CODEWITHSUNDEEP
c++classstlvectordictionary
user188276
user188276

Reputation:

Can I use vector as index in map structure in c++?

I attempted to do something like this but it does not compile:

class point
{
    public:
        int x;
        int y;
};



int main()
{

    vector<point> vp1;
    vector<point> vp2;
    vector<point> vp3;

    map < vector<point>, int > m;

    m[vp1] = 1;
    m[vp2] = 2;
    m[vp3] = 3;

    map < vector<point>, int >::iterator it;
    for (it=m.begin(); it!=m.end(); it++)
    {
        cout<<m[it->first]<<endl;
    }
    return 0;
}

Upvotes: 2

Views: 831

Answers (5)

Potatoswatter
Potatoswatter

Reputation: 137930

Yes, you can. Vectors, like all containers, are comparable. The resulting map will sort the vectors in lexicographic order.

The problem is that point is not comparable. You have to define a sort order for points, and then this will in turn define lexicographic order over vector<point>.

class point
{
    public:
        int x;
        int y;
};

bool operator<( point const &l, point const &r ) {
    return l.x < r.x? true
         : r.x < l.x? false
         : l.y < r.y;
}

A simpler solution is to use std::pair instead of defining your own point.

typedef pair< int, int > point; // point::first = x, point::second = y
   // pair is already comparable; order defined as in previous example
typedef vector<point> pointvec; // OK

Upvotes: 1

ablaeul
ablaeul

Reputation: 2798

You have to declare the operator<. It would look like this (please keep in mind, that the three vectors in your sample code actually look the same):

bool operator<(const vector<point>& left, const vector<point>& right)
{
    return left.size() < right.size();
}

Upvotes: -1

Alex Martelli
Alex Martelli

Reputation: 882611

You can use anything as the index type into a std::map as long as it supports an operator< (which could could define as a free-standing function -- doesn't have to be a member function, as long as you can write a < b for a and b being instances of your type of interest) with the usual semantics (antireflexive, transitive, ...). Or, you can pass a binary function with the same semantics to use in lieu of <, if that suits you better.

Upvotes: 6

Jerry Coffin
Jerry Coffin

Reputation: 490623

You can, but the type used as a key in a map needs to be comparable, either using operator<, or using a comparison function/functor you supply as the third template parameter for the map type.

Upvotes: 4

wheaties
wheaties

Reputation: 35980

You haven't defined a function to compare a vector<point> Maps make requiremets of keys checking of equivalence and comparison.

Upvotes: 0

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