//文件名:CHAPTER9-23.cpp
#include <set>
#include <iostream>
#if _MSC_VER > 1020   // if VC++ version is > 4.2
   using namespace std;  // std c++ libs implemented in std
#endif
int main( )
{  set <int> s1;
   int i;
   s1.insert( 1 );
   s1.insert( 1 );
   // Keys must be unique in set, so duplicates are ignored
   i = s1.count( 1 );
   cout << "The number of elements in s1 with a sort key of 1 is: "<< i << "." << endl;
   i = s1.count( 2 );
   cout << "The number of elements in s1 with a sort key of 2 is: "<< i << "." << endl;
}