chapter11-5.cpp

C++STL程序员开发指南

//文件名:CHAPTER11-5.cpp
#include <vector>
#include <algorithm>
#include <iostream>
// The function object multiplies an element by a Factor
template <class Type>
class MultValue
{
private:
   Type Factor;   // The value to multiply by
public:
   // Constructor initializes the value to multiply by
   MultValue ( const Type& _Val ) : Factor ( _Val ) {   }
   // The function call for the element to be multiplied
   void operator ( ) ( Type& elem ) const {elem *= Factor;}
};
// The function object to determine the average
class Average
{
private:
   long num;      // The number of elements
   long sum;      // The sum of the elements
public:
   // Constructor initializes the value to multiply by
   Average ( ) : num ( 0 ) , sum ( 0 )   {   }
   // The function call to process the next elment
   void operator ( ) ( int elem )
   {   num++;      // Increment the element count
      sum += elem;   // Add the value to the partial sum
   }
   // return Average
   operator double ( ) {return  static_cast <double> (sum) / static_cast <double> (num);}
};
int main( )
{
   using namespace std;
   vector <int> v1;
   vector <int>::iterator Iter1;
   // Constructing vector v1
   int i;
   for ( i = -4 ; i <= 2 ; i++ )  { v1.push_back(  i );}
   cout << "Original vector  v1 = ( " ;
   for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ ) cout << *Iter1 << " ";
   cout << ")." << endl;
   // Using for_each to multiply each element by a Factor
   for_each ( v1.begin ( ) , v1.end ( ) , MultValue<int> ( -2 ) );
   cout << "Multiplying the elements of the vector v1\n "<<  "by the factor -2 gives:\n v1mod1 = ( " ;
   for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ ) cout << *Iter1 << " ";
   cout << ")." << endl;
   for_each (v1.begin ( ) , v1.end ( ) , MultValue<int> (5 ) );
   cout << "Multiplying the elements of the vector v1mod\n "
        <<  "by the factor 5 gives:\n v1mod2 = ( " ;
   for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ ) cout << *Iter1 << " ";
   cout << ")." << endl;
   double avemod2 = for_each ( v1.begin ( ) , v1.end ( ) , Average ( ) );
   cout << "The average of the elements of v1 is:\n Average ( v1mod2 ) = "
        << avemod2 << "." << endl;
}