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POC# double precision issue, how to detect and handle in a safe way
 

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  1. POC# double precision issue, how to detect and handle in a safe way
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    <p>I was looking for satisfactory and safe workaround to my double precision issue specified to this problem:</p> <p><img src="https://i.stack.imgur.com/SUPn4.png" alt="enter image description here"></p> <p>This program tries to find how many small circle can fit into a large circle. It fills the large circle and then culls those that intersect the large circumference. using this formula:</p> <pre><code>distance_small_pos_from_center + small_radius &lt; big_radius </code></pre> <p>All calculations were in double, except for screen output on WinForms which takes int for coords.</p> <p><img src="https://i.stack.imgur.com/OtsgM.png" alt="enter image description here"></p> <p>The above image shows the result of the culling. You can see that it is not symmetric when it should really be because the constraint is that there must be one small circle exactly in the center. I step through the code and find that this is because some calculations yield, for example, </p> <pre><code>99.9999999 &lt; 100 </code></pre> <p>This answer <a href="https://stackoverflow.com/questions/6875007/c-double-precision">C++ double precision and rounding off</a> says we should use all the precision available, but in this case, I had to do a <code>Math.Round(distance_small_pos_from_center + small_radius, 3)</code> using 3 arbitarily.</p> <p>The result of the culling differs very much without Math.Round. In retrospect, this is one kind of bug that is hard to detect <strong>if I had not drawn it out</strong>. Maybe I did something wrong, or didn't understand doubles as much as I thought I had.</p> <p>So, anyone has solutions or tips to avoid this kind of problem?</p>
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    1. COMy guess is that double can't represent 100 exactly. I suggest choosing a figure very close to 100 that double can represent and comparing against that.
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      1. USGeorge Duckett
    2. CO99.99999 < 100 was just one case I found out. If i set the large diameter to 600 and small diameter to 40 (increase by factor 2) the same problem happens. Since the input can be any decimal number, i think it is pretty hard to know when the issue will happen.
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    3. COActually, thinking about it i'm not sure what i said makes sense (since the 100 would be converted to a double to do the comparison anyway). Would be interested to hear other people's take on this.
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