array.push produces different result than console.log for Heap's Algorithm in Javascript

Question

I am trying to make an array of all the permutations for a given string using Heap's Algorithm. The function prints the correct array results when I use console.log to test. However, when I try to push all of the arrays into a new array it pushes the same initial array every time.

In this example it pushes [a, a, b] 6 times.

First of all, why is it doing this? Second, how do I get around it and push all of the correct results into my new array.

function permAlone(str) {
//make str into array
var arr = str.split('');
var allArrays = [];
//Heap's Algorithm to make new array with all permutations

var swap = function (array, pos1, pos2) {
    var temp = array[pos1];
    array[pos1] = array[pos2];
    array[pos2] = temp;

};

var permute = function(array, n) {
    n = n || array.length;
    if (n === 1 ) {
        console.log(array);
        //allArrays.push(array);
    }
    else {
        for (var i=1; i <= n; i += 1) {
            permute(array, n-1);
            if (n%2) {
                var j=1;
            }
            else {
                j=i;
            }
            swap(array, j-1, n-1);
        }
    }

};
permute(arr);
//console.log(allArrays);
}

permAlone('aab');

didn;t pay much attention to your code but it sounds like you need to use a closure. — I wrestled a bear once.
– I wrestled a bear once., Commented Nov 8, 2016 at 21:37

Paul · Accepted Answer · 2016-11-08 21:37:42Z

1

You only ever have one array (not counting allArrays), so you've pushed the same array into allArrays six times.

You should make a copy of your array before pushing it. You can do that with slice:

allArrays.push(array.slice(0));

answered Nov 8, 2016 at 21:37

Paul

142k28 gold badges285 silver badges272 bronze badges

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

3 Comments

andrewgi Over a year ago

Thanks that got it! Since it runs 6 times and array changes each time, I was assuming that it would push the different version of the array into the new array. I still don't quite understand why it doesn't.

Paul Over a year ago

@andrewgi Objects (which includes arrays) aren't copied like primitive values are when they are passed or assigned. Try this for instance: var arr1 = []; var arr2=arr1; arr1.push('test'); console.log( arr2 ); You'll see that arr2 and arr1 are literally the same array, so when you log arr2 you see the value test that was pushed into arr1. If you had var arr2=arr1.slice(0); to make a copy instead then you'd have two separate arrays and wouldn't see 'test'.

andrewgi Over a year ago

Ahhh ok I understand now, it's stored permanently and there is no catching a copy or "screenshot" of it when its running through a loop. Thanks!!

Collectives™ on Stack Overflow

array.push produces different result than console.log for Heap's Algorithm in Javascript

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related