Can Javascript create a multi-dimensional array on the fly

Question

I am iterating over a JSON array (rec'd from a webworker AJAX call)

var dataReceived = JSON.parse(xhr.responseText);//JSON verified as ok
                                               //dataReceived.length verified 
    var dataProcessed = []; 
   for (var i = 0; i < dataReceived.length; i++) {
    for ( var h = 0; h < dataReceived[i].length; h++) {
                dataProcessed[i][h][0]=((dataReceived[i][h][0])*30)-30;
                dataProcessed[i][h][1]=((dataReceived[i][h][1])*30)-30;
        }
    }
    postMessage(dataProcessed);

But i get the error

dataProcessed[i] is undefined

Does Javascript not create multidimensional arrays on the fly?

Yes its three dimensional, no nulls

EnglishAdam
– EnglishAdam

2014-02-24 10:08:42 +00:00
Commented Feb 24, 2014 at 10:08 — EnglishAdam
– EnglishAdam, Commented Feb 24, 2014 at 10:08
Question title changed

EnglishAdam
– EnglishAdam

2014-02-24 10:10:15 +00:00
Commented Feb 24, 2014 at 10:10 — EnglishAdam
– EnglishAdam, Commented Feb 24, 2014 at 10:10

T.J. Crowder · Accepted Answer · 2014-02-24 10:14:57Z

7

Does Javascript not create multidimensional arrays on the fly?

No, you have to create it:

for (var i = 0; i < dataReceived.length; i++) {
    dataProcessed[i] = [];        // <============= Here
    for ( var h = 0; h < dataReceived[i].length; h++) {
        dataProcessed[i][h] = []; // <============= And here

Side note: You can make that more efficient by factoring out your repeated lookups; also, you can create and initialize the innermost array simultaneously:

var dataReceived = JSON.parse(xhr.responseText);
var dataProcessed = []; 
var recEntry, procEntry;
for (var i = 0; i < dataReceived.length; i++) {
    procEntry = dataProcessed[i] = [];
    recEntry = dataReceived[i];
    for ( var h = 0; h < recEntry.length; h++) {
        procEntry[h] = [
            ((recEntry[h][0])*30)-30,
            ((recEntry[h][1])*30)-30
        ];
    }
}
postMessage(dataProcessed);

edited Feb 24, 2014 at 10:14

answered Feb 24, 2014 at 10:09

T.J. Crowder

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1 Comment

EnglishAdam Over a year ago

Thanks, like your thinking about how to solve, +1 and will accept in a minute or too

DhruvPathak · Accepted Answer · 2014-02-24 10:07:48Z

3

No, javascript does not create multidimensional arrays on the fly. You will have to look for the edge case .e.g. first iteration of the loop, and then create an empty array.

Or you can also initialize the array using || operator

for (var i = 0; i < dataReceived.length; i++) {

dataProcessed[i] = []; // array

for ( var h = 0; h < dataReceived[i].length; h++) {
            dataProcessed[i][h] = []; // array
            dataProcessed[i][h][0]=((dataReceived[i][h][0])*30)-30;
            dataProcessed[i][h][1]=((dataReceived[i][h][1])*30)-30;
    }
}
postMessage(dataProcessed);

answered Feb 24, 2014 at 10:07

DhruvPathak

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1 Comment

EnglishAdam Over a year ago

Thanks, changed the question title to help being found too, +1

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