unique_copy elements value from map to vector - c++

Question

I have a map<K, V> and I want to use unique_copy to put the values into a vector<V>.
I tried this but it doesn't work:
#include <iostream>
#include <map>
#include <algorithm>
#include <vector>
#include <functional>
using namespace std;
using namespace placeholders;
int main() {
map<std::string, int> mp;
mp["1"] = 1;
mp["2"] = 2;
mp["3"] = 3;
mp["4"] = 3;
mp["5"] = 3;
mp["6"] = 4;
mp["7"] = 2;
vector<int> vec;
unique_copy( mp.begin(), mp.end(),
back_inserter(vec),
bind(&map<std::string, int>::value_type::second, _1) );
for( auto& i : vec )
cout<< i <<" ";
}
Expected output:
1 2 3 4 2
All google searches return ways to use transform but I need unique_copy. Is there any way to make this work?

Answer 1

There's no way to do this directly, because the value type of the map iterator and the vector iterator are not compatible. You really need a transform of some sort, or an iterator adaptor.
You could use boost::transform_iterator for this:
auto getValue = [](const std::map<std::string, int>::value_type &pair) { return pair.second; };
unique_copy(
boost::make_transform_iterator(mp.begin(), getValue),
boost::make_transform_iterator(mp.end(), getValue),
back_inserter(vec)
);
If you cannot use Boost, you'll have to write such an iterator adaptor yourself:
template <class T_PairIterator>
struct SecondIterator
{
typedef T_PairIterator PairIterator;
typedef typename std::iterator_traits<PairIterator>::iterator_category iterator_category;
typedef typename std::iterator_traits<PairIterator>::value_type::second_type value_type;
typedef typename std::iterator_traits<PairIterator>::difference_type difference_type;
typedef value_type *pointer;
typedef value_type &reference;
PairIterator it;
SecondIterator() {}
explicit SecondIterator(PairIterator it) : it(it) {}
pointer operator-> () const { return &it->second; }
reference operator* () const { return it->second; }
SecondIterator& operator++ () { ++it; return *this; }
SecondIterator operator++ (int) { SecondIterator ret(*this); ++it; return ret; }
};
template <class T>
bool operator== (const SecondIterator<T> &lhs, const SecondIterator<T> &rhs)
{ return lhs.it == rhs.it; }
template <class T>
bool operator!= (const SecondIterator<T> &lhs, const SecondIterator<T> &rhs)
{ return !(lhs == rhs); }
template <class T>
SecondIterator<T> makeSecondIterator(const T &it)
{ return SecondIterator<T>(it); }
You could then use it like this:
unique_copy(
makeSecondIterator(mp.begin()),
makeSecondIterator(mp.end()),
back_inserter(vec)
);
Of course, the adaptor could be made a bit more generic (perhaps usable for first as well), and/or more encapsulated (it needn't be public); it would also need proper handling for const-iterators. But the above should be enough to give you the idea.

Answer 2

std::copy followed by vec.erase( std::unique( vec.begin(), vec.end() ), vec.end() ); will end up in the same place as unique_copy with a transform would. It may use extra memory in the middle (before removing redundant entries) but is pretty clean.
A manual copy is another option:
if(!src.empty()){
auto* prev = &src.begin()->second;
dest.push_back(*prev);
for(auto it = std::next(src.begin()); it != sec.end(); ++it){
if (it->second == *prev) continue;
prev=&it->second;
dest.push_back(*prev);
}
}
with the note thay prev is technically redundant (use dest.back() instead of *prev except when push_backing -- there use ->second).

Answer 3

As Vijay says in his comment, you could use a std::set instead of a vector (if you don't need the individual features of a vector, of course, like contiguous storage, custom ordering or random access). In this case the std::set::insert will already take care of removing duplicates and you can use std::transform in the way you tried:
std::set<int> set;
std::transform( mp.begin(), mp.end(), std::inserter(set),
bind(&std::map<std::string, int>::value_type::second, _1) );
As to your modified question
std::set won't insert the value if it is already in the set but I
don't want consecutive duplicates.
Well, in this case just do a normal copy (i.e. std::transform) into the vector and do a std::unique afterwards, which will just remove all consecutive duplicates:
std::transform( mp.begin(), mp.end(), std::inserter(vec),
bind(&std::map<std::string, int>::value_type::second, _1) );
vec.erase( std::unique(vec.begin(), vec.end()), vec.end() );