How to run the loop inside the loop faster in javascript

Question

I have two array objects which is of different lengths in which data.rows is of length 955 and place_names is of length 287. I am running loop inside the loop which takes around 3 minutes to run. Is there any easy and fastest way to run the code below?

for (i = 0; i < place_names.length; i++) {
  for (j = 0; j < data.rows.length; j++) {
    if (place_names[i].name === data.rows[j].name) {
      geom.push(data.rows[j].st_asgeojson);
      geom1.push({
        name: data.rows[j].name,
        geometry: data.rows[j].st_asgeojson
      });
      matched_average_value.push(place_names[i].average);
      matched_sum_value.push(place_names[i].sum);
      matched_minimum_value.push(place_names[i].minmum);
      matched_maximum_value.push(place_names[i].maximum);
    }
    else {
      console.log("no matches found");
    }
  }
}

You can try this and compare. But I don't think it is gonna make much difference. Fiddle: jsfiddle.net/xoowvcf9 — Rayon
– Rayon, Commented Dec 17, 2015 at 12:43
are i and j local variables ? you could sort both list by name. — Hacketo
– Hacketo, Commented Dec 17, 2015 at 12:43
@Hacketo, Sorting will go through another loop right ? But that won't be nested one. Kind of helpful! Also you somewhere need to break a loop I suppose. if (place_names[i].name === data.rows[j].name) here ? — Rayon
– Rayon, Commented Dec 17, 2015 at 12:46
@RayonDabre sorting would allow to stop inner loop when reach the last item that match, and you could start the inner loop at the last item that previously match. — Hacketo
– Hacketo, Commented Dec 17, 2015 at 12:46
remove the console.log ... where you have it will result in up to 274000+ pointless console.logs — Jaromanda X
– Jaromanda X, Commented Dec 17, 2015 at 12:47

Sriram · Accepted Answer · 2015-12-17 12:57:11Z

You have two lists place_names and data.rows. The field you're interested in is the name field.

Depending on where you get this data from, it might be better to use a dictionary for this:

var dict = {};
place_names.forEach(
    function(place_name) {
        dict[place_name.name]=place_name;
    }
);

Now you can loop through your data.rows:

data.rows.forEach(
    function(row) {
        if(row.name in dict) {
            var place_name = dict[row.name];
            // Do what needs to be done with "row" and "place_name"
        }
    }
);

This should reduce your n^2 algorithm to a n log n or even n (Depending on JavaScript's dictionary indexing complexity, I'm not sure). This solution assumes that all place_names[i].name values are unique. If they aren't, then you might want to store a list of all matching place_names in your dict.

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