c++ Initialising 2 different iterators in a for loop [duplicate]

Question

Possible Duplicate:
Can I declare variables of different types in the initialization of a for loop?

I'd like to have a for loop in c++ which constructs 2 different kinds of vector iterator in the initialisation.

Here is a rough idea of what I would like:

std::vector<double> dubVec;
std::vector<int> intVec;
double result = 0;

dubVec.push_back(3.14);
intVec.push_back(1);

typedef std::vector<int>::iterator intIter;
typedef std::vector<double>::iterator dubIter;

for (intIter i = intVec.begin(), dubIter j = dubVec.begin(); i != intVec.end(); ++i, ++j)
{
  result += (*i) * (*j);
}

Anyone know what is the standard to do in this situation? I can't just use a vector of double for the intVec because I'm looking for a general solution. [i.e. I might have some function f which takes int to double and then calculate f(*i) * (*j)]

Answer 1

You can't declare variables of different types inside a for loop.

Just declare them outside:

intIter i = intVec.begin();
dubIter j = dubVec.begin();
for (; i != intVec.end(); ++i && ++j)
{
}

Answer 2

You could declare a std::pair with first and second as the iterator types:

for (std::pair<intIter, dubIter> i(intVec.begin(), dubVec.begin());
     i.first != intVec.end() /* && i.second != dubVec.end() */;
     ++i.first, ++i.second)
{
    result += (*i.first) * (*i.second);
}

Answer 3

Check out the zip iterator. It does exactly what you want: parallel iterate over two or more sequences simultaneously. Using that, I'd write it as:

using namespace boost;

for (auto i=make_zip_iterator(make_tuple(dubVec.begin(), intVec.begin())),
          ie=make_zip_iterator(make_tuple(dubVec.end(), intVec.end()));
          i!=ie; ++i)
{
  // ...
}

Admittedly, this get's a little more complicated if you don't have support for auto or other type inference in your specific case, but it can still be quite nice with a typedef.