SOURCE FILE: ex0810.cpp

//   
// 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.h>
#include <ctype.h>   
#include "boolean.h"   
   
int main(void)   
{   
   int GetNonnegative(void);   
   
   int nonneg_int;      // nonnegative integer user enters 
      
   cout << "Enter a nonnegative integer and press return.\n";
   cout << "Do not use a sign and only type digits.\n";
   cout << "Your nonnegative integer: ";
   nonneg_int = GetNonnegative();   
   if (nonneg_int < 0)   
      cout << "Sorry, input line is invalid.\n";
   else   
      cout << "Your nonnegative integer is " << nonneg_int << ".\n";
   
   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(void)   
{   
   const int NEWLINE = '\n';   
   int nonneg_int;      // nonnegative integer user enters 
   boolean_t 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 
   nonneg_int = 0;   
   InputOK = TRUE;   
   
   while (!isspace(ch) && InputOK)   
   {   
      if (isdigit(ch))   // in number, accumulate and cont. 
      {   
         nonneg_int = nonneg_int * 10 + ch - '0';   
         cin.get(ch);   
      }   
      else              // Error: character not a digit 
      {   
         cout << ch << " is not a digit.\n";
         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.\n";
         InputOK = FALSE;   
      }      
   }   
   
   // Error detected.  Last processed character not NEWLINE 
   if (!InputOK)   
   {   
      nonneg_int = -1;   
      cin.ignore(256, '\n');   
   }   
      
   return nonneg_int;   
}