Hacker News Reader

Comment by brandonbloom | Hacker News Reader

xg15's comment still makes sense if you replace "copy all ..." with "slide the remaining elements down by the number of removed elements" to match the mutative behavior.

However, a bigger problem is concurrent modification: What happens if your reject! predicate has a side effect which modifies the array? Ideally you'd receive a "ConcurrentModificationException" or the like.

View on HN

That would still require O(n) space (to store the list of elements to keep or remove) so why not simply use the regular `reject` ?

Because you're saving the O(N) time copy...

iirc, Ruby's array doesn't expose the distinction between length and capacity, but internally it may decide to amortize the cost of such copies. For example, if reject only removes a small % of elements, it may choose not to shrink capacity.

You're not saving the O(N) time copy, just replacing it with an O(N) slide.

(That's assuming you copy at all, instead of simply updating the reference.)

I meant O(N) space, as you said "That would still require O(n) space" - but a slide does not require additional space.

No, before sliding elements, you set a local variable "dirty" to true. You iterate over all elements and keep the ones that are not rejected by moving them at the end of the current max index. When you code finishes normally, dirty is set to false.

You have an "ensure" block (finally) protecting the code which handles the cases where dirty is true (moving all the remaining elements after the current max). Then you truncate the array.

Reply on news.ycombinator.com