从键盘上输入x和y的值，计算y = ln( 2x – y )

#include <iostream>
#include <cmath>
using namespace std;
double f( double x,double y );
int main()
{
double x,y;
      try
{
  cout << "输入x和y的值：";
      cin >> x >> y;
         cout << f( x,y ) << endl;    
}
      catch( char * )
       {
    cout << "负数不能求对数！" << endl;
}
}
double f( double x,double y )
{
  if( 2*x-y < 0 )  
        throw  "error";
      else
        return log( 2*x - y );
}