javascript 2D array - sorting and merging

Question

I have a 2D array like below:

1230 | this is a test
1278 | my new test
1230 | test2
7654 | testing...

I want to transform the array so that values are unique in the first column and the second column stores the concatenated text associated with that particular value. See example below.

1230 | this is a test -- test2
1278 | my new test
7654 | testing...

I understand I should first sort the array by its first column and then do the merging.

Here is the code for sorting the array by the first column:

var x = exp_arr.sort(function(a,b){ return a[0] > b[0] ? 1 : -1; });
alert(x);

I am a bit lost about how to do the merging. Your suggestions are most welcome

Merge by copying the items to a new array if the "id" is not present and merging if it is. Sort the resulting array passing a custom comparator function. — Prusse
– Prusse, Commented Sep 30, 2014 at 15:41

mccannf · Accepted Answer · 2014-09-30 15:47:45Z

4

You can use the reduce command like so:

var array = [[1230, "this is a test"], [1278, "my new test"], [1230, "test2"], [7654, "testing..."]];

var result =
    array.reduce(function(ob, ar) {
              if (!(ar[0] in ob.nums)) {
                  ob.nums[ar[0]] = ar
                  ob.result.push(ar);
              } else
                  ob.nums[ar[0]][1] = (ob.nums[ar[0]][1]) + " -- " + ar[1];

              return ob
          }, {nums:{}, result:[]}).result
    .sort(function(a,b) {
        return a[0] - b[0];
    });

Fiddle here.

Reduce documentation here.

answered Sep 30, 2014 at 15:47

mccannf

16.6k3 gold badges54 silver badges64 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Kim Over a year ago

Thank you so much. The link to the documentation was very helpful indeed :)

score 1 · Accepted Answer · 2014-09-30 16:09:14Z

First group array elements with the same first value into object whose keys are the first element, and values are arrays of second elements:

groups = array.reduce(function(result, elt) {
    result[elt[0]] = (result[elt[0]] || []).concat(elt[1]);
    return result;
});

Then map the object back into an array, by joining the members of each key's array with --.

Object.keys(groups).map(function(key) { return [key, groups[key].join(' -- '); })

Or, taking a leaf out of @friedi's book, taking advantage of sorting the array first, but using reduce instead of filter, which is slightly simpler because it means we do not need a IIFE to close around a previous item:

array.sort(sortfunc).reduce(result, item) {
    var prev = result[result.length-1] || [];
    if (prev[0] === item[0]) {
        prev[1] += " -- " + item[1];
    } else {
        result.push(item);
    }
}, []);

Either is far more readable than the accepted solution.

friedi · Accepted Answer · 2014-09-30 16:16:11Z

1

You can do this:

var x = exp_arr.sort(function(a,b){ return a[0] > b[0] ? 1 : -1; }).filter((function() {
    var prev = [null];
    return function(item) {
        if(prev[0] === item[0]) {
            prev[1] += ' -- ' + item[1];
            return false;
        }
        prev = item;
        return true;
    };
}()));

Explanation:
The filter function takes advantage of the fact that the array is already sorted. So every element is compared with the previous element (herefore a closure was used to save the previous element). If the previous element has the same id, we append the string to the previous element.

Here is the jsfiddle-demo

edited Sep 30, 2014 at 16:16

user663031

answered Sep 30, 2014 at 15:55

friedi

4,3601 gold badge15 silver badges19 bronze badges

Collectives™ on Stack Overflow

javascript 2D array - sorting and merging

3 Answers 3

1 Comment

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related