Hi all, In the course of testing 5.8.0, I was puzzled by an example hard-coding a value for pi, rather than re-using one from some public header as I expected. To my surprise, the standard library only provides M_PI as a POSIX extension. It turns out we do have a qmath.h that, sensibly enough, does
#ifndef M_PI #define M_PI (3.14159265358979323846) #endif Despite this, 302 other places in my Qt-dev source tree supply their own values for pi. There is some diversity of values. Let's start with the correctly-rounded ones: 3.14159 3.14159265 (enough for float - see below) 3.14159265358979 3.141592653589793 (enough for double - see below) 3.1415926535897932 3.141592653589793238 3.14159265358979323846 (same as M_PI in qmath.h) 3.1415926535897932384626433832795 3.14159265358979323846264338327950288 3.14159265358979323846264338327950288419717 3.14159265359 3.1415927 Then the truncated ones that aren't correct roundings (in each case, the last digit needs +1): 3.1415 3.141592 3.1415926 3.1415926535 3.141592653589 Finally, the values that look suspiciously like they were meant to be pi, but aren't (inserting a space after the last matching digit): 3.1415 2 (missing 9) In several parameters to sin(...) 3.14159 62 (last two digits swapped) In some JS benchmarks 3.14159265358979 23846 (missing 3) In an ECMA JS test I am fairly sure the parameters to sin were meant to use pi; and I suspect changing the JS tests is out-of-scope as they look like imports (i.e. third-party). Wikipedia gives the following first 49 digits (followed by a 0): 3.1415926535897932384626433832795028841971693993751 Experiment with gcc on 64-bit Intel reveals that there is no point supplying more than the following digits, for each type: long double: 3.1415926535897932385 double: 3.141592653589793 float: 3.1415927 So our M_PI is almost good. It coincides with what Linux's <math.h> provides, except that we put parentheses round it - can anyone give me a good reason for that ? I doubt it matters. However, neither M_PI puts an L suffix on it, so its last four digits are effectively ignored (because a floating literal is double by default). It looks like it would make sense to: i) Change qmath.h's M_PI to (3.14159265358979323846L) i.e. make it a long double, let context downgrade it as needed ii) Change all other hand-crafted approximations to use M_PI. Can anyone offer me a sane reason to not do this ? We have have little arithmetic irregularities creeping in from our inconsistencies ... Eddy. _______________________________________________ Development mailing list Development@qt-project.org http://lists.qt-project.org/mailman/listinfo/development