2-D arrays

A grid of rows and columns

• A 2-D array is a list whose items are themselves lists.
• It models a grid or table: rows of columns.
• Each inner list is one row.

grid = [[1, 2, 3],
        [4, 5, 6]]
print(grid)

Reach one cell

• Use two indexes: first the row, then the column.
• grid[row][col] — both count from 0 in Python.

grid = [[1, 2, 3], [4, 5, 6]]
print(grid[0][1])    # 2
print(grid[1][2])    # 6

A nested loop visits every cell

• The outer loop walks the rows; the inner loop walks the columns.
• for row in grid: gives one row (a list) at a time.

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

Add up a grid

• Keep a running total and add every cell with a nested loop.

grid = [[1, 2, 3], [4, 5, 6]]
total = 0
for row in grid:
    for value in row:
        total = total + value
print(total)         # 21

In the exam: ARRAY[1:rows, 1:cols]

• Pseudocode writes a 2-D array with two ranges, counting from 1.

DECLARE Grid : ARRAY[1:2, 1:3] OF INTEGER
FOR r ← 1 TO 2
    FOR c ← 1 TO 3
        OUTPUT Grid[r, c]
    NEXT c
NEXT r

Common mistakes

• Use two indexes: grid[row][col].
• The outer loop walks the rows, the inner loop the columns.

Now you try

• Use a nested loop or two indexes.
• Press Check answer to test your code.