreadsheet, a game of tic-tac-toe: all of them are just a list of rows.

Reading a cell is a two-step move: grid[1] picks the row, then [2] picks the column within it. Watch the two steps happen.

So grid[row][col] — row first, then column. Assigning works the same way: grid[1][2] = 99 changes exactly one cell.

You rarely type a grid out by hand. A nested comprehension builds one at any size, and two nested for loops are the natural way to walk every cell.

One trap to remember from the references lesson: [[0] * 3] * 2 does not make two rows — it makes two references to the same row, so editing one edits both. The comprehension gives each row its own list. Run this to see the difference.