19. Nested loops — Homework solutions

The .lua 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 = 1, 3 do
    local line = ""
    for col = 1, 5 do
        line = line .. "#"
    end
    print(line)
end

Problem 2 — Coordinate pairs

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

Worked solution.

for x = 1, 3 do
    for y = 1, 3 do
        print("(" .. x .. ", " .. y .. ")")
    end
end

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

Common mistakes.

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

Problem 3 — Times table

Problem. A 5×5 multiplication grid.

Worked solution.

for row = 1, 5 do
    local line = ""
    for col = 1, 5 do
        line = line .. (row * col) .. "\t"
    end
    print(line)
end

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 = 1, 6 do
    local line = ""
    for star = 1, row do
        line = line .. "*"
    end
    print(line)
end

-- shrinking
for row = 6, 1, -1 do
    local line = ""
    for star = 1, row do
        line = line .. "*"
    end
    print(line)
end

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

Common mistakes.

• Looping the inner range 1, 6 (fixed) instead of 1, row. 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.