Re: [boost] [Review:Algorithms] is_ordered return value

Subject: Re: [boost] [Review:Algorithms] is_ordered return value
From: Phil Endecott (spam_from_boost_dev_at_[hidden])
Date: 2011-09-24 12:42:41

Andrew Sutton wrote:
>> With the RV convention change that I proposed on github (always point
>> before the first element that is out-of-order with respect to its
>> predecessor, rather than the first that is out-of-order with respect
>> to its successor), it would be exactly the same, so I think we may be
>> stuck with this name, even though I agree that it sounds like it
>> should return a bool.
>
> I'm not sure I understand the recommendation. Are you saying that if I write:
>
> auto i = is_ordered(f, l, r)
>
> Then I should have:
>
> is_increasing(f, i);
>
> Or, writing using std algorithms:
>
> auto i = is_sorted_until(f, l, r)
> assert(is_sorted(f, i));

I wondered about this when I looked at the docs and was going to 
suggest "the docs should be explicit about which iterator they return"; 
then I decided that there was (obviously!) only one thing that could 
sensibly be returned.  I.e. if the input is 1,2,3,4,5,0,42,12 then the 
result should be an iterator referring to the 0 element.  Surely this 
what it does, no?

> On a side note, I think the algorithm can be trivially implemented
> using adjacent_find, and should have the same iterator requirements.
>
> return adjacent_find(f, l, not2(pred));

Right, except that that returns the "wrong" iterator of the pair, i.e. 
it would return an iterator returning to the 5 in the example above.  
You can fix that by incrementing the iterator, but that has to consider 
the end case specially.

Regards,  Phil.