Why filling 2D array in javascript fails?

Question

I want to fill a 2d array, and I have this code in javascript:

n=2; //rows
m=3; //columns
z=0;
array1=[];
for(i=0;i<n;i++){
    for(j=0;j<m;j++){
        array1[i]=[];
        array1[i][j] = z;
        console.log(i+","+j+"="+z); //debug
        z++;
    }

}

console.log(array1);

But instead of getting this expected result;

[[0, 1, 2], [3, 4, 5]]

I get:

[[undefined, undefined, 2], [undefined, undefined, 5]]

why!? I don't understand why I am getting undefined for all the items in each "inner" array, except the last one which is correct (here, the 2 and 5).

I made a debug step logging each i,j pair and the assigned z value, and i DO get the correct pair-values each time (i,j=z):

0,0=0
0,1=1
0,2=2
1,0=3
1,1=4
1,2=5

so, I think I am correctly filling the array using arr[i][j]=z each time, so why does it get undefineds for those cases? I also tried using the arr=new Array() instead of arr=[] syntax in both cases, but with the same result.

Any ideas? I'm almost sure it will be a trivial error, but I can't find what is wrong!.

Thanks.

You'd probably spot the error faster if you used meaningful variable names, such as row, rows, column, columns, and counter; instead of i, n, j, m, and z. — Patrick McElhaney
– Patrick McElhaney, Commented Jan 24, 2013 at 23:21

Interrobang · Accepted Answer · 2013-01-24 23:17:13Z

7

You are reinitializing the empty array in every loop iteration:

array1[i]=[];

When you should only be doing this once per value of i. Move the initialization out of the inner-most loop.

answered Jan 24, 2013 at 23:17

Interrobang

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1 Comment

DiegoDD Over a year ago

OH! I know it would be trivial, now I fell dumb. Thanks!

Ibu · Accepted Answer · 2013-01-24 23:17:19Z

0

Here is how it should be

n=2; //rows
m=3; //columns
z=0;
array1=[];
for(i=0;i<n;i++){
    array1[i]=[];
    for(j=0;j<m;j++){
        array1[i].push(z); // Here is my change
        console.log(i+","+j+"="+z); //debug
        z++;
    }

}

answered Jan 24, 2013 at 23:17

Ibu

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