Nested loops & 2-D lists

A loop inside a loop

• You can put one loop inside another loop.
• For every pass of the outer loop, the inner loop runs fully.
• So an outer of 2 and an inner of 3 runs the body 2 × 3 = 6 times.

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

A list of lists

• A 2-D list is a list whose items are themselves lists.
• It models a grid or table: rows of columns.
• Reach one cell with two indexes: grid[row][col].

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

Loop over a grid

• Use a nested loop to visit 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)

In Cambridge pseudocode

• A 2-D list is the exam's 2-D array, written ARRAY[1:rows, 1:cols].

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

• The inner loop finishes all its turns for each single turn of the outer loop.
• Index a grid row first, then column: grid[row][col].

Now you try

• Use a nested loop in each task.
• Press Check answer to test your code.