Objective

Four programs with functions in them, each with exactly one thing wrong. Two crash, two do not, and in three of the four the error message names a line that is not where the mistake is.

That gap — between where a program notices and where it went wrong — is most of what makes debugging a skill rather than a lookup.

The Four Programs

Bug 1

This prints 10 and then stops with an error about arithmetic on a nil value.

1
local function double(n)
2
    print(n * 2)
3
end
4

5
local answer = double(5)
6

7
print("Answer plus one: " .. answer + 1)

Bug 2

The player loses a life. It prints Lives left: 3.

1
local lives = 3
2

3
local function loseALife(lives)
4
    lives = lives - 1
5
    return lives
6
end
7

8
loseALife(lives)
9

10
print("Lives left: " .. lives)

The function is correct. Something around it is not.

Bug 3

This should add up the table and print Total: 20. It prints Total: 4.

1
local function total(t)
2
    local sum = 0
3

4
    for i, v in ipairs(t) do
5
        sum = sum + v
6
        return sum
7
    end
8
end
9

10
print("Total: " .. total({4, 6, 10}))

Bug 4

This stops immediately with attempt to call global 'greet' (a nil value).

1
print(greet("Ada"))
2

3
local function greet(name)
4
    return "Hello " .. name
5
end

Submit

For each program, give:

• One sentence on what was wrong
• The corrected line of code
• Which line the error names, and which line the mistake is on — for the two that produce errors

Bug 2 is the one to spend time on. There is nothing wrong inside the function, and writing down exactly what happened to the value is worth more than the fix.

Worth 5 points, graded on completion.