19. Nested loops — Homework solutions

The .py solution files are in exercises/19/homework/solutions/.

Problem 1 — Rectangle of hashes

Problem. A 5-wide, 3-tall block of #.

How to think about it. Outer loop = rows (3). Inner loop = columns (5): build a row of five #, then print it.

Worked solution.

for row in range(1, 4):
    line = ""
    for col in range(1, 6):
        line = line + "#"
    print(line)

Problem 2 — Coordinate pairs

Problem. Print every (x, y) for x and y from 1 to 3.

Worked solution.

for x in range(1, 4):
    for y in range(1, 4):
        print(f"({x}, {y})")

Output: (1, 1), (1, 2), (1, 3), (2, 1), ... — nine lines.

Common mistakes.

• Reusing x for both loops. Each loop needs its own variable.

Problem 3 — Times table

Problem. A 5×5 multiplication grid.

Worked solution.

for row in range(1, 6):
    line = ""
    for col in range(1, 6):
        line = line + str(row * col) + "\t"
    print(line)

Output:

1   2   3   4   5
2   4   6   8   10
3   6   9   12  15
4   8   12  16  20
5   10  15  20  25

Challenge — Right triangle

Problem. A 6-row triangle where row n has n stars, then upside down.

Worked solution.

# growing
for row in range(1, 7):
    line = ""
    for star in range(1, row + 1):
        line = line + "*"
    print(line)

# shrinking
for row in range(6, 0, -1):
    line = ""
    for star in range(1, row + 1):
        line = line + "*"
    print(line)

The two differ only in the outer loop's direction: range(1, 7) up, range(6, 0, -1) down. The inner loop is identical.

Common mistakes.

• Using a fixed inner range range(1, 7) instead of range(1, row + 1). A fixed inner range gives a rectangle; the triangle comes from tying the inner count to the current row.

Done?

You can build grids and shapes from loops inside loops. The last chapter of Part 4 — Loop patterns — names the jobs loops do most: adding up, counting, searching, and flagging.