Essential C++

adjacent_find()

By default, looks for the first adjacent pair of duplicate elements. A binary operator can override the built-in equality operator. Returns an iterator that addresses the first element of the pair.

#include <algorithm> 
class TwiceOver { 
public: 
   bool operator() ( int val1, int val2 ) 
        { return val1 == val2/2 ? true : false; } 
}; 

piter = adjacent_find( ia, ia+8 ); 
iter  = adjacent_find( vec.begin(), vec.end(), TwiceOver() );