[Cs3] using extraction operation in conditions

[Mikhail Nesterenko]

CS3 students, 

There was a question on using the extraction operator in a conditional
expression like inside the while loop. For example:

   int i;
   while(filestream >> i)
       // do something with i

Somehow, I was never comfortable doing that since to me it looks
unintuitive. The reason that it works is this: extraction operator
above is an alliterative form of

      iostream& operator >> (iostream&, i)

This operator returns a stream so that it can be stacked. If used in
an expression, the compiler implicitly invokes a bool() operator on
this stream

    http://en.cppreference.com/w/cpp/io/basic_ios/operator_bool

that returns true if there were no errors in reading from the stream
and false otherwise. One of the errors is end-of-file condition.

Now, using eof() to check for the end of file, such as

     while(!filestream.eof()){
        int i;
	cin >> i;
	// do something with i
     }
     

although used in some places such as the textbook we use for CSI, is
discouraged since it only catches end-of-file condition and may skip
another file reading error.

I recoded the roster example we used in class.

thanks,
--
Mikhail