Re: [Boost-users] [range] count vs count_if

Subject: Re: [Boost-users] [range] count vs count_if
From: Bill Buklis (boostusr_at_[hidden])
Date: 2012-02-13 12:26:16

On 2/12/2012 7:03 AM, John M. Dlugosz wrote:
> On 2/10/2012 11:39 AM, Bill Buklis wrote:
>>>
>>>>
>>>> What am I missing? How can count_if be properly expressed as count. 
>>>> To me it seems like it
>>>> can't.
>>>>
>
> I agree.  "count returns the number of elements x in rng where x == 
> val is true."
> If you filter the original range using the predicate instead, you 
> don't want to then compare them all against some constant.  If you did 
> so, you would need a special value such that every one of them 
> compared true.
>
>
> You just want to count the number of items in the range you get after 
> filtering.  The obvious boost::size requires a bidirectional range, 
> and is probably less efficient then we would want.
>
> Nathan mentioned boost::distance, which is hard to find in the 
> documentation.  But I see it listed on the synopsis under Forward 
> Range functions.  I guess it's missing from the page 
> doc/range/concepts/forward_range.html
>
> In general, it is true that having two algorithms "do something with 
> or to each element" and "same, but selecting elements via a predicate" 
> can be generalized to using the first (only) with a range filter.
>
> But count and count_if are not such a pair.  count also has a built-in 
> predicate, but of the form el==value, rather than acting implicitly on 
> all the elements.
>
> —John

Indeed, the documentation must be wrong. Hopefully it can be 
appropriately fixed soon.

That is an excellent suggestion about using distance. I had not thought 
of that, nor, come to think of it, did I even know that there was a 
boost::distance function. But, it makes sense. On the other hand, I do 
have to wonder if using "distance" for "count" would obfuscate the code. 
In general, I think I would rather use count_if just for clarity. But, 
at least the statement that the "_if functions aren't required", still 
holds true even if it may not be rather obvious which one to use.

-- 
Bill