Javascript multidim array error

Question

I have a array like this:

var ret = ["-37.8497", "144.968", "Albert Park Grand Prix Circuit", "2.76083", "101.738", "Sepang International Circuit","26.0325", "50.5106", "Bahrain International Circuit",...]

and i want it to be like this:

var markers = [
  {latLng: [45.616944, 9.2825], name: 'Italian Grand Prix'},
  {latLng: [52.0732605, -1.0166426], name: 'British Grand Prix'},
  {latLng: [43.7345, 7.4214], name: 'Monaco Grand Prix'},
  {latLng: [49.332, 8.58], name: 'German Grand Prix'}, ...]

I implemented this function:

var j=0;
for (i = 0; i < ret.length; i+=3)
{
  markers[j].latLng[0] = ret[i];
  markers[j].latLng[1] = ret[i+1];
  markers[j].name = ret[i+2];
  j++;
 }

And i get this error:

Uncaught TypeError: Cannot read property '0' of undefined

in the first line of the for.

I googled for a while but it seemed to be that javascript has not a true support for multiples arrays. Can you help me with the correct implementation?

raina77ow · Accepted Answer · 2015-12-23 15:59:30Z

1

You'll have to create an object manually. Here's one possible way to do it:

var markers = [];
for (var i = 0; i < ret.length; i+=3) {
  markers.push({
    latLng: [ret[i], ret[i+1]],
    name: ret[i+2]
  });
}

Here I dropped j variable, swapping it for push; this way seems to be more concise and readable.

As for the error - see, there's no such thing as auto-vivification in JavaScript. This expression...

markers[j].latLng[0]

... is processed this way:

first, JS attempts to find a value of markers variable,
then it attempts to take that value's property which name is equal to j (0 at the first step of the loop)
then it attempts to take latLng property of the value found at the previous step
finally, this value's 0 property is filled with ret[i] value.

Apparently, if any of those steps results in undefined, subsequent attempt to query any property of undefined causes an error.

edited Dec 23, 2015 at 15:59

answered Dec 23, 2015 at 15:48

raina77ow

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3 Comments

Andreas Over a year ago

You're missing the closing ] for the latLng array. Also in the example the values in latLng are numbers not strings.

raina77ow Over a year ago

You're right, of course; even markers won't be auto-created. Updated the answer (and went to more simple .push).

Andreas Over a year ago

If markers wouldn't be an array the error message would be slightly different TypeError: Cannot set property '0'.... So please forget this part of my comment :)

Nina Scholz · Accepted Answer · 2015-12-23 15:58:46Z

You need two small changes:

var ret = ["-37.8497", "144.968", "Albert Park Grand Prix Circuit", "2.76083", "101.738", "Sepang International Circuit", "26.0325", "50.5106", "Bahrain International Circuit"],
    markers = [],
    i, j = 0;

for (i = 0; i < ret.length; i += 3) {
    markers[j] = {};                // add this for a new object
    markers[j].latLng = [];         // add this for a new array
    markers[j].latLng[0] = ret[i];
    markers[j].latLng[1] = ret[i + 1];
    markers[j].name = ret[i + 2];
    j++;
}

document.write('<pre>' + JSON.stringify(markers, 0, 4) + '</pre>');

Or in a single line:

var ret = ["-37.8497", "144.968", "Albert Park Grand Prix Circuit", "2.76083", "101.738", "Sepang International Circuit", "26.0325", "50.5106", "Bahrain International Circuit"],
    markers = [],
    i;

for (i = 0; i < ret.length; i += 3) {
    markers.push({ latLng: [ret[i], ret[i + 1]], name: ret[i + 2] });
}

document.write('<pre>' + JSON.stringify(markers, 0, 4) + '</pre>');

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