Cannot set property '3' of undefined in 2 dimensional array

Question

I am trying to create a 2 dimensional array but am getting this error.

I loop an object and try to assign them but it won't let me assign a value to the second dimension.

This is what I have:

//this is globally set
var gcollision = new Array();

function create() {
    for (var X in sdata) {
        X = parseInt(X);
        for (var Y in sdata[X]) {
            Y = parseInt(Y);
            width = parseInt(sdata[X][Y][2]);
            height = parseInt(sdata[X][Y][3]);
            for (i = X; i != X + width; i++) {
                //error occurs here "Uncaught TypeError: Cannot set property '3' of undefined"
                gcollision[i][Y] = 1
                for (j = Y; j != Y + height; j++) {
                    gcollision[X][j] = 1
                }
            }
        }
    }

How do I make it set the value of properly?

EDIT sdata looks like this:

var sdata = {"4":{"7":["1","7","3","3"]},"3":{"3":["2","8","1","1"]}};

what does your sdata look like?

Jonathan Payne
– Jonathan Payne

2012-04-06 18:22:15 +00:00
Commented Apr 6, 2012 at 18:22 — Jonathan Payne
– Jonathan Payne, Commented Apr 6, 2012 at 18:22

Authman Apatira · Accepted Answer · 2012-04-06 18:21:08Z

3

//this is globally set
var gcollision = new Array();

function create(){
    for (var X in sdata) {
        X = parseInt(X);
        for (var Y in sdata[X]) {
            Y = parseInt(Y);
            width = parseInt(sdata[X][Y][2]);
            height = parseInt(sdata[X][Y][3]);

for (i=X; i!= X+width; i++) {
                 if( typeof gcollision[i] == 'undefined' ) gcollision[i] = new Array();
                 gcollision[i][Y] = 1

for (j=Y; j!=Y+height; j++) {
                 if( typeof gcollision[X] == 'undefined' ) gcollision[X] = new Array();
                 gcollision[X][j] = 1
                }
            }
        }
    }

Basically, even though you created your array, those indices do not exist yet, so you cannot reference them as arrays until after you define them as such.

If you set up your for loops a bit more optimally, you don't have to do isset and can just create the gcol[index] = Array(); the right before the inner loop where it is first accessed.

answered Apr 6, 2012 at 18:21

Authman Apatira

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4 Comments

Sir Over a year ago

One final question, is making an array for this better than having objects ? Which would be more efficient for doing checks later on ?

Sir Over a year ago

Also i do not understand what you mean about the setup of loops more optimally?

Authman Apatira Over a year ago

In Javascript, you wont notice much of a difference between the two (arrays and objects). Internally, they're probably handled the same way anyway. As for the optimizations, we're currently checking gcollision[]'s type in the internal loop because I don't know what type of data you have in the width & height variables; if you could restructure your function such that the gcol[index] arrays are created outside of that loop, you would save yourself an order of if-operations.

Sir Over a year ago

There is a slight issue, see screenshot: i.imgur.com/eSBzi.jpg , i also added sdata format to the question at the bottom. Is it deffinately listed as you would have expected, im not sure i can see how X:Y link together to do checks?

Ry- · Accepted Answer · 2012-04-06 18:21:09Z

1

You'll need to initialize the first level of arrays, first.

function create() {
    for (var X in sdata) {
        X = parseInt(X);
        gcollision[X] = [];

        for (var Y in sdata[X]) {
            Y = parseInt(Y);
            width = parseInt(sdata[X][Y][2]);
            height = parseInt(sdata[X][Y][3]);
            for (i = X; i < X + width; i++) {
                gcollision[i][Y] = 1;

                for (j = Y; j < Y + height; j++) {
                    gcollision[X][j] = 1;
                }
            }
        }
    }

answered Apr 6, 2012 at 18:21

Ry-♦

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2 Comments

Sir Over a year ago

Is this the most efficient way to do it ?

Ry- Over a year ago

@Dave: Possibly. It shouldn't really matter that much, though.

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