Question

If I have two maps which are guaranteed to have exactly the same set of keys, how can I efficiently iterate through both maps?

For example, say I have the following maps:

std::map<std::string, int> iMap;
std::map<std::string, std::vector<int> > vMap;

At some point they both end up with exactly the same set of keys. I now need to update all values of vMap based on the corresponding iMap value. The first thing that comes to mind is something this:

typedef map<string, int> map_t;
BOOST_FOREACH(map_t::value_type &p, iMap) {
  vMap[p.first].push_back(p.second);
}

However, it seems rather wasteful that we have to lookup each value of vMap[n] considering we’re effectively going through the keys in order. Is there any way we can take advantage of this?

Answer 1

If you’re absolutely sure that the keys are identical, you can iterate over both maps in lockstep:

auto it1 = iMap.begin();
auto it2 = vMap.begin();

while (it1 != iMap.end())
{
    it2->second.push_back(*it1);
    ++it1;
    ++it2;
}