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POWhy does this unit test fail when comparing two doubles?
 

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  1. POWhy does this unit test fail when comparing two doubles?
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    <p>I have the following code in vb.net that calculates the amount before tax was applied:</p> <pre><code>Public Shared Function CalculateRateBeforeTax(ByVal rate As Decimal, ByVal tax As Decimal) As Decimal Dim base As Decimal = rate / (1 + (tax / 100.0)) Return Math.Round(base,2) End Function </code></pre> <p>Some scenarios I setup were:</p> <p>Rate = 107, Tax = 7%, Base = 100</p> <p>Rate = 325, Tax = 6.5%, Base = 305.16</p> <p>Rate = 215, Tax = 125%, Base = 95.55</p> <p>I put the above scenarios into some unit tests using c# and using the nunit testing framework. The first scenario passes, but the other fails and I am not sure how I can get it to pass. Here is are my tests:</p> <pre><code>[TestFixture] class TaxTests { [Test] public void CalculateRateBeforeTax_ShouldReturn100_WhenRateIs107AndTaxIs7Percent() { decimal expected = 100.0m; decimal actual = TaxUtil.CalculateRateBeforeTax(107.0m, 7.0m); Assert.AreEqual(expected,actual); } [Test] public void CalculateRateBeforeTax_ShouldReturn305point16_WhenRateIs325AndTaxIs6point5Percent() { decimal expected = 305.16m; decimal actual = TaxUtil.CalculateRateBeforeTax(325.0m, 6.5m); Assert.AreEqual(expected, actual); } [Test] public void CalculateRateBeforeTax_ShouldReturn95point55_WhenRateIs215AndTaxIs125Percent() { decimal expected = 95.55m; decimal actual = TaxUtil.CalculateRateBeforeTax(215.0m, 125.0m); Assert.AreEqual(expected, actual); } } </code></pre> <p>As I said before, the first test passes, but the results of the other tests are:</p> <p>Second Test expected 305.1600000000000003d But was: 305.1643192488263d</p> <p>Third Test expected 95.54999999999997 But was: 95.55555555555555557d</p>
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      1. VTUpMod
    1. COFor monetary you should really be using decimal
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    2. CO[What every computer scientist should know about floating-point arithmetic](http://portal.acm.org/citation.cfm?id=103163)
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