Use lower_bound to locate the first point at which 5 can be inserted in order : lower_bound « STL Algorithms Binary search « C++ Tutorial

1. C++ Tutorial
2. STL Algorithms Binary search
3. lower_bound

#include <iostream>
#include <algorithm>
#include <vector>

using namespace std;

int main()
{
   const int SIZE = 10;
   int a1[] = { 2, 2, 4, 4, 4, 6, 6, 6, 6, 8 };
   vector< int > v( a1, a1 + SIZE );

   vector< int >::iterator lower;

   cout << "\n\nUse lower_bound to locate the first point\n" << "at which 5 can be inserted in order";
   lower = lower_bound( v.begin(), v.end(), 5 );
   cout << "\n   Lower bound of 5 is element " << ( lower - v.begin() ) << " of vector v";

   return 0;
}