lua: attempt to index a nil value (field '?')

Question

I'm getting this error in this file:

-- mid point circle drawing algoritm taken from here:
-- https://stackguides.com/questions/17163636/filled-circle-in-matrix2d-array
-- code translated from java

-- initates 2d array
matrix = {}

for i = 1, 2 do
    matrix[i] = {}
end

-- given matrix and diameter populates array
function mCircle(matrix, d)
    local startX = d/2
    local startY = d/2
    local r = d/2
    mCircleRec(matrix, d, startX, startY, r, r)

    --prints matrix
    for i = 1, d do
        for j = 1, d do
            print(matrix[x][y])
        end
    end

end

function mCircleRec(matrix, d, startX, startY, x, y)
    if (x >= d or y >= d or x < 0 or y < 0 or matrix[x][y] == 1)
    then 
        return
    elseif (matrix[x][y] == 9)
    then 
        return
    end

    local r = d/2

    if (((x - startX) * (x - startX) + (y - startY) * (y - startY)) <= (r * r))
    then
        matrix[x][y] = 1
    else
        matrix[x][y] = 9
    end

    mCircleRec(matrix, d, startX, startY, x+1, y);  -- down
    mCircleRec(matrix, d, startX, startY, x, y+1);  -- right
    mCircleRec(matrix, d, startX, startY, x-1, y);  --up
    mCircleRec(matrix, d, startX, startY, x, y-1);  --left
    mCircleRec(matrix, d, startX, startY, x-1, y-1); -- diagonal up-left
    mCircleRec(matrix, d, startX, startY, x+1, y+1); -- diagonal right-down
    mCircleRec(matrix, d, startX, startY, x+1, y-1);  -- diagonal left-down
    mCircleRec(matrix, d, startX, startY, x-1, y+1);  -- diagonal right-up
end

mCircle(matrix, 20)

The complete error message:

lua: circle.lua:27: attempt to index a nil value (field '?')
stack traceback:
        circle.lua:27: in function 'mCircleRec'
        circle.lua:16: in function 'mCircle'
        circle.lua:54: in main chunk
        [C]: in ?

This is a simple filled circle algoritm that creates a 2d array and places inside a filled circle, the code was translated from java and the original code is in one of the answers here .

I'm new to lua and I looked in every other question about the same error but none of them was relevant. Thanks in advance for the help

edit: typo

edit: While troubleshooting I discovered another error: I got wrong the 2d array creation and modified it:

- initates 2d array

local radius = 20
local diameter = radius * 2

-- Create a diameter x diameter array
matrix = {}
for i = 1, diameter do

    matrix[i] = {}

    for j = 1, diameter do
        matrix[i][j] = 0      -- Fill the values here
    end

end

Changing this didn't change the error message

The error is in the first line of the body of mCircleRec. You might need to find a text editor that shows you line numbers. — luther
Thanks @EgorSkriptunoff , I fixed the typo but this didn't change the error. — Francesco Mazzucco
Sorry @luther the error was a line 27, I wrote that comment directly on the stack overflow editor and chose the wrong line — Francesco Mazzucco
@luther because it is not a syntax error. it defines a local nil value start and a global Y assigned d/2 so there is nothing to catch. but there were other errors and not the one he reported when I posted his code so I presume he didn't copy paste the code that causes the index error — Piglet

Piglet Piglet · Accepted Answer · 2021-02-12T08:54:23

The code you provided does not cause the quoted error message.

First you had this

local start Y = d/2

which you fixed already

Then there is a broken if statement

if (x >= d or y >= d or x < 0 or y < 0 or matrix[x][y] == 1)  --the error is in this line (27) apparently
    then
        matrix[x][y] = 1
    then 
        return
    elseif (matrix[x][y] == 0)
    then 
        return
    end

where you have two thens

Only after fixing these errors you will get to the point where you see the quoted error.

Inside mCircle x and y are nil. So matrix[x][y] is not possible.

lua: attempt to index a nil value (field '?')

2 Answers