Modify a variable locally but not globally

Question

I need to save a copy of a globally set array variable, while I'm modifying it locally. For some reason the code below doesnt seem to do this.

var tileArray;
        .
        .
        .
function simplify (tiles){

    tileArray = tiles;  //tileArray is the global variable passed to simplify()
    for (var i = 0; i < tiles.length; i++){
      if (tiles[i].numeratorDim == ""){tiles[i].numeratorDim = 1};
      if (tiles[i].denominatorDim == ""){tiles[i].denominatorDim = 1};
    };
    while (thereIsAMatch(tiles)){ // a bunch of code modifying tiles, but not tileArray
        for (var i = 0; i < tiles.length; i++){
            for (var j = 0; j < tiles.length; j ++) {
                if (unitsMatch(tiles[i].numeratorUnit, tiles[j].denominatorUnit)) {
                    tiles[i].numeratorDim = parseInt(tiles[i].numeratorDim) - 1; 
                    if (tiles[i].numeratorDim == 0){tiles[i].numeratorUnit = ""}
                        tiles[j].denominatorDim = parseInt(tiles[j].denominatorDim) - 1;
                    if (tiles[j].denominatorDim == 0){tiles[j].denominatorUnit = ""}
                  } // if unitsMatch
              } // for j
          } //for i
        } //while thereIsAMatch
    console.log(tileArray === tiles);  //returns TRUE!! But tileArray hasnt been modified!!

    return tiles;
} // function simplify

@PaulThomas doesn't really matter as assigning an array reference to a variable does not make a copy of the array. — Pointy, Dec 20 '18 at 22:40
`while, for, for, if, if` <-- This screams poor design and is definitely an anti-pattern. — Scott Marcus, Dec 20 '18 at 22:41
actually, since it's an array of objects, you probably want to clone each object: `let tiles = tileArray.map(({...props}) => ({...props}))` — ic3b3rg, Dec 20 '18 at 22:46
Possible duplicate of [What is the most efficient way to deep clone an object in JavaScript?](https://stackoverflow.com/questions/122102/what-is-the-most-efficient-way-to-deep-clone-an-object-in-javascript) — Taplar, Dec 20 '18 at 22:54

score 0 · Answer 1 · answered Dec 20 '18 at 22:46

0

You need to do a deep copy on this array because right now tiles and tileArray are pointing to the same piece of memory (reference). You can do this with tileArray = Arrays.from(tiles)

Here's a helpful article: https://medium.com/@gamshan001/javascript-deep-copy-for-array-and-object-97e3d4bc401a

answered Dec 20 '18 at 22:46

Michael

49
3

`Array.from()` does not disjoin the values, as noted on that link. – Taplar Dec 20 '18 at 22:52
1

Array.from() did not achieve the deep copy i needed, but you definitely pointed me in the right direction! Thanks! I ended up making new Tile objects in a for loop and populating its methods and properties one-at-a time. (I'm sure there are *way* more elegant ways to do this, but I am just a hobbiest trying to get my program to work, by hook or by crook,) Anyway - really appreciate your willingness to help! – RobertHauss Dec 21 '18 at 15:24
Not a problem! :) – Michael Dec 21 '18 at 16:42

score -1 · Answer 2 · answered Dec 20 '18 at 22:44

-1

Basically, tileArray refers to the same array, instead of creating a copy for that,

Use :

tileArray = tiles.slice()

slice(), operation clones the array and returns the reference

answered Dec 20 '18 at 22:44

Ankur Goel

311
1
11

This only disjoins the array references. It does not disjoin the array values themselves. – Taplar Dec 20 '18 at 22:49

score -1 · Answer 3 · answered Dec 20 '18 at 22:45

-1

Have a look at this question

To clone an array try something like:

var dup_array = original_array.slice();

But note what they say about it being a shallow copy.

answered Dec 20 '18 at 22:45

Kelvin

333
2
7

Modify a variable locally but not globally

3 Answers3