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POFloating point rounding when truncating
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copied!<p>This is probably a question for an x86 FPU expert:</p> <p>I am trying to write a function which generates a random floating point value in the range [min,max]. The problem is that my generator algorithm (the floating-point Mersenne Twister, if you're curious) only returns values in the range [1,2) - ie, I want an inclusive upper bound, but my "source" generated value is from an exclusive upper bound. The catch here is that the underlying generator returns an 8-byte double, but I only want a 4-byte float, and I am using the default FPU rounding mode of Nearest.</p> <p>What I want to know is whether the truncation itself in this case will result in my return value being inclusive of max when the FPU internal 80-bit value is sufficiently close, or whether I should increment the significand of my max value before multiplying it by the intermediary random in [1,2), or whether I should change FPU modes. Or any other ideas, of course.</p> <p>Here's the code I am currently using, and I did verify that 1.0f resolves to 0x3f800000:</p> <pre><code>float MersenneFloat( float min, float max ) { //genrand returns a double in [1,2) const float random = (float)genrand_close1_open2(); //return in desired range return min + ( random - 1.0f ) * (max - min); } </code></pre> <p>If it makes a difference, this needs to work on both Win32 MSVC++ and Linux gcc. Also, will using any versions of the SSE optimizations change the answer to this?</p> <p><strong>Edit:</strong> The answer is yes, truncation in this case from double to float is sufficient to cause the result to be inclusive of max. See Crashworks' answer for more.</p>

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