How to get range for single just inserted element?

DList!int list;
...
list.insertBack(5);
auto r = ??? // get range for single last element
...
list.insertBack(something);
...
list.remove(r);

On Monday, 5 November 2012 at 16:41:16 UTC, Jack Applegame wrote:
> How to get range for single just inserted element?
>
> DList!int list;
> ...
> list.insertBack(5);
> auto r = ??? // get range for single last element
> ...
> list.insertBack(something);
> ...
> list.remove(r);

Something like

>>auto r = list[$ - 1];

On Monday, 5 November 2012 at 18:06:44 UTC, Michael wrote:
> Something like
>
> auto r = list[$ - 1];

Error: no [] operator overload for type DList!(int)

On Monday, 5 November 2012 at 16:41:16 UTC, Jack Applegame wrote:
> How to get range for single just inserted element?
>
> DList!int list;
> ...
> list.insertBack(5);
> auto r = ??? // get range for single last element
> ...
> list.insertBack(something);
> ...
> list.remove(r);

http://forum.dlang.org/thread/cfkllwgfushidyuwzuwp@forum.dlang.org

On Monday, 5 November 2012 at 18:52:24 UTC, cal wrote:
> http://forum.dlang.org/thread/cfkllwgfushidyuwzuwp@forum.dlang.org
I read it. It's absolutely useless for my question.

On 05.11.2012 17:41, Jack Applegame wrote:
> How to get range for single just inserted element?
>
> DList!int list;
> ...
> list.insertBack(5);
> auto r = ??? // get range for single last element
> ...
> list.insertBack(something);
> ...
> list.remove(r);

There is no way to do that in constant time with current dlist interface. (You could do something like list[].drop(walkLength(list)-1).takeOne), which is embarrassing.

I think DList should offer a native backwards range, so that you can do list.retro.takeOne.

On Monday, November 05, 2012 17:41:15 Jack Applegame wrote:
> How to get range for single just inserted element?
> 
> DList!int list;
> ...
> list.insertBack(5);
> auto r = ??? // get range for single last element
> ...
> list.insertBack(something);
> ...
> list.remove(r);

If you want to remove an element, do

auto range = find(list[], elem);
list.remove(take(range, 1));

Unfortunately, it looks like insert* makes the choice of returning the number of elements inserted instead of a range starting at the newly inserted element (C++ std::list returns an iterators pointing to that element, which seems a lot more useful to me). So, you can't easily get at a range to the most recently inserted element.

However, since you know that it's the last element, if you want to remove it, it's easy. Just use

list.removeBack();

If you want a range to the last element for something other than removing it, then you'd probably have to do something like

auto range = retro(take(retro(list[]), 1));

though that won't be the original range type. If you need that, you'd probably have to pop elements off until there's only one left (probably by saving before every pop and then returning the saved range when the range is empty).

- Jonathan M Davis

On Monday, November 05, 2012 19:35:57 Jack Applegame wrote:
> On Monday, 5 November 2012 at 18:06:44 UTC, Michael wrote:
> > Something like
> > 
> > auto r = list[$ - 1];
> 
> Error: no [] operator overload for type DList!(int)

The range returned by list isn't random access, and it can't be random access, so that won't work. Regardless, that would have given you the last element, not a range over just the last element. It would have to be list[$ - 1 .. $] to give you a range over just the last element, but that requires a sliceable random-access range (which DList's range can't be). If all you want is the last element, then use list[].back, since DList's range is bidirectional.

- Jonathan M Davis

On Monday, 5 November 2012 at 19:31:25 UTC, Jonathan M Davis wrote:
> If all you want is the
> last element, then use list[].back, since DList's range is bidirectional.
>
> - Jonathan M Davis
No. I want after inserting element, remember its "position" (in C++ I used iterator) and remove it later even if other elements was inserted after it.

On Monday, November 05, 2012 21:59:05 Jack Applegame wrote:
> On Monday, 5 November 2012 at 19:31:25 UTC, Jonathan M Davis
> 
> wrote:
> > If all you want is the
> > last element, then use list[].back, since DList's range is
> > bidirectional.
> > 
> > - Jonathan M Davis
> 
> No. I want after inserting element, remember its "position" (in C++ I used iterator) and remove it later even if other elements was inserted after it.

If you're using insertBack like you're original post lists, then you know that it's at the back. However, unfortunately, DList made the choice of returning the number of elements inserted rather than a range starting at the element inserted, so if you want a range to that element, you're either going to have to do

auto range = retro(take(retro(list[]), 1));

or

typeof(list[]) targetRange = list[];

for(auto range = list[]; list.empty; list.popFront())
 targetRange = range.save;

The first is obviously more efficient, but it won't give you the same type as list[] (whether that matters depends on what you're doing). The second will given you the correct type but is obviously suboptimal and only works because you know that you inserted it at the end. If you were inserting in the middle somewhere, you'd have to use find to find the new element (which could be problematic if the list contains duplicates), or you'd have to somehow know how deep into the list the element is and pop that many elements from list[].

Personally, I think that it was a serious mistake for insert* to not return a range like C++ returns an iterator, but at least for the moment, that's what it does. The basic design for std.container is solid, but a number of the details definitely need some work, particularly since this is the first major attempt to have containers using ranges exclusively without anything like an iterator, and it's one area where ranges aren't always better than iterators.

- Jonathan M Davis