/*-----------------------------------------------------*/
/*  Program about data files                           */
/*  This program reads data pairs from the             */
/*  the file sensor.dat and writes valid data pairs    */
/*  to the file checkedSensor.dat.  Valid data pairs   */
/*  may not be negative.  Invalid data is written to   */
/*  to standard error(cerr)                            */ 

#include<iostream> //Required for cerr
#include<fstream>  //Required for ifstream,ofstream
using namespace std;

int main()
{
  //Define file streams for input and output.
  ifstream fin("sensor.dat");
  ofstream fout("checkedSensor.dat");

  //Check for possible errors.
  if(fin.fail())
  {
     cerr << "could not open input file sensor.dat\n";
     exit(1);
  }
  if(fout.fail())
  {
     cerr << "could not open output file checkedSensor.dat\n";
     exit(1);
  }
  
  //All files are open.
  double t, motion;
  fin >> t >> motion;
  while(!fin.eof())
  {
    //Write valid data to output file.
    if(t >= 0 && motion >= 0)
    {
       fout << t << " " << motion << endl;
    }
    //Write invalid data to standard error ouput.
    else 
    {
       cerr << "meet negative number in the pair: "  
            << t << " " << motion << endl;
    }

    //Input next data pair.
    fin >> t >> motion;
  }//end while

  return 0;
}