# Re: I'd like to use STL algorithms

From:
"kanze" <kanze@gabi-soft.fr>
Newsgroups:
comp.lang.c++.moderated
Date:
22 Sep 2006 08:46:09 -0400
Message-ID:
<1158915948.378994.300040@m73g2000cwd.googlegroups.com>
Alberto Ganesh Barbati wrote:

kanze ha scritto:

His iteration actually involves the numerical value of the
index. As such, any conversion to the STL is likely to be
obfuscation, or at least more work, requiring that he maintain
the index manually anyway. In general, when the numerical value
of the index is significant, the STL (nor any other library
based on iterators) is not the answer.

I believe it should be easy to implement an iterator adapter
that produces pairs (index, it). Basically it might be similar
to boost::counting_iterator holding two incrementables instead
of one. Then you could use something like:

std::transform(
make_double_counting_iterator(tokens.begin(), 0), // 0 is start index
make_double_counting_iterator(tokens.end(), -1), // -1 is a dummy
back_inserter(val),
convert_adapter());

where convert_adapter is

struct convert_adapter
{
template <class It, class Diff>
void convert(const std::pair<It, Diff>& p)
{ convert(p.second, *p.first); }
};

Apart from the necessity of the adapter, I don't see this code
so obfuscated. How would you rate this design?
double_counting_iterator is not tied to this problem domain
and could also be useful as a generic tool.

There's one thing I'm having trouble understanding here: if the
problem is stated in terms of 'for index i, do convert(i)', why
do you want to get rid of the index? Any removal of the index
here is obfuscation; it hides the basic statement of the problem
we are trying to solve. Why do you want to add all the
complexity of a new type of iterator, and an adapter, for what
is basically a simple problem?

When the numeric value of the index is significant, any use of
iterators seems wrong to me. The whole idea behind an iterator,
as I see it, is to free the concept of visiting a sequence of
objects in succession from the need of a numeric value. If you
need the numeric value, you don't want an iterator, if you can
avoid it.

--
James Kanze GABI Software
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