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POImplementing a half precision floating point number in C++
 

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  1. POImplementing a half precision floating point number in C++
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    <p>I am trying to implement a simple half precision floating point type, entirely for storage purposes (no arithmetic, converts to double implicitly), but I get weird behavior. I get completely wrong values for <code>Half</code> between -0.5 and 0.5. Also I get a nasty "offset" for values, for example 0.8 is decoded as 0.7998.</p> <p>I am very new to C++, so I would be great if you can point out my mistake and help me with improving the accuracy a bit. I am also curious how portable is this solution. Thanks!</p> <p>Here is the output - double value and actual decoded value from the half:</p> <pre><code>-1 -1 -0.9 -0.899902 -0.8 -0.799805 -0.7 -0.699951 -0.6 -0.599854 -0.5 -0.5 -0.4 -26208 -0.3 -19656 -0.2 -13104 -0.1 -6552 -1.38778e-16 -2560 0.1 6552 0.2 13104 0.3 19656 0.4 26208 0.5 32760 0.6 0.599854 0.7 0.699951 0.8 0.799805 0.9 0.899902 </code></pre> <p>Here is the code so far:</p> <pre><code>#include &lt;stdint.h&gt; #include &lt;cmath&gt; #include &lt;iostream&gt; using namespace std; #define EXP 4 #define SIG 11 double normalizeS(uint v) { return (0.5f * v / 2048 + 0.5f); } uint normalizeP(double v) { return (uint)(2048 * (v - 0.5f) / 0.5f); } class Half { struct Data { unsigned short sign : 1; unsigned short exponent : EXP; unsigned short significant : SIG; }; public: Half() {} Half(double d) { loadFromFloat(d); } Half &amp; operator = (long double d) { loadFromFloat(d); return *this; } operator double() { long double sig = normalizeS(_d.significant); if (_d.sign) sig = -sig; return ldexp(sig, _d.exponent /*+ 1*/); } private: void loadFromFloat(long double f) { long double v; int exp; v = frexp(f, &amp;exp); v &lt; 0 ? _d.sign = 1 : _d.sign = 0; _d.exponent = exp/* - 1*/; _d.significant = normalizeP(fabs(v)); } Data _d; }; int main() { Half a[255]; double d = -1; for (int i = 0; i &lt; 20; ++i) { a[i] = d; cout &lt;&lt; d &lt;&lt; " " &lt;&lt; a[i] &lt;&lt; endl; d += 0.1; } } </code></pre>
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    1. COThere's a similar question here: http://stackoverflow.com/questions/3316130/changing-float-type-to-short-but-with-same-behaviour-as-float-type-variable
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      1. POImplementing a half precision floating point number in C++
      1. USMike Weir
    2. COTry converting 0.8 to binary and use only the number of bits you have for storage. than try to convert it back to decimal and see the result. If you only have 2 decimal bits you can only use 1/2 and 1/4 thus trying to store e.g. .8 would be represented as 1/2+1/4 = .75 this is closer to .8 than 1 but still you have what you call `offset`
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      1. USted
 

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