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up vote 0 down vote favorite

The following code results in the console printing "Z".

<script>
var x = new Array(new Array());
x[0][0] = "Z";
console.log(x[0][0]);
</script>

But if I replace x[0][0] = "Z" with a nested for loop of arbitrary dimensions ...

<script>
var x = new Array(new Array());
for(i=0;i<5;i++){
    for(j=0;j<3;j++){
        x[i][j] = "Z";
    }
}
console.log(x[0][0]);
</script>

... it gets stuck on the very zeroth iteration, despite that being the same instruction (as far as I see) as given in the first one. According to both Chrome and Firefox output, it suddenly considers x[0][0] to be undefined.

What's the deal?

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1  
I think the first iteration is fine, but x[1][0] will throw an error, since your outer array only has one element. –  Felix Kling Nov 18 '13 at 2:50
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1 Answer

up vote 0 down vote accepted

JavaScript doesn't have 2D arrays. What you can do is create an array of arrays. In your example you did exactly that, you created an array which contains another array.

var x = new Array(new Array());
console.log(x.length); // 1
console.log(x[0]); // []

x is an array with one element which happens to be an array. If you are trying to access x[1], i.e. the second element in the array, you will get an error because such element does not exist:

console.log(x[1]); // undefined

What you have to do is assign an array to the ith position in the outer array first:

var x = []; // create outer array
for(i=0;i<5;i++){
    x[i] = []; // create inner array at i
    for(j=0;j<3;j++){
        x[i][j] = "Z";
    }
}
console.log(x[0][0]);
share|improve this answer
    
Ah, that makes sense. I knew it was really an array of arrays, but I wasn't properly understanding the ways that made it fundamentally different from a regular array. –  Lenoxus Nov 18 '13 at 3:04
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