/* 
   This method takes a String representation of a (proper or improper)
   fraction and returns a corresponding instance of Fraction.  If the
   String is invalid null is returned.
*/

public static Fraction parseFraction(String input)
{
   boolean negative = input.charAt(0) == '-';     // negative value?
   int numberStartIndex = (negative ? 1 : 0);     // beginning of digits
   int spaceIndex = input.indexOf(' ');    // separates whole and frac parts
   int slashIndex = input.indexOf('/');
   int intPart = 0;                        // whole number part of the input
   if (slashIndex == -1)      // input is a whole number
   {
      try
      {
         // read input as a whole number: no fraction part
         intPart = Integer.parseInt(input.substring(numberStartIndex));
         if (negative)
            intPart = -intPart;           // correct for sign
      }
      catch (NumberFormatException e)
      {
         return null;
      }
      return new Fraction(intPart, 1);   // whole number
   }
   int fractionStart = 0;       // where the fraction starts in input
   if (spaceIndex != -1)        // input is a whole number and a fraction
   {
      // read whole number part
      fractionStart = spaceIndex + 1;  // fraction part starts here
      try
      {
         intPart = 
           Integer.parseInt(input.substring(numberStartIndex, spaceIndex));
      }
      catch (NumberFormatException e)
      {
         return null;
      }
   }
   // read the fraction part
   int num, denom;
   try
   {
      // read numerator
      if (input.charAt(fractionStart) == '-')
         fractionStart++;      // may have '-' if not a mixes fraction
      num = Integer.parseInt(input.substring(fractionStart, slashIndex));
   }
   catch (NumberFormatException e)
   {
      return null;
   }
   try
   {
      // read denominator
      denom = Integer.parseInt(input.substring(slashIndex + 1));
   }
   catch (NumberFormatException e)
   {
      return null;
   }
   int numerator = intPart*denom + num;      // total numerator
   if (negative)
      numerator = -numerator;                // correct sign if necessary
   return new Fraction(numerator, denom);
}