# Re: count_unique or unique_count - why does it not exist?

From:
"Gene Bushuyev" <spam@spamguard.com>
Newsgroups:
comp.lang.c++.moderated
Date:
21 Oct 2006 16:44:06 -0400
Message-ID:
<PUn_g.16755\$e66.11901@newssvr13.news.prodigy.com>
"Alberto Ganesh Barbati" <AlbertoBarbati@libero.it> wrote in message
news:waa_g.11087\$uv5.79027@twister1.libero.it...

Stephen Howe ha scritto:

Hi

Consider a sequence container that is sorted by some criteria
I am surprised that there is not a count_unique() (or unique_count()) that
counts between a range of iterators, considering duplicate elements as 1
element. I don't see that I can use count_if() as that would mean a
predicate with a state.

Or is there? Have I overlooked an algorithm in the C++ library?

The relationship between count() and count_unique() would be analogous to
the SQL aggregate functions:
COUNT(somefield) and COUNT(DISTINCT somefield)

There is no count_unique() algorithm because there are several ways to
implement it, with very different memory requirements, computational
complexity and pre-conditions. It's not very smart to think about having some
general-purpose algorithm for that.

You are right about many ways of implementing it. But why whould anybody
want an
algorithm with suboptimal complexity and memory characteristics? - like
the one
using slow and expensive copying to a set, while for sorted ranges there
is an
obvious algorithm with linear complexity and no copying? Absense of such
implementation of unique counting for sorted ranges in the standard
looks like
an oversight. But again, the standard library doesn't claim to be complete.
It looks to me the easiest way of solving this task would be simply
writing an
explicit loop, comparing data and incrementing counter. But it can be also
implemented in terms of inner_product. Below is the code off the top of
so it may contain errors, but the idea I think should be clear:

std::size_t sum(std::size_t count, bool equal)
{
return equal ? count : count + 1;
}

template<class It>
std::size_t unique_count(It first, It last)
{
return first != last ?
std::inner_product(first++, last, first, 1, sum,
std::equal_to<typename
It::value_type>())
: 0;
}

P.S. the above won't work with input iterators.
P.P.S. Is there a faster than linear algorithm (without caching the
count during
sorting)?

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