javascript TypeError: 'undefined' when evaluating a 2d array

Question

this is my code :

var y = new Array(); // it have 500 entries.
var z = [[]];
    var counter =0;
    for(var i =0; i<49;i++){
        for(var j=0; j<9;j++){
            z[i][j] = y[counter];
            counter +=1;
        }
    }
    console.log(z);

and I am getting this error : TypeError: 'undefined' is not an object (evaluating 'z[i][j] = y[counter]')

what am I doing wrong ?

You are trying to access z[0][1] which doesn't exist. var z = [[]]; creates an array with a single element (another array). JavaScript doesn't have multidimensional arrays, you have to create every element of an array explicitly. — Felix Kling
– Felix Kling, Commented Mar 17, 2015 at 15:08

Pointy · Accepted Answer · 2015-03-17 15:08:32Z

4

You have to initialize each row of the two-dimensional array. Your declaration:

var z = [[]];

only creates row 0.

So:

for(var i =0; i<49;i++){
    z[i] = []; // <===== initialize the row
    for(var j=0; j<9;j++){
        z[i][j] = y[counter];
        counter +=1;
    }
}

answered Mar 17, 2015 at 15:08

Pointy

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