Javascript object losing attribute

Question

I'm having a bit of a problem with this function:

function create_enemies(rows, columns) {
    var i, j;

    var x_position = 0;
    var y_position = 0;
    var id = 0;

    enemies_array = new Array(rows);

    for (i = rows - 1; i >= 0; i--) {
        enemies_array[i] = new Array(columns);
        for (j = columns - 1; j >= 0; j--) {
            x_position = j * (enemy_squadron_width / 4) + (ship_width / 2);
            y_position = i * (enemy_squadron_height / 4);

            enemies_array[i, j] = {
                x : x_position,
                y : y_position,
                width : ship_width,
                height : ship_height,
                speed : 2,
                id : id
            };

            id++;
            console.log("this one's fine: " + enemies_array[i, j].y);
        }
    }

    for (i = rows - 1; i >= 0; i--) {
        for (j = columns - 1; j >= 0; j--) {
            console.log("This one's not fine:  " + enemies_array[i, j].y);
        }
    }
}

What's happening is that on the first console.log, the Y attribute is being correctly printed, but on the second console.log, every Y in every element of the array is set at 0. Somehow the Y attribute is lost between the first outer for-loop and the second.

I'm surely missing something very obvious, and starting to feel a little insane.

Any ideas?

Thank you very much.

edit - I should mention that every other attribute is fine. Only the Y is being reset

What do you expect enemies_array[i, j] do? Did you mean enemies_array[i][j]? — JJJ
– JJJ, Commented Jul 14, 2015 at 11:59
Isn't that how we access multidimensional arrays? enemies_array[i][j] gives me an undefined error. Besides, all the other attributes are fine, it's just the Y that is being reset to 0. If I do enemies_array[i, j].x the values are fine, as with the rest of the attributes — Ehpansei
– Ehpansei, Commented Jul 14, 2015 at 12:04
No, it's not. enemies_array[i, j] is exactly equivalent to enemies_array[j] (see the comma operator). You'll have to change it everywhere, not only on the last line. — JJJ
– JJJ, Commented Jul 14, 2015 at 12:06

Juan Marcos Armella · Accepted Answer · 2015-07-14 12:20:02Z

1

Here you go:

function create_enemies(rows, columns) {

    var x_position = 0;
    var y_position = 0;
    var enemy_squadron_width = 100;
    var enemy_squadron_height = 100;
    var ship_width = 100;
    var ship_height = 100;


    var id = 0;

    var enemies_array = new Array(rows);

    for (var i = rows - 1; i >= 0; i--) {
        enemies_array[i] = new Array(columns);
        for (var j = columns - 1; j >= 0; j--) {
            var x_position = j * (enemy_squadron_width / 4) + (ship_width / 2);
            var y_position = i * (enemy_squadron_height / 4);
            enemies_array[i][j] = {
                x : x_position,
                y : y_position,
                width : ship_width,
                height : ship_height,
                speed : 2,
                id : id
            };

            id++;
            console.log("this one's fine: " + enemies_array[i][j].y);
        }
    }

    for (var i = rows - 1; i >= 0; i--) {
        for (var j = columns - 1; j >= 0; j--) {
            console.log("This one's not fine:  " + enemies_array[i][j].y);
        }
    }
}

create_enemies(10,10);

You need to acces the elements of the array by a[i][j].

Fiddle: http://jsfiddle.net/bL4mwgez/1/

answered Jul 14, 2015 at 12:20

Juan Marcos Armella

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1 Comment

Ehpansei Over a year ago

Thank you, Juhana already helped me earlier. I'll accept this answer nonetheless.

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