在Java中减去负分数的问题

Question

For some reason, I'm getting the correct result, but my negative sign is having issues. For example, if I do 1/4 - (-2/4), I get (-3/4).

Here is my minus method for subtracting fractions.

   /**
   Subtracts a fraction from another fraction.
   @param toUse, the fraction to subtract.
   @return minusFraction, the result after subtraction.
   */   
   public Fraction minus(Fraction toUse)
   {
      int newNum = ((toUse.numerator * denominator)
                     - (numerator * toUse.denominator));
      int newDen = denominator * toUse.denominator;
      Fraction minusFraction = new Fraction(newNum, newDen);

      return minusFraction;
   }

Here is my reduce() method, just in case...

  /**
   Reduces the fraction, if possible, to it's simplest form.
   Converts negative fractions to the form -x/y, or if -x/-y --> x/y
   */   
   private void reduce()
   {
      int lowest = Math.abs(numerator);
      int highest = Math.abs(denominator);

      if (lowest > highest)
      {
         int temp = highest;
         highest = lowest;
         lowest = temp;
      }

      while (lowest != 0)
      {
         int temp = lowest;
         lowest = highest % lowest;
         highest = temp;
      }

      numerator /= highest;
      denominator /= highest;

      if (denominator < 0)
      {
         numerator *= -1;
         denominator *= -1;
      }
   }

I only switched an operator from my previous addition method, given here as well. I think only switching the + to a - may have caused my issue.

   /**
   Adds two fractions together.
   @param toUse, the fraction to be added.
   @return plusFraction, the sum of the two fractions.
   */   
   public Fraction plus(Fraction toUse)
   {
      int newNum = ((toUse.numerator * denominator)
                     + (numerator * toUse.denominator));
      int newDen = denominator * toUse.denominator;
      Fraction plusFraction = new Fraction(newNum, newDen);

      return plusFraction;
   }

Answer 1

i think the problem is in your Fraction Minus function:

public Fraction minus(Fraction toUse)
{
  int newNum = ( (numerator * toUse.denominator)-
                       (toUse.numerator * denominator)
                 );
  int newDen = denominator * toUse.denominator;
  Fraction minusFraction = new Fraction(newNum, newDen);

  return minusFraction;
}

ex: 1/4-(-2/4)..... here the toUse.numerator is -2 and toUseDenominator is 4 what your code was doing it made your toUse fraction(-2/4) as base fraction and was getting subtracted from the original base Fraction(1/4) ie (-2/4)-(1/4)...hence the result -3/4

hopefully it works

Answer 2

Two problems:

1. You are subtracting in the wrong order.

   Your javadoc says:

    /** Subtracts a fraction from another fraction. @param toUse, the fraction to subtract. @return minusFraction, the result after subtraction. */ 

   Which implies this.numerator - toUse.numerator

   But here you do the opposite:

    int newNum = ((toUse.numerator * denominator) - (numerator * toUse.denominator)); 

   Which, when you call 1/4.minus(-2/4) ends up computing:

    int newNum = (-2*4) - (1*4) = -9 int newDen = 4 * -4 = -16 // Your new fraction will have -9/-16 which you reduce to -3/-4 // (which is how you hold -3/4) 

   Just switch the code to:

    /** Subtracts a fraction from another fraction. @param toUse, the fraction to subtract. @return minusFraction, the result after subtraction. */ public Fraction minus(Fraction toUse) { int newNum = ((numerator * toUse.denominator) - (toUse.numerator * denominator)); int newDen = denominator * toUse.denominator; Fraction minusFraction = new Fraction(newNum, newDen); return minusFraction; } 

   Now when

    int newNum = (1*4) - (-2*4) = 9 int newDen = 4 * -4 = -16 

2. This brings us to the second problem. Your reduce method relies on the denominator to determine if the number is negative or positive. But it is the numerator that should really carry the sign because you do the subtraction on the numerator.