Sorting of array by another (not related) array

Question

I have two arrays of the same length. First array is static with data of such format a = [['q', 10], ['s', 20], ['z', 40], ['d', 15]] and another array b = [1, 0, 0, 1]. So ['q', 10] in mind is related to 1, ['s', 20] - to 0, ['z', 40] to 0, ['d', 15] to 1. Then I do some sorting of the first array and receive, for example, a = [['d', 15], ['s', 20], ['q', 10], ['z', 40]]. So in what way can I change second array to receive b = [1, 0, 1, 0]?

Important. It is not possible to add/change any data in first array.

look up a suitable sorting algorithm and implement it taking into account that you need to rearrange both arrays. — akonsu
– akonsu, Commented Jun 27, 2013 at 15:13
What do you mean, "it is not possible" to change the first array? What if you change it before sorting and then change it back? — Pointy
– Pointy, Commented Jun 27, 2013 at 15:14
What if you change it before sorting and then change it back? ok, such way is possible :) — user1931780
– user1931780, Commented Jun 27, 2013 at 15:15
I don't know why you'd need to do this, why not put the arrays togther eg. [['q', 10, 1], ['s', 20, 0], ['z', 40, 0], ['d', 15, 1]] — Blaine
– Blaine, Commented Jun 27, 2013 at 15:17

Pointy · Accepted Answer · 2013-06-27 15:17:39Z

2

If you can change the first array temporarily, you can do this:

Augment the first array with an index value for each element:

for (var i = 0; i < a.length; ++i) {
  a[i] = { value: a[i], index: i };
}

Sort the array. If your sort function is "compare", then you can do it like this:
```
a.sort(function(e1, e2) {
  return compare(e1.value, e2.value);
});
```

Now you can make a new "b" by arranging it according to the indexes:

var newB = [];
for (i = 0; i < a.length; ++i)
  newB[i] = b[a[i].index];
b = newB;

Now restore the values of "a":

for (i = 0; i < a.length; ++i)
  a[i] = a[i].value;

answered Jun 27, 2013 at 15:17

Pointy

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3 Comments

akonsu Over a year ago

or you can just create a property on each element of the original array a that holds the corresponding value from the original array b.

Pointy Over a year ago

@akonsu yes, that'd work, unless the original array elements are numbers or strings. Also, even if they're objects, that's risky because there might be a name collision. Of course, in the (common) case where one has complete knowledge of the nature of the array values, those may not be problems to worry about.

user1931780 Over a year ago

thanx. i used your idea. one correction, in step 1 'index' must be so: index: b[i]. and I also combined steps 3 and 4

wirey00 · Accepted Answer · 2013-06-27 15:22:43Z

1

You can keep an array of the return values of the first sort..

var c = [];
var newa = a.sort(function(a,b){
    var ret = a[1] - b[1];// whatever you're sorting on
    c.push(ret); // save return value
    return ret;
});

Then do the same with the second sort.

// go through and sort b the same way
var i=0;
var newb = b.sort(function(){
   return c[i++];
});

answered Jun 27, 2013 at 15:22

wirey00

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Comments

Samuel Reid · Accepted Answer · 2013-06-27 15:20:47Z

If you want a function to use in sorting that will swap elements in both arrays at the same time,

function swap(array1,array2,a,b) {
  temp = array1[a];
  array1[a] = array1[b];
  array1[b] = temp;
  temp = array2[a];
  array2[a] = array2[b];
  array2[b] = temp;
}

So then you can do

a = [0,1,2,3];
b = [0,1,2,3];
swap(a,b,0,1);

And you'll end up with

a: [1,0,2,3];
b: [1,0,2,3];

Otherwise, if you're actually looking to sort one array in ascending or descending order and have the other array be sorted according to how the first was sorted, check out php.js's array_multisort here: http://phpjs.org/functions/array_multisort/

And for some extra documentation on array_multisort, http://php.net/manual/en/function.array-multisort.php

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