how to copy elements from a map to a std:: string - c++

Question

I am writing a program that implements a uni-directional graph using std::map. I have to search for the key and then copy the vector values corresponding to that key into a new location in the map. The problem is that the key I search for is extracted from yet another location in the map. I understand why I am getting the error as std::copy_n does not copy from vector to string (although it works the other way) but I don't know how to fix this.
I am copying a snippet of the code below:
The keys to search for are stored in a vector corresponding to key mykey.
for (
vector<string>::iterator itr = mymap.find(mykey)->second.begin()
; itr != mymap.find(mykey)->second.end()
; itr++
)
{
string find_key = "";
// error C2664 :cannot convert parameter 1 from 'std::_Vector_iterator<_Myvec>'
// to 'const std::basic_string<_Elem,_Traits,_Ax> &'
std::copy_n(itr, 1, find_key.begin());
if (mymap.find(find_key) == mymap.end())
cout << "key not found" << endl;
else
mymap[mykey].insert(
mymap.find(find_key)->second.begin()
, mymap.find(find_key)->second.end()
);
}
Edit: using stringstream to extract key from map
for (vector::iterator itr =mymap.find(mykey)->second.begin(); itr!=mymap.find(mykey)->second.end();++itr){
stringstream ss;
ss<< *itr;
string find_key = ss.str();
if (mymap.find(find_key)==mymap.end())
cout<<"key not found"<<endl;
else
for (vector<string>::iterator mapit= mymap.find(find_key)->second.begin(); mapit!=mymap.find(find_key)->second.end(); ++mapit)
mymap[mykey].push_back(*mapit);}
}
I am now getting a "vector not incrementable" error. Can you please tell me why?Also, what happens to duplicate values in a map? Thanks a lot for your help!

Answer 1

copy_n with a size one copy only one element (of course), but you're mixing an input iterator on string and an output iterator on char.
So do you want to read the first character of the current string or the whole string? I guess the second makes more sense in your context. So you could simple deference the iterator:
string const& find_key = *itr;
Regarding performance, even though I do not recommend to look into optimizations too earlier in the development of your software, here you perform some searches way too many times.
For example, in the first loop, at every iteration you'll look for mykey in your map. You could rewrite it using C++11 features:
for (auto const& str : mymap.find(mykey)->second)
This will call find once only and iterate on the (assuming to be existing) associated vector of string.
Here I guess that str is what you called find_key in your original post.
Then again you call three times mymap.find(find_key). That can be optimized quite easily. Additionally, does the insert compile? I couldn't find a matching function in the doc...
auto it = mymap.find(find_key);
if (it == mymap.end()) {
cout << "key not found" << endl;
} else {
auto const& vec = *it; // a ref to vector of strings to be copied
auto& dest = mymap[mykey];
dest.insert(std::end(dest), std::begin(vec), std::end(vec));
}
And when we put everything together we get:
for (auto const& find_key : mymap.find(mykey)->second) {
auto it = mymap.find(find_key);
if (it == mymap.end()) {
std::cout << "key not found" << std::endl;
} else {
auto const& vec = *it; // a ref to vector of strings to be copied
auto& dest = mymap[mykey];
dest.insert(std::end(dest), std::begin(vec), std::end(vec));
}
}
Doesn't that make more sense? ;-)