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POHow does a C# evaluate floating point in hover over and intermediate window versus compiled?
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  1. POHow does a C# evaluate floating point in hover over and intermediate window versus compiled?
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    copied!<p>I am seeing something odd with storing doubles in a dictionary, and am confused as to why.</p> <p>Here's the code:</p> <pre><code> Dictionary&lt;string, double&gt; a = new Dictionary&lt;string, double&gt;(); a.Add("a", 1e-3); if (1.0 &lt; a["a"] * 1e3) Console.WriteLine("Wrong"); if (1.0 &lt; 1e-3 * 1e3) Console.WriteLine("Wrong"); </code></pre> <p>The second if statement works as expected; 1.0 is not less than 1.0. Now, the first if statement evaluates as true. The very odd thing is that when I hover over the if, the intellisense tells me false, yet the code happily moves to the Console.WriteLine.</p> <p>This is for C# 3.5 in Visual Studio 2008.</p> <p>Is this a floating point accuracy problem? Then why does the second if statement work? I feel I am missing something very fundamental here.</p> <p>Any insight is appreciated.</p> <p><strong>Edit2</strong> (Re-purposing question a little bit):</p> <p>I can accept the math precision problem, but my question now is: why does the hover over evaluate properly? This is also true of the intermediate window. I paste the code from the first if statement into the intermediate window and it evaluates false.</p> <p><strong>Update</strong></p> <p>First of all, thanks very much for all the great answers.</p> <p>I am also having problems recreating this in another project on the same machine. Looking at the project settings, I see no differences. Looking at the IL between the projects, I see no differences. Looking at the disassembly, I see no apparent differences (besides memory addresses). Yet when I debug the original project, I see: <a href="http://i30.tinypic.com/ega874.png" rel="nofollow noreferrer">screenshot of problem http://i30.tinypic.com/ega874.png</a></p> <p>The immediate window tells me the if is false, yet the code falls into the conditional.</p> <p>At any rate, the best answer, I think, is to prepare for floating point arithmetic in these situations. The reason I couldn't let this go has more to do with the debugger's calculations differing from the runtime. So thanks very much to Brian Gideon and stephentyrone for some very insightful comments.</p>
 

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