Referencing An Integer Grid

Any reference to pixelGrid through pixelGrid[#][#] makes the code stop working. I've declared pixelGrid in several different ways, without any improvement. Assigning 0 to a value and showing it works, but assigning pixelGrid[0][0], also equal to 0, abruptly breaks things at that point. It doesn't matter whether I access the array in a condition, to update a value, or elsewhere.


var pixelGrid;
for (let row = 0; row < 9; row++) {
    for (let column = 0; column < 9; column++) {
        pixelGrid[row][column] = 0;
    }
}
function redrawPixels() {
    // Shortened for simplicity
    ImageData.data[0] = 0; // Works fine.
    ImageData.data[0] = pixelGrid[0][0]; // Function stops, and ctx.putImageData no longer updates canvas.
}

The conversation continues 2

Follow the reasoning

Tess11 Oct Permalink
Before treating a variable as an array, you must initialise it with an array value, for example
```
var pixelGrid = []; // <- add this
for (let row = 0; row < 9; row++) {
    pixelGrid[row] = []; // <- and add this
    for (let column = 0; column < 9; column++) {
        pixelGrid[row][column] = 0;
    }
}
console.log(pixelGrid[0][0]);
console.log(pixelGrid[0]);
console.log(pixelGrid);
```
The result is:

0

[0, 0, 0, 0, 0, 0, 0, 0, 0]

[[0, 0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0]]

Note: Checking errors in the console helps you identify problems like these.
Bea11 Oct Permalink
You've declared pixelGrid without ever initializing it to a two-dimensional array, which causes the problem.

Right now you wrote:
```
var pixelGrid;
```
That means pixelGrid is actually: undefined

This statement then breaks:
```
pixelGrid[row][column] = 0;
```
since what JavaScript effectively attempts is:
```
undefined[row][column]
```
Initialize the array before doing that:
```
var pixelGrid = [];
```

Referencing An Integer Grid

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