//
// Example 8.10. Program to read a nonnegative integer one character
// at a time and to print the integer or an error message 
//

#include <iostream>
#include <cctype>

using namespace std;

int main()
{
   int getNonnegative();

   int nonnegInt; 	// nonnegative integer user enters

   cout << "Enter a nonnegative integer and press return." << endl;
   cout << "Do not use a sign and only type digits." << endl;
   cout << "Your nonnegative integer: ";
   nonnegInt = getNonnegative();
   if (nonnegInt < 0)
	cout << "Sorry, input line is invalid." << endl;
   else
	cout << "Your nonnegative integer is " << nonnegInt << "." << endl;

   return 0;
}

//
// Function to read characters, verifying that they are digits,
// and to accumulate the digits into a nonnegative integer.
// The function does not check for integer overflow.
// Pre:  none
// Post: A nonnegative integer was returned; or in the case
// 	of an error, -1 was returned.
//

int getNonnegative()
{
   const int NEWLINE = '\n'; 
   int nonnegInt; 	// nonnegative integer user enters
   bool inputOK;	// boolean value for continuing loop
   char ch; 	// character input from user

   cin >> ch;  // skip over white space at beginning of line

   // Verify each character input is a digit and accumulate
   nonnegInt = 0;
   inputOK = true;

   while (!isspace(ch) && inputOK)
   {
	if (isdigit(ch))   // in number, accumulate and cont.
	{
	   nonnegInt = nonnegInt * 10 + ch - '0';
	   cin.get(ch);
	}
        else 	           // Error: character not a digit
	{
	   cout << ch << " is not a digit." << endl;
	   inputOK = false;
	}
   }

   // If number valid, make sure nothing else on line. 
   while (ch != NEWLINE && inputOK) 
   {
         if (isspace(ch)) 
            cin.get(ch); 
	 else 	            // Error: additional values on line 
	 {
	    cout << "Have only one integer per line." << endl; 
	    inputOK = false; 
         } 
   }

   // Error detected. Last processed character not NEWLINE
   if (!inputOK)
   {
      nonnegInt = -1; 
      cin.ignore(256, '\n'); 
   }
   
   return nonnegInt; 
}