Removing Items From Array

[leedsleedsleedsleeds]

Hello,

I am at a loss as to how my function can work, except for the number ‘-45’.
Please can someone explain why?

Kindest regards.

const myArr = [-10, 20, -21, 10, -23, -45, -44, 13, 5, -1, 15, 1, -25, -45, 17, 16];

const removeNeg = (arr) => {
  for (let i = 0; i < arr.length; i++) {
    if (arr[i] < 0) {
      arr.splice(i, 1);
    }
  }
  
  return arr;
}

console.log(removeNeg(myArr));

[kevinSmith]

The issue has nothing to do with the number -45. You are changing the loop on which you are doing math, while you are doing that math.

Consider this:

const myArr = [-1, -2, -3, -4, -5, -6];

const removeNeg = (arr) => {
  for (let i = 0; i < arr.length; i++) {
    console.log('\ni', i)
    console.log('arr[i]', arr[i])
    console.log('arr.length', arr.length)
    if (arr[i] < 0) {
      arr.splice(i, 1);
    }
  }
  
  return arr;
}

console.log(removeNeg(myArr));

Examine the output closely and it may make it clear what the issue is.

[leedsleedsleedsleeds]

Ah, of course. By removing the element from the array I am skipping over the next element when incrementing ‘i’.

It was just by chance that ‘-45’ was the only number to suffer the consequences multiple times.

Much appreciated

[kevinSmith]

Right. Something to consider is that there is a way to avoid this. Hint: Does the problem exist if we do it backwards? And you could also fix what you were doing with a tiny bit of math, but that gets a little messy code, imho.

And what Randy says is true - in fact that is a better approach because manipulating arrays is costly, having to shift all those elements each time - Randy’s approach would avoid that by just creating a new array. But it’s good to understand what is happening with the approach you were trying.