Fastest way to "ignore" elements from array without removal

[z]

On Tuesday, 16 February 2021 at 06:03:50 UTC, H. S. Teoh wrote:
> It depends on what your goal is.  Do you want to permanently 
> remove the items from the array?  Or only skip over some items 
> while iterating over it?  For the latter, see 
> std.algorithm.iteration.filter.
The array itself is read only, so it'll have to be an array of 
pointers/indexes.

> For the former, you can use the read-head/write-head algorithm: 
> keep two indices as you iterate over the array, say i and j: i 
> is for reading (incremented every iteration) and j is for 
> writing (not incremented if array[i] is to be deleted).  Each 
> iteration, if j < i, copy array[i] to array[j].  At the end of 
> the loop, assign the value of j to the length of the array.
>
> Example:
>
> 	int[] array = ...;
> 	size_t i=0, j=0;
> 	while (i < array.length)
> 	{
> 		doSomething(array[i]);
> 		if (!shouldDelete(array[i]))
> 			j++;
> 		if (j < i)
> 			array[j] = array[i];
> 		i++;
> 	}
> 	array.length = j;
>
> Basically, the loop moves elements up from the back of the 
> array on top of elements to be deleted.  This is done in tandem 
> with processing each element, so it requires only traversing 
> array elements once, and copies array elements at most once for 
> the entire loop.  Array elements are also read / copied 
> sequentially, to maximize CPU cache-friendliness.
>
>
> T

This is most likely ideal for what i'm trying to 
do.(resizes/removes will probably have to propagate to other 
arrays)
The only problem is that it does not work with the first element 
but i could always just handle the special case on my own.[1]
I'll probably use .filter or an equivalent for an initial first 
pass and this algorithm for the rest, thank you both!

[1] https://run.dlang.io/is/f9p29A (the first element is still 
there, and the last element is missing. both occur if the first 
element didn't pass the check.)