C++ Maps

[khal]

Hi, I am having difficultly understanding how to print the value of the vector inside a map.

map<string,vector<string> > test;

 map<string,vector<string> >::iterator itr;
    for(itr = test.begin(); itr != test.end(); itr++)
    {
        for(int i = 0; i != itr->second.size(); i++)
        {
            cout << itr->second[i] << endl;
        }
    }

This is what I have but it just seem strange. Is their a better method?

[Catfish2]

You could use iterators for vectors too:

for (vector<string>::const_iterator vi = itr->second.begin();
    vi != itr->second.end();
    ++vi
    )
    cout << *vi << endl;

If you're using Visual Studio 2010+, or GNU GCC 4.6+, they may offer support for C++11's range based for loop, and automatic type inference.

Then your code could look like:

map<string, vector<string> > test;

for (auto aPair: test)
    for (string s: aPair.second)
        cout << s << endl;

Disclaimer: code not tested, yet.

[andywestken]

Unfortunately, range-based fors didn't turn up with Visual Studio 2010. But they are there in 2012.

[khal]

So it would always have to be a nested loop in order to get the value in the vector?

[andywestken]

Well, yes. But the loops don't have to actually be nested, or even your own. For example, you could factor out the vector output code into a helper function?

template<typename TElem>
inline void write_vector(ostream& os, const vector<TElem>& vec) {
	typedef vector<TElem>::size_type size_type;
	const size_type size = vec.size();
	for(size_type i = 0; i < size; ++i) {
		os << vec[i] << endl;
	}
}

and then use it like

	typedef map<string,vector<string> >::iterator iter_type;
	for(iter_type itr = test.begin(), itr_end = test.end(); itr_end != itr; ++itr) {
		write_vector(cout, itr->second);
	}

or use an algorithm

	typedef map<string, vector<string> >::iterator iter_type;
	for(iter_type itr = test.begin(), itr_end = test.end(); itr_end != itr; ++itr) {
		const vector<string>& vec = itr->second;
		ostream_iterator<string> out_it(cout, "\n");
		copy( vec.begin(), vec.end(), out_it);
	}

or even use a iterative solution?

template<typename TIter>
inline void write_iteratively(TIter itr, const TIter itr_end, ostream& os) {
	if(itr != itr_end) {
		os << *itr << endl;
		write_iteratively(itr + 1, itr_end, os);
	}
}

used like this

	typedef map<string, vector<string> >::iterator iter_type;
	for(iter_type itr = test.begin(), itr_end = test.end(); itr_end != itr; ++itr) {
		const vector<string>& vec = itr->second;
		write_iteratively(vec.begin(), vec.end(),cout);
	}

[Catfish2]

... well you could use the horrible std::for_each() from the algorithm library, together with C++11 lambdas.

(Prepare a barf bag.)

#include <algorithm>

// ...

for_each(test.begin(), test.end(), [](decltype(*test.begin()) aPair) {
    for_each(aPair.second.begin(), aPair.second.end(), [](string s) {
        cout << s << ' ';
    });

    cout << endl;
});

[khal]

Thank You all for the help I understand it now.