Grokking std.container and Ranges

[Steven Schveighoffer]

On Mon, 28 Jun 2010 23:01:42 -0400, Mike Parker <aldacron at gmail.com> wrote:

> I thought I understood ranges until I actually started trying to use  
> them. Now I'm having difficulties with the new range-based containers.  
> So I've got two issues right now, grokking ranges and understanding the  
> container interfaces.
>
> Given an SList, I want to do the following:
>
> foreach(obj; list)
> {
>     if(obj.pleaseKillMe)
>        somehow_remove_the_object_from_the_list();
> }
>
> Part of my problem is I'm not entirely clear what's going on with the  
> foreach. I know it's iterating a range, but is it the SList itself being  
> iterated, or is it a range returned by SList.opSlice? I assume the  
> latter, which at one point led me to try this (thinking of Java's  
> iterator.remove()):
>
> auto r = list[];
> foreach(obj; r)
> {
>     if(obj.pleaseKillMe)
>        r.popFront();
> }
>
> Which, of course, didn't work. I see that popFront doesn't actually  
> 'pop' anything off of, or out of, the range as it would in a traditional  
> stack or a queue. The foreach doc says about range.popFront:
>
> move the left edge of the range right one
>
> Meaning, it's more like a next() than the pop() I'm familiar with  
> (recalibrating all those years of C and Java terminology is not easy).  
> And even if it did, changes to the range do not propagate to the  
> underlying container. I understand that, at least (now).
>
> So apparently I want something like the list.stableRemove*() variants,  
> which the docs say remove an item from a container without invalidating  
> any ranges currently iterating the container. Great! Only, there's no  
> variant that accepts a single item. SList has removeFront and removeAny,  
> and ranges can be removed via linearRemove. I can insert individual  
> items fine. But how do I remove them?

linearRemove is it for SList.  Sucks, but that's what's available, until  
Andrei can figure out a better way to remove a range.

With a singly linked list, you can't remove the front of a range, because  
you have to alter the element *before* the front element.  What SList  
needs is a function to allow removing all but the *first* element in the  
range.  Something like removeTail.  I think it would be an SList-specific  
function, not much use outside there.

Anything you use will have to avoid foreach, removing elements from  
anything while using foreach on that thing is not a good idea (unless the  
foreach is doing it for you, see below).  You have to manually handle the  
ranges.

You may want to give dcollections a try.  They support purging (the  
operation you are trying to write) natively with foreach via opApply.  The  
linked list is dual-linked, so there are no issues with removing arbitrary  
elements (removal of a single element is an O(1) operation):

LinkList!T list;

...

foreach(ref doPurge, obj; &list.purge)
{
    doPurge = obj.pleaseKillMe;
}

http://www.dsource.org/projects/dcollections

Please note, the online docs are D1 only, the D2 docs are included in the  
distribution, but aren't as pretty.

-Steve