JavaScript combining two dimensional arrays

Question

Let's say I have two arrays:

var a = [[1, 10], [2, 10], [3, 10], [4, 10], [5, 10]];
var b = [[1, 20], [3, 20], [4, 20]];

I want to combine these two into a new array:

var c = [[1, 10, 20], [2, 10], [3, 10, 20], [4, 10, 20], [5, 10]];

What's the best way to go about doing this. I'll update my question in a second with what I've tried to do.

var c = [];
for(var i = 0; i < a.length; i++) {
  for(var j = 0; j < b.length; j++) {
    if(a[i][0] == b[j][0]) {
      // Push b value into a
      a[i][0].push(b[j][1]);
    } else {
      // Don't do anything...
    }
  }
}

c = a;  // Useless code here but just wanted to return c
return c;

You could use map on the first array, loop through each of the second array, compare the first numbers and then append the first array with the numbers it doesn't already contain? — JakeSidSmith
– JakeSidSmith, Commented May 26, 2014 at 15:38
don't make yourself life harder by giving your iterators arbitrary names. use ai, bi, ci instead of i,j,... to avoid confusion (I started naming them like you myself and have to now force myself to overcome this bad habit) — Winchestro
– Winchestro, Commented May 26, 2014 at 15:43

JakeSidSmith · Accepted Answer · 2014-05-26 17:59:44Z

2

Example

var a = [[1, 10], [2, 10], [3, 10], [4, 10], [5, 10]];
var b = [[1, 20], [3, 20], [4, 20]];
var c = [];

// Loop over a
for (var n = 0; n < a.length; n++) {
    // Create copy of a's value
    var subA = a[n].slice(0);
    // Loop over b
    for (var i = 0; i < b.length; i++) {
        var subB = b[i];
        // Check if a contains b
        if (subA.indexOf(subB[0]) === 0) {
            // Add b to new a value
            subA.push(subB[1]);
        }
    }
    // Add new value to c
    c.push(subA);
}

console.log(a)
console.log(c);

Edit 1: updated for loop

Edit 2: If first item equals first item

Edit 3: Slice a to retain original values

edited May 26, 2014 at 17:59

answered May 26, 2014 at 15:52

JakeSidSmith

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7 Comments

Bergi Over a year ago

Also, don't use for in loops on arrays

JakeSidSmith Over a year ago

That wasn't in the question when I started writing my answer.

JakeSidSmith Over a year ago

This still creates an entirely new array with the same structure, and is easier to understand.

Bergi Over a year ago

Oh, I see. Yet, it's not an entirely new array, it does modify the subA arrays instead of copying them. Also, OP doesn't really want if a contains b but rather subA[0] == subB[0]

JakeSidSmith Over a year ago

You have a good point about modifying "a" actually. Didn't think about that.

|

Community · Accepted Answer · 2017-05-23 12:11:03Z

1

Why don't you use plain arrays? They can be sparse.

var a = [10, 10, 10, 10, 10];
var b = [20,   , 20, 20];

Now get them into one array:

var c = new Array(Math.max(a.length, b.length));
for (var i=0; i<c.length; i++)
    if (i in a && i in b)
        c[i] = [a[i], b[i]];
    else if (i in a)
        c[i] = [a[i]];
    else if (i in b)
        c[i] = [b[i]];

For the case that your objects (the items really don't have to be arrays) consist of a non-integer indexing values, you might use a simple standard merge algorithm on the two sorted lists. It will run in O(n+m), not in O(n*m) as your solution. See this answer for an example implementation and further pointers.

edited May 23, 2017 at 12:11

CommunityBot

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answered May 26, 2014 at 15:38

Bergi

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1 Comment

sir_thursday Over a year ago

My example above is simplified from what I'm actually doing. In my case, each array is actually a 2-D array of dates and values.

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