Unit test fails AreEquals() and ToString()

Question

My main method is:

using System;
namespace Rational
{
   public class Rational
   {
       public int Denom { get; private set; }
       public int Numer { get; private set; }
       public Rational(int numerator = 0, int denominator = 1)
       {
           Denom = denominator;
           Numer = numerator;
       }
       public void IncreaseBy(Rational num)
       {
           Numer = Numer * num.Denom + num.Denom * Denom;
           Denom = Denom * num.Denom;
       }
       public void DecreaseBy(Rational num)
       {
           Numer = Numer * num.Denom - num.Denom * Denom;
           Denom = Denom * num.Denom;
       }
       override public String ToString() { return (Numer + "/" + Denom); }
   }

My Program that calls on my method is:

using System;
namespace Rationals
{
   public class Rationals

   {

       class Program
       {
           static void Main(string[] args)
           {
               Rational num1 = new Rational(10, 8);
               Rational num2 = new Rational(8, 10);
               Console.WriteLine("1st number is:   " + num1);
               Console.WriteLine("2nd number is:   " + num2);
               num1.IncreaseBy(num2);
               Console.WriteLine("1st number + 2nd number = " + num1);
               num1.DecreaseBy(num2);
               Console.WriteLine("1st number - 2nd number = " + num2);
           }
       }
   }
}

I have to Write a Unit test to test my IncreaseBy() as well as my DecreaseBy() and finally ToString(). can someone please help me?!!?

This is what i have so far in the UnitTest please crap on it as much as you want. I AM UNABLE TO MAKE THE TEST PASS IT KEEPS COMING UP WITH A DIFFERENT ANSWER ALSO THE ToString() brings up an error.

using Microsoft.VisualStudio.TestTools.UnitTesting;
using Rationals_123456789;

namespace ratonalstest
{
   [TestClass]
   public class UnitTest1
   {
       [TestMethod]
       public void DecreaseByTest()
       {
           int num = 1, den = 6, num1 = 2, den1 = 9;
           Rational fraction1 = new Rational(num, den);
           Rational fraction2 = new Rational(num1, den1);
           Rational expectedFraction = new Rational(-1, 18);

           //when
           fraction1.DecreaseBy(fraction2);

           //then
           Assert.AreEqual(fraction1.Numer, expectedFraction.Numer);
           Assert.AreEqual(fraction1.Denom, expectedFraction.Denom);

       }
       [TestMethod]
       public void IncreaseByTest()
       {
           int num = 1, den = 4, num1 = 1, den1 = 4;
           Rational fraction1 = new Rational(num, den);
           Rational fraction2 = new Rational(num1, den1);

           Rational expectedFraction = new Rational(7, 18);

           //when
           fraction1.IncreaseBy(fraction2);

           //then
           Assert.AreEqual(fraction1.Numer, expectedFraction.Numer);
           Assert.AreEqual(fraction1.Denom, expectedFraction.Denom);

           //when
           fraction1.DecreaseBy(fraction2);

           //then
           Assert.AreEqual(fraction1.Numer, expectedFraction.Numer);
           Assert.AreEqual(fraction1.Denom, expectedFraction.Denom);

       }

       [TestMethod]
       public void ToString()
       {
           int num = 1, den = 6, num1 = 2, den2 = 9;
           Rational fraction1 = new Rational(num, den);
           Rational fraction2 = new Rational(num1, den2);

           string expectedString = "1/6";
           string outputString = fraction1.ToString();

          //then
           Assert.AreEqual(expectedString, outputString);

       }
   }
}

Answer 1

There are two issues with your code.

1. You calculate the result wrong
2. You don't normalize the fractions

First problem is that these two expressions:

Numer = Numer * num.Denom + num.Denom * Denom;
Numer = Numer * num.Denom - num.Denom * Denom;

should use num.Numer instead of num.Denom so these are correct:

Numer = Numer * num.Denom + num.Numer * Denom;
Numer = Numer * num.Denom - num.Numer * Denom;

Now, this will still not make your tests pass, but let's try your code now:

Rational fraction1 = new Rational(1, 6);
Rational fraction2 = new Rational(2, 9);

fraction1.DecreaseBy(fraction2);
Console.WriteLine(fraction1.ToString());

this outputs:

-3/54

Your test expects -1/18 , but these are in fact the same number. Your tests have to handle this discrepancy, either by introducing fraction normalization or by comparing in a different manner.

---

Lastly I have some tips:

1. Make the Rational type a readonly immutable struct
2. Implement proper operators that don't mutate the existing value but instead compute and return the result as a new Rational value
3. Add support for normalization, this is usually done by dividing numerator and denominator by the GCD value, and making sure that at most only the numerator is negative (no negative denominator)