How to create a 2D array in Javascript

Question

I'm trying to create a 2D array with javascript that I can ultimately put inside a nested loop to extract X/Y information.

What am I doing wrong here?


function createMatrix(){
    let colcount = 0;
    let rowcount = 0;

    var pixel_array = new Array();
    var y_array = new Array();

    for (var i = 0; i <= 100; i++){
        var checker = (i+1) % 10;
        if(checker == 0){
            y_array.push(i);
            //create new X array
            pixel_array[rowcount] = [];
            //push column data into row array
            pixel_array[rowcount].push(y_array);
            //create new Y array 
            var y_array = new Array();
            //increment row counter
            //reset column counter
            parseInt(rowcount++);
            colcount = 0;
        }else{
            y_array.push(i);
            parseInt(colcount++);
        }
    }

    //sanity check: spit out the matrix
    for (var x=0; x<10;x++){
        for(var y=0; y<10; y++){
            console.log(pixel_array[x][y]);
        }
    }

}

I was expecting to call a specific X/Y 'coordinate' and extract the information from that 'cell'. However, I'm getting an error that basically says the [Y] part of the array is not defined.

Looking at console.log and console.table - I can see the X array is filled, but it isn't like I'd expect, just a list of numbers not another array.

Edit: To be more specific, my goal is to create a 2D array from a single For loop. The nested for loop at the bottom of the code is shown as an example of how I would like to call the 'cells' [X][Y].

T.J. Crowder · Accepted Answer · 2019-07-31 09:22:17Z

3

This code:

pixel_array[rowcount] = [];
//push column data into row array
pixel_array[rowcount].push(y_array);

creates a new array, stores it in pixel_array[rowcount], and then pushes y_array into it. So at that point, you have an array (pixel_array) with an entry that's an array (the one you created via []), with an entry that's an array (y_array). That's closer to a 3D array than a 2D array.

You may be overcomplicating it a bit. I can't quite make out what you want your final array to be, so here's an example of creating a 3x4 "2D array" (it isn't really¹, it's an array of arrays, but...) with the numbers 1-12 in it, see comments:

// Create the outer array
var array = [];
for (var x = 0; x < 4; ++x) {
    // Create the inner array for this row and store it in `array[x]`
    var inner = array[x] = [];
    for (var y = 0; y < 3; ++y) {
        // Set the value of the inner array at `y`,
        // which is also `array[x][y]`
        inner[y] = (x * 3) + y + 1;
    }
}
console.log(array);

.as-console-wrapper {
    max-height: 100% !important;
}

In a comment you've said:

I want to create a 2D array from a single For loop. So in my example I'm looping 0-99, with the expected result being a 10x10 'matrix' (0-9, 10-19, 20-29, etc).

Here's an example doing that:

// Create the outer array
var array = [];
var inner;
for (var n = 0; n < 100; ++n) {
    // Time to create a new inner array?
    if (n % 10 === 0) { // It's important that this condition is true on the first loop iteration
        inner = [];
        array.push(inner);
    }
    // Add this `n` to the current inner array
    inner.push(n);
}
console.log(array);

.as-console-wrapper {
    max-height: 100% !important;
}

¹ "it isn't really, it's an array of arrays" - JavaScript doesn't have multi-dimensional arrays. It has arrays that can contain other arrays. For many purposes, the distinction doesn't really matter, but it means (amongst other things) that the arrays can be jagged: Not all entries in the outer array have to have the same length. (In fact, they don't even have to be arrays.)

edited Jul 31, 2019 at 9:22

answered Jul 31, 2019 at 8:50

T.J. Crowder

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4 Comments

moshacoder Over a year ago

Thanks for the reply T.J. - I now relize I should have been more specific in my question: I want to create a 2D array from a single For loop. So in my example I'm looping 0-99, with the expected result being a 10x10 'matrix' (0-9, 10-19, 20-29, etc). Ultimately I will use this with a Nth For loop to create an NxN matrix.

T.J. Crowder Over a year ago

@edwardrford - Ah, that's simpler than what I was doing. :-) I've added an example to the answer.

moshacoder Over a year ago

(insert face palm emoji) - Wow, I was thinking waaaay too hard about that one. Thanks T.J. - I really appreciate the answer.

T.J. Crowder Over a year ago

@edwardrford - LOL, we've all done it. Happy coding!

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