--- Gio 24/12/09, Linh Phan ha scritto: > Hi, > > I have wondering why when I do floating-point math > in Cygwin 1.7, Cygwin does not output the "correct" result, > eg: > > main () { > double qcb[4] = {0.41585680180318363, > 0.70823637211274604, 0.089200955545759006, > -0.56347808399291521}; > double qba[4] = {0.09648538897087118, > -0.37032037362514358, > 0.89403246842889339, 0.23293633462432001}; > double qca[4]; > > qca[0]= qcb[3]*qba[0] + qcb[2]*qba[1] - > qcb[1]*qba[2] + qcb[0]*qba[3]; > qca[1]= -qcb[2]*qba[0] + qcb[3]*qba[1] + > qcb[0]*qba[2] + qcb[1]*qba[3]; > qca[2]= qcb[1]*qba[0] - qcb[0]*qba[1] + > qcb[3]*qba[2] + qcb[2]*qba[3]; > qca[3]= -qcb[0]*qba[0] - qcb[1]*qba[1] - > qcb[2]*qba[2] + qcb[3]*qba[3]; > > // I am expecting the result to be: > > printf("qca = [-0.623718486146499718, > 0.736824293298044886, -0.260654850643024127, > 0.011147182658310384] CORRECT (LINUX/SOLARIS)\n"); > > // I get the above result on both Linux/Solaris AND > even when running under Cygwin's gdb and using gdb's printf > > // but when I compile and run on cygwin, I get the > wrong result:
s/wrong/different > printf("qca = [%.18f, %.18f, %.18f, %.18f] NOT > CORRECT (CYGWIN)\n",qca[0],qca[1],qca[2],qca[3]); > } > > Here is the output when I compiled and run on cygwin: > > qca = [-0.623718486146499718, 0.736824293298044886, > -0.260654850643024127, 0.011147182658310384] CORRECT > (LINUX/SOLARIS) > qca = [-0.623718486146499607, 0.736824293298044886, > -0.260654850643024072, 0.011147182658310373] NOT CORRECT > (CYGWIN) > > Here's the output when running under cygwin's gdb (it > matches Linux/Solaris outputs but not cygwin’s output): > > (gdb) printf "%.18f\n", qcb[3]*qba[0] + qcb[2]*qba[1] - > qcb[1]*qba[2] + qcb[0]*qba[3] > -0.623718486146499718 > (gdb) printf "%.18f\n", -qcb[2]*qba[0] + qcb[3]*qba[1] + > qcb[0]*qba[2] + qcb[1]*qba[3] > 0.736824293298044886 > (gdb) printf "%.18f\n", qcb[1]*qba[0] - qcb[0]*qba[1] + > qcb[3]*qba[2] + qcb[2]*qba[3] > -0.260654850643024127 > (gdb) printf "%.18f\n", -qcb[0]*qba[0] - qcb[1]*qba[1] - > qcb[2]*qba[2] + qcb[3]*qba[3] > 0.011147182658310384 > (gdb) > > These errors accumulate and eventually builds up so much > that the my simulation result is totally wrong when running > under cygwin. > > Please help. I will bet http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323 see http://hal.archives-ouvertes.fr/hal-00128124 for further detailed info on rounding if so it is not a bug is a feature :-) by the way on (cygwin) octave that round as you expect I have printf("%.18g\n",qca) -0.623718486146499718 0.736824293298044886 -0.260654850643024127 0.0111471826583103839 > Thanks, > > Linh > Marco -- Problem reports: http://cygwin.com/problems.html FAQ: http://cygwin.com/faq/ Documentation: http://cygwin.com/docs.html Unsubscribe info: http://cygwin.com/ml/#unsubscribe-simple