# Re: Traversing multiple vectors "in parallel"

From:
Ulrich Eckhardt <eckhardt@satorlaser.com>
Newsgroups:
microsoft.public.vc.stl
Date:
Tue, 31 Oct 2006 09:39:52 +0100
Message-ID:
<qnej14-h4c.ln1@satorlaser.homedns.org>
Raphael Simon wrote:

Would you mind elaborating on how one would go writing an
iterator that would say add the 4 integers that are in each index of 4
integer vectors?

The important thing about an iterator is mainly that it provides operator++,
operator!= (and maybe operator==) and unary operator*, some algorithms
might need a nested value_type typedef and maybe some further code to make
iterator_traits work. Haven't played with the latter though, so I can't
tell you more.

Now:

struct my_iterator
{
typedef std::vector<unsigned>::iterator iterator_base;
my_iterator( iterator_base i1, iterator_base i2):
m_i1(i1), m_i2(i2)
{}

unsigned operator*() const
{ return *m_i1+*m_i2; }
void operator++() //TODO: correct returntype
{
++m_i1;
++m_i2;
}
friend bool operator==(my_iterator const& i1, my_iterator const& i2)
{
bool b1 = i1.m_i1==i2.m_i1;
bool b2 = i1.m_i2==i2.m_i2;
// if this fires, iterators are shifted, i.e. the sequences
// have different sizes.
assert(b1==b2);
return b1;
}
friend bool operator!=(my_iterator const& i1, my_iterator const& i2)
{ return !(i1==i2); }
private:
iterator_base m_i1;
iterator_base m_i2;
};

vector<unsigned> v1, v2;
....
for( my_iterator it(v2.begin(), v2.begin()), end(v1.end(), v2.end());
it!=end;
++it)
{
std::cout << "sum = " << *it << std::endl;
}

Now, Boost's zip_iterator rules. This here is basically the same, but much,
much more premitive.

Uli

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