How to take a vector of points and grab only 'y' of those points

Unfortunately my math abilities and objective-c/c/c++ isn't developed enough to understand how to do this.

I have a vector set up like this: [2,2,3,3,4,4,5,5,6,6] <-- i think thats how vectors are set up correct?

Th开发者_Python百科is is sort of what i think it should be set up:

vector<CGPoint>::iterator i;
    vector<CGPoint>* dp = xGraph.GraphPoints;
    for(i = dp->begin(); i != dp->end(); ++i){
        /* grab y points only code here*/
    }

Now I want to write some kind of for statement that takes xGraph.GraphPoints and only grabs the y coords. and... i guess puts it into another vector of only y coords, which looks like [2,3,4,5,6] after the code is finished

Can someone help me out here?

Cheers

---

Ok, my interpretation of your question is that you have a vector that contains CGPoint objects and you want to extract only the y coordinate from all of the points. In that case you want something like (using C++11 lambdas):

std::vector<CGFloat> ycoord;
std::transform( dp->begin(), dp->end(), std::back_inserter( ycoord ), 
                []( CGPoint const & p ){ return p.y; } );

If the compiler does not support lambdas, you can write a simple function to perform the extraction:

CGFloat extractY( CGPoint const & p ) {
   return p.y;
}
std::transform( dp->begin(), dp->end(), std::back_inserter( ycoord ), 
                &extractY );

Or functor:

struct extractYfunctor {
   CGFloat operator()( CGPoint const & p ) const {
      return p.y;
   }
};
std::transform( dp->begin(), dp->end(), std::back_inserter( ycoord ), 
                extractYfuctor() );

If you can use the boost libraries:

std::transform( dp->begin(), dp->end(), std::back_inserter( ycoord ),
                boost::bind( &CGPoint::y, _1 ) );

Or with a plain loop:

for ( std::vector< CGPoint >::const_iterator it = dp->begin(); it != dp->end(); ++it ) {
   ycoord.push_back( it->y );
}

---

Your vector is not set up like [2,2,3,3,4,4,5,5,6,6]. It's set up like this: [(2,2),(3,3),(4,4),(5,5),(6,6)]. That is, it's a list of pairs of numbers, not just a list of numbers. To get the y component of each element in the vector, you can write a loop like this:

vector<CGPoint>* dp = xGraph.GraphPoints;
for(i = dp->begin(); i != dp->end(); ++i){
    /* grab y points only code here*/
    CGFloat current_y = i->y;
}

---

vector<CGPoint>::iterator i;
vector<CGPoint>* dp = xGraph.GraphPoints;
vector<CGFloat> dp = yPoints;
for(i = dp->begin(); i != dp->end(); ++i){
    yPoints.push_back(i->y);
}

vectors are indeed laid out like that in memory, but not logically. Don't overthink things. It's still a vector of CGFloat objects.