Input

1. Just let the number of x's be an input parameter n:

      #include <iostream.h>
      int main()
      { 
       int n;
       float x;
       cout << "Number of x's" << endl;
       cin >> n;
       int i;
       for(i=0; i<n; i++)
       {  
         cout<<"Enter x" << endl;
         cin >> x;
       }
       return 0;
      }
   

2. Use control-D to end the program:

      #include <iostream.h>
      int main()
      {
       float x;
       while(cin>>x)
        //do whatever
       return 0;
      }
   

3. Have a character at the end of the stream, like ``q''. Then use

      #include <iostream.h>
      int main()
      { 
        char c;
        float x;
        while (1)
        {
          cin >> c;
          if(c == 'q') 
            {exit(1);}
          else
            {
            cin.putback(c);             //put c back in the input stream
            cin >> x;
            }
        }
         return 0;
       }
   

4. You can use argc and argv if you enter everything on the command line of your program. argc is the number of items listed in the command line and argv is an array of those items where each item is regarded as a string.

      #include <stdlib.h>
      int main(int argc, char **argv)
      {
         int i;
         float x;
         for(i=1; i<argc; i++)         //start at i=1 because the executable 
                                       //is not a number
          {
           x=atof(argv[i]);            //atof turns ascii into float
          }
         return 0;
      }
   

   The way you run this is:

   moses% a.out 24.3 534.5 664.3
   

   Here argc = 4 and argv[0] is a.out.

Input and Output Files The quick and dirty way to read from one file and write to another file is to use cout and cin as before, but on the command line type

moses% a.out < input.dat > output.dat

A better way to open files to read from and write to:

  #include <fstream.h>
   
   int main() {
    ifstream infile("input.dat");     //input.dat is the name of the file
                                      //in your directory
    ofstream outfile("output.dat");   //output.dat is the name of the file
                                      //that will be created in your directory
    float x;
      while(infile >> x)              //detects end-of-file and exits loop
         { outfile << "x = " << x << endl; }
    infile.close();
    outfile.close();
    return 0;
   }

We can open a file so that anything written to it is appended to the end:

  #include <fstream.h>

   int main() {
    ifstream infile("input.dat");     
    ofstream outfile("output.dat",ios::app);  //append to output.dat
    float x;
      while(infile >> x) {
          outfile << "x = " << x << endl; }
    infile.close();
    outfile.close();
    return 0;
   }

It is also possible to open a file for both input and output. For example,

  #include <fstream.h>

   int main() {
    fstream inout("input.dat",ios::in|ios::out);
    float x;
      inout >> x; 
      inout<< endl << "x = " << x << endl; 
    inout.close();
    return 0;
   }

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