CODEWITHSUNDEEP
javascriptalgorithmsortingfor-loopiteration
mariaalouisaa
mariaalouisaa

Reputation: 46

For loop not iterating through whole array (Javascript)

I am trying to solve a problem that takes an array and moves all of the zeros to the end, preserving the order of the other elements.

This isn't iterating through the entire array, any ideas on what I'm doing wrong would be greatly appreciated.

let moveZeros = function (arr) {
  let newArr = [];
  for (let i = 0; i < arr.length; i++) {
    if (arr[i] === 0) {
      newArr.push(arr[i]);
      arr.splice(i, 1);
    }
  }
  return arr.concat(newArr);
};

moveZeros([9, 0, 9, 1, 2, 1, 1, 3, 1, 9, 0, 0, 9, 0, 0, 0, 0, 0, 0, 0]);

// expexted to return [9,9,1,2,1,1,3,1,9,9,0,0,0,0,0,0,0,0,0,0]
// currently returns  [9,9,1,2,1,1,3,1,9,0,9,0,0,0,0,0,0,0,0,0]

// Strangely... sending a smaller array to the function seems to work.

//moveZeros([false,1,0,1,2,0,1,3,"a"])
// correctly returns[false,1,1,2,1,3,"a",0,0]

Upvotes: 1

Views: 736

Answers (2)

Nina Scholz
Nina Scholz

Reputation: 386560

You could keep the array and move all not zero values to the start and fill the rest with zeroes.

let moveZeros = function(array) {
        let i = 0,
            j = 0;

        while (i < array.length) {
            if (array[i] !== 0) array[j++] = array[i];
            i++;
        }

        while (j < array.length) array[j++] = 0;

        return array;
    };

console.log(...moveZeros([9, 0, 9, 1, 2, 1, 1, 3, 1, 9, 0, 0, 9, 0, 0, 0, 0, 0, 0, 0]));
console.log(...moveZeros([false, 1, 0, 1, 2, 0, 1, 3, "a"]));

Upvotes: 0

Josh Regev
Josh Regev

Reputation: 379

The problem isn't related to the length, it occurs when you have more than one consecutive 0. For example [0,0,9,0,0,9] will come out [0,9,0,9,0,0].

In the first iteration of the loop (i=0), you remove the first 0 from the array, leaving you with [0,9,0,0,9]. On the second iteration (i=1), it's now looking at the second element in the array, which is 9. The first 0 is skipped. This will happen later in the array too as the loop progresses.

In general it can be problematic to modify an array as you loop through it. There are a number of ways you could perform a sort like this. But to keep it close to your original you could do this:

let moveZeros = function(arr) {
    let arrA = [];
    let arrB = [];
    for (let i = 0; i < arr.length; i++) {
        if (arr[i] === 0) {
            arrB.push(arr[i]);
        } else {
            arrA.push(arr[i]);
        }
    }
    return arrA.concat(arrB);
};

Now the function keeps the original array intact as it goes through it. Non-0 items are pushed to arrA, and 0s are pushed to arrB, and then those two are concatenated.

Upvotes: 2

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