Column Major or Row Major

[NA]

Is the data in DataTables stored in a column major or row major
format? Put another way, if I'm looping over all the elements, is it
faster to do:

foreach row:
foreach col:
table.getValue(r,c)
end
end

or

foreach col:
foreach row:
table.getValue(r,c);
end
end

thanks,

[Riccardo Govoni]

I would recommend against relying on a specifying datatable implementation (which anyway can change without notice), but instead perform your own profiling depending on your own use case.

For example, a test like the one at the bottom, with a 2000x2000 datatable, yields the following results on Chrome 13 (current beta):

colmajor iteration alone : ~1sec

rowmajor iteration alone: ~400ms

rowmajor followed by colmajor: ~500ms  followed by ~1.2sec

colmajor followed by rowmajor : ~1s followed by ~800ms

(mind that I've run the tests only a few times, so numbers are not necessarily stable).

The last one in particular suggests that Chrome, at least, performs some optimizations / caching out of your control (the same code behaves differently depending on which order we iterate first), hence I insist that your best option is to profile your own code and see what option works best in your scenario.

/R.

benchmark:

    console.time('creation');

    var n = 2000;

    var d = [];

    var header = [];

    for (var i = 0; i < n; i++) {

        header.push('hdr');

    }

    d.push(header);

    for (var i = 0; i < n; i++) {

        var row = [];

        for (var j = 0; j < n; j++) {

            row.push(Math.random());

        }

        d.push(row);

    }

    data = google.visualization.arrayToDataTable(d);

    console.timeEnd('creation');

    console.log(data.getNumberOfRows());

 

    console.time('colmajor');

    res = [];

    for (var col = 0; col < n; col++) {

        for (var row = 0; row < n; row++) {

            res.push(data.getValue(row, col));

        }

    }

    console.timeEnd('colmajor');

    console.time('rowmajor');

    res = [];

    for (var row = 0; row < n; row++) {

        for (var col = 0; col < n; col++) {

            res.push(data.getValue(row, col));

        }

    }

    console.timeEnd('rowmajor');