Making an independent copy of a reversed array in JavaScript

Question

Here is my fiddle: http://jsfiddle.net/sepoto/Zgu9J/1/

I'm starting with a reverse function:

function reverseArr(input) {
    var ret = new Array;
    for(var i = input.length-1; i >= 0; i--) {
        ret.push(input[i]);
    }

    //I tried changing the return value to
    //return ret.slice(0) which has no effect on making
    //an independent copy
    return ret;
}

The second array I make pointOrigins2 is not an independent copy of pointOrigins1. In other words modifying pointOrigins2 is also modifying pointOrigins1 which is not what I need to achieve. From my reading on StackOverflow I have tried a few options such as using slice or using a for loop however nothing seems to be working yet so I made a fiddle.

Is there a way to make an independent copy of the reversed array?

http://stackoverflow.com/questions/4518616/append-an-arrays-reverse-to-itself — adeneo, May 14 '14 at 23:26
This dupe question ended up with better answers https://stackoverflow.com/questions/30610523/reverse-array-in-javascript-without-mutating-original-array — Simon_Weaver, Nov 18 '17 at 09:38

score 2 · Accepted Answer · answered May 14 '14 at 23:14

2

You are making a new independent array, but you are not making independent copies of the items that fill your arrays. You need to do something like:

function reverseArr(input) {
    var ret = new Array;
    for(var i = input.length-1; i >= 0; i--) {
        ret.push({
            positionx: input[i].positionx, 
            positiony: input[i].positiony, 
            index: input[i].index
        });
    }

    return ret;
}

so that you are generating new objects (with the same properties) as well as the new array.

answered May 14 '14 at 23:14

lincolnk

11,218
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What's the point of cloning the objects when they are not manipulated? – Bergi May 14 '14 at 23:17
I don't know, that's just how I'm reading OP's question. His function is making a new array, but modifying the property of an array element would also affect the other array. – lincolnk May 14 '14 at 23:19
Ah, I see now - he has mentioned that in the fiddle, but not in the question. – Bergi May 14 '14 at 23:22

score 1 · Answer 2 · edited May 23 '17 at 11:46

1

All you have to do is clone your array before you reverse it! :)

The shortest way to type is:

var b = a.slice();

But using concat is correct as well. See:

Javascript fastest way to duplicate an Array - slice vs for loop

If slice does not create a copy for you, you can use numpy arrays and create a deep copy this way:

np.copy(a);

see: http://docs.scipy.org/doc/numpy/reference/generated/numpy.copy.html

edited May 23 '17 at 11:46

Community

1
1

answered May 14 '14 at 23:14

cgnorthcutt

3,890
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I had no luck using splice before calling reverseArr. – Eae May 14 '14 at 23:33
Make sure you call your function on the copy created by slice. It should work (if your function works). – cgnorthcutt May 14 '14 at 23:36
use np.copy(a); if slice doesnt create a copy for you – cgnorthcutt May 15 '14 at 05:17

score 1 · Answer 3 · answered May 15 '14 at 09:33

1

use this function

function reverseArray(input) {
var newArray = new Array();
for(var i = input.length-1; i >= 0; i--) {
    newArray.push(input[i]);
   }
   return newArray;
}

answered May 15 '14 at 09:33

Ramesh Boddepalli

191
1
7

Bergi · Answer 4 · 2014-05-14T23:26:51.367

Your function is working correctly, even if it could be abbreviated to

return input.slice().reverse();

//This will show that pointOrigins2 is not an independent copy of pointOrigins1
//When pointOrigins2 is modified pointOrigins1 is also being modified
pointOrigins2[0].index++;

No. You are not modifying the pointOrigins2 array here, you are only accessing it and modifying the point object - the same point that was in the pointOrigins1 array. Yet, pointOrigins1 !== pointOrigins2.

You can modify the array independent from the other, e.g. like

pointOrigins2[0] = {
    positionx: pointOrigins2[0].positionx, 
    positiony: pointOrigins2[0].positiony, 
    index:     pointOrigins2[0].index + 1
};

and pointOrigins1 will stay untouched. So if you want to have points whose properties you can modify without being reflected somewhere else, you will need to create new points.

score 0 · Answer 5 · answered May 14 '14 at 23:23

0

function reverseArr(input) {
    return input.slice().reverse();
}

answered May 14 '14 at 23:23

cgnorthcutt

3,890
34
41

Did you try my other suggestion? Just use slice to create a copy and then run the copy through your original method. That's the easiest solution by far. See: http://stackoverflow.com/a/23666706/3570653 – cgnorthcutt May 14 '14 at 23:34
I tried calling slice before calling my original reverseArr. No luck. An independent copy was not created. – Eae May 14 '14 at 23:41
You can create a deep copy of an numpy array then using np.copy(arr): http://docs.scipy.org/doc/numpy/reference/generated/numpy.copy.html – cgnorthcutt May 15 '14 at 05:16

Making an independent copy of a reversed array in JavaScript

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