Quote:Original post by alvaro
This is not really portable, but it will probably do what you want:
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#include <cstdio>#include <float.h>float previous(float x) { int u = *reinterpret_cast<int *>(&x); u--; return *reinterpret_cast<float *>(&u);}float previous_alt(float x) { return x*(1.0f-FLT_EPSILON);}int main() { int i=10; float f=*reinterpret_cast<float *>(&i); std::printf("%.20g\n",f); std::printf("%.20g\n",previous(f)); std::printf("%.20g\n",previous_alt(f)); std::printf("%.20g\n",1.0f); std::printf("%.20g\n",previous(1.0f)); std::printf("%.20g\n",previous_alt(1.0f));}1.4012984643248170709e-441.2611686178923353638e-441.4012984643248170709e-4410.999999940395355224610.99999988079071044922Quote:Original post by floatingwoods
But my question was: I need a value DIFFERENT from the original, but just slightly smaller.
Quote:Original post by alvaro
Note that multiplying by (1.0f-FLT_EPSILON) will not work for very small numbers. It also seems to go down two steps instead of one even for the simplest case (1.0f).
*** Source Snippet Removed ***1.4012984643248170709e-441.2611686178923353638e-441.4012984643248170709e-4410.999999940395355224610.99999988079071044922
Quote:Original post by ZahlmanQuote:Original post by floatingwoods
But my question was: I need a value DIFFERENT from the original, but just slightly smaller.
... why?
Quote:Original post by all_names_takenActually, splitting a float into its sign, exponent and significand pieces using bitwise operations is pretty easy. I've done it before. From there it's a simple decrement to the significand, and if necessary decrement the exponent. No loops required, and not even that many lines of code.
A general solution to the problem most likely requires bitwise operations with the binary representation of the floating point number, and would become a rather large function with several conditional statements and maybe also some loops.
