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POWhy do I lose precision while multiplying and dividing whole ints?
 

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  1. POWhy do I lose precision while multiplying and dividing whole ints?
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    <p>I had thought Python3 is supposed to be able to handle numbers of arbitrary length, but I'm running into a problem where they don't seem to act consistently.</p> <p>After multiplying then dividing, my int seems to have changed it's internal representation, and no longer evaluates as a match for it's former self.</p> <p>I'm using whole numbers, without any decimals or fractions, but it's acting almost as if it's losing precision to rounding..?</p> <p>I'd appreciate any insight on why this is happening, and if there's something I should be doing differently. I have workarounds for my code, but since the result is counter-intuitive, I'd love to know what's behind the behavior ;)</p> <pre><code>Python 3.3.2 (default, Jul 30 2013, 00:52:04) [GCC 4.2.1 Compatible Apple LLVM 4.2 (clang-425.0.28)] on darwin Type "help", "copyright", "credits" or "license" for more information. &gt;&gt;&gt; a = 313585730727764141482831584863 &gt;&gt;&gt; a 313585730727764141482831584863 &gt;&gt;&gt; b = a*2 &gt;&gt;&gt; c = b /2 &gt;&gt;&gt; a 313585730727764141482831584863 &gt;&gt;&gt; c 3.1358573072776415e+29 &gt;&gt;&gt; a == c False </code></pre> <p>This seems to work if I use floor division, however-</p> <pre><code>&gt;&gt;&gt; c = b//2 &gt;&gt;&gt; c 313585730727764141482831584863 &gt;&gt;&gt; a == c True </code></pre> <p>Python 2.7 also seems to avoid this scenerio, keeping them in <code>longs</code></p> <pre><code>&gt;&gt;&gt; a = 313585730727764141482831584863 &gt;&gt;&gt; a 313585730727764141482831584863L &gt;&gt;&gt; b = a*2 &gt;&gt;&gt; c = b /2 &gt;&gt;&gt; a 313585730727764141482831584863L &gt;&gt;&gt; c 313585730727764141482831584863L &gt;&gt;&gt; a == c True </code></pre> <p>I'd appreciate any insight! Thank you!</p>
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    1. CODon't forget to accept Martijn's answer.
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