Extract groups of items with same values from javaScript array

Question

I have an array of nested arrays containing coordinates. I would like to create a new array containing nested arrays of coordinates based on if they have the same latitude. The description might be a little confusing, so here some examples and code

Initial array (latitude is the second value of the number pairs)

const coordinateArray = [[46,11], [38,11], [44,9], [81,15], [55,15]];

Expected outcome:

const newArray = [
                  [[46,11],[38,11]],
                  [[81,15],[55,15]], 
                  [[44,9]]
                 ];

I tried this, but it returns every coordinate in a new array instead of pairing the ones with same latitude:

const rowArrays = [];
coordinateArray.map(c => {
    const row = [c];
    for (let i = 0; i < coordinateArray.length; i++) {
      console.log(c[1], coordinateArray[i][1]);
      if (c[1] === [1]) {
        row.push(coordinateArray[i]);
        coordinateArray.splice(0, 1);
      }
    }
    return rowArrays.push(row);
  });

Would be grateful for any suggestions

Tyler Roper · Accepted Answer · 2019-09-19 14:35:58Z

4

Your solution is close, but as you mentioned, it unconditionally creates a new array with each iteration through coordinateArray. Because your input-to-output is not 1:1 and instead you're looking to change the shape, reduce would be preferable to map.

If you reduced the array into an object based on the latitude, you could then use Object.values to achieve the result in your desired shape.

const coordinateArray = [[46,11], [38,11], [44,9], [81,15], [55,15]];
const matched = coordinateArray.reduce((out,arr) => {
  out[arr[1]] = out[arr[1]] //Have we already seen this latitude?
    ? [arr, ...out[arr[1]]] //Yes: Add the current coordinates to that array
    : [arr];                //No: Start a new array with current coordinates
  return out;
  }, {});

//const matched looks like this:
//{
//  "9":  [[44,9]],
//  "11": [[38,11],[46,11]],
//  "15": [[55,15],[81,15]]
//}
  
console.log(Object.values(matched)); //We only care about the values

If you favor brevity and/or have a bone to pick with your fellow developers, the reduce could be converted to a single expression using spread operators (...) and a falsy coalesce (out[arr[1]] || []).

const coordinateArray = [[46,11], [38,11], [44,9], [81,15], [55,15]];
const matched = coordinateArray.reduce((out,arr) => ({...out, [arr[1]]: [arr, ...(out[arr[1]] || [])]}), {});
console.log(Object.values(matched));

edited Sep 19, 2019 at 14:35

answered Sep 19, 2019 at 14:05

Tyler Roper

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1 Comment

danji_ma90 Over a year ago

Thanks a lot! This is what I needed

TheMaster · Accepted Answer · 2019-09-19 14:09:18Z

0

Create a map and group:

const coordinateArray = [[46,11], [38,11], [44,9], [81,15], [55,15]];
let map = coordinateArray.reduce((map,coord)=>{
  map.has(coord[1]) ? map.get(coord[1]).push(coord) : map.set(coord[1], [coord]);
  return map;
},new Map())
console.info([...map.values()])

answered Sep 19, 2019 at 14:09

TheMaster

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Comments

Maria · Accepted Answer · 2019-09-19 14:25:00Z

There are definitely better ways to write this with less lines of code posted above, but this might be a little bit easier for you to follow:

const coordinateArray = [[46,11], [38,11], [44,9], [81,15], [55,15]];

const rowArrays = [];

coordinateArray.forEach(c => {
    // set match to false at the beginning of every new coordinate
    let match = false;
    rowArrays.forEach (r => {
      // check if your coordinate lattitude already exists in rowArrays
      // if it does, push it to that array, and set match to true
      if (r[0][1] === c[1]) {
        r.push(c);
        match = true;
      }
    });
    // after iterating through every item in rowArrays, if you still
    // haven't found a match, push the new array to rowArrays.
   if(!match){
    rowArrays.push([c])
   }
  });

console.log(rowArrays)

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