Question 170565 in Javascript Question posted in
A very good W3school tutorial can be found here.

Why does pushing an array into an array in recursion give me an empty result in Javascript?

var subsets = function(nums=[1,2,3]) {
    nums.sort( (a, b) => a - b);
    
    return dfs(nums, 0, [], []);
};

var dfs = function(nums, pos, tmp, res) {
    if(nums.length === pos) {
        res.push(tmp.slice());
        return;
    }

    // 選
    tmp.push(nums[pos]);
    dfs(nums, pos+1, tmp, res);

    // 不選
    tmp.pop()
    dfs(nums, pos+1, tmp, res);

    return res;
}

console.log(subsets());

I want to find all subsets of nums.
If nums=[1, 2, 3], I hope output is [[],[1],[2],[1,2],[3],[1,3],[2,3],[1,2,3]]

In my code,tmp is a subset of nums, and res is used to save all possible subsets of nums.

In the 9 line, if I write res.push(tmp), then I will print empty array.
But if I write res.push(tmp.slice()), I can print array with values.

if(nums.length === pos) {
    console.log(tmp);
    console.log(tmp.slice());
    res.push(tmp.slice());
    return;
}

If I print both tmp and tmp.slice() like above, they both have same value.

I am so confused, how to explain this situation?

Maybe in recusive, res array doesn’t reuse value ?

Tags:

2

Answers


  1. 0

    From the docs, Array.prototype.slice() returns a shallow copy of the array being sliced. If you want to push a shallow copy to the result array, you could replace the line containing the slice method with:

    res.push([...tmp]);

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  2. 0

    When you pass tmp to dfs in the recursive call (e.g. dfs(nums, pos+1, tmp, res)), no copy of it is made; you’re essentially passing a reference to that array and thus tmp always refers to the same one array throughout the execution of the program. Consequently, res.push(tmp) always adds the same one array to res, so at the end you end up with multiple references to the same array in res.

    After dfs is complete, tmp is cleared because all elements are popped. res consists of multiple references to tmp which all reflect changes to that array, so you see an array containing multiple empty arrays.

    Array#slice makes a shallow copy of the array, so res.push(tmp.slice()) will add a different array each time, avoiding the issue.

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