JS Snippets

In-place

This is some code which removes consequtive duplicates from an array.

function removeDuplicates(a) {

  let i = 0;
  for (j = 1; j < a.length; j++) {
      if (a[i] != a[j]) {
          i++;
          a[i] = a[j];
      }
  }

  return i + 1;

}

How does this work? Say we want to remove duplicates from an array such as [0, 1, 1, 2]. The code above steps through the following.

step		i	j	(a[i] != a[j])	0,1,1,2
1	start	0	1	0 != 1 > true	0,1,1,2
	end	1	1	a[1] = a[1]	0,1,1,2
2	start	1	2	1 != 1 > false	0,1,1,2
	end	1	2	no action	0,1,1,2
3	start	1	3	1 != 2 > true	0,1,1,2
	end	2	3	a[2] = a[3]	0,1,2,2
4	start	2	4	2 != - > true	0,1,2,2
	end	3	4	a[3] = a[4]	0,1,2

step 1 does seem to be pointles as the original array has the element at index position 1 set to equal the (same!) element at index position 1. Obviously this results in no change to the original array.

step 2 also results in no change. This time the condition in the if clause is not met and so i is not incremented and also no change is made to the array.

step 3 is the first step which results in a change to the array. The if clause evaluates to true so i is incremented by 1 and a[2] (1) is set equal to a[3] (2). So, now the array is [0,1,2,2].

step 4 results in the last element in the array being deleted. The if clause evaluates to true so i is incremented by 1 to equal 3 and then a[3] is set equal to a[4]. However, a[4] does not exist. So, the effect of setting a[3] to a non existant element is that a[3] now ceases to exist ie. a[3], the last element in the array, is removed from the array.

return value There are no more steps as the condition j < a.length is no longer true. The value of i +1 (4) .