Bug 121051 - incorrect result of "Calculate integral"
Summary: incorrect result of "Calculate integral"
Status: RESOLVED FIXED
Alias: None
Product: kmplot
Classification: Applications
Component: general (show other bugs)
Version: unspecified
Platform: Gentoo Packages Linux
: NOR normal
Target Milestone: ---
Assignee: Klaus-Dieter M
URL:
Keywords:
Depends on:
Blocks:
 
Reported: 2006-01-30 18:03 UTC by Zeliboba
Modified: 2007-05-06 20:31 UTC (History)
1 user (show)

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Description Zeliboba 2006-01-30 18:03:26 UTC 
Version:           1.2.0 (using KDE KDE 3.5.0)
Installed from:    Gentoo Packages
Compiler:          gcc version 3.4.4 (Gentoo 3.4.4-r1, ssp-3.4.4-1.0, pie-8.7.8) Configured with: /var/tmp/portage/gcc-3.4.4-r1/work/gcc-3.4.4/configure --prefix=/usr --bindir=/usr/x86_64-pc-linux-gnu/gcc-bin/3.4.4 --includedir=/usr/lib/gcc/x86_64-pc-linux-gnu/3.4.4/include --datadir=/usr/share/gcc-data/x86_64-pc-linux-gnu/3.4.4 --mandir=/usr/share/gcc-data/x86_64-pc-linux-gnu/3.4.4/man --infodir=/usr/share/gcc-data/x86_64-pc-linux-gnu/3.4.4/info --with-gxx-include-dir=/usr/lib/gcc/x86_64-pc-linux-gnu/3.4.4/include/g++-v3 --host=x86_64-pc-linux-gnu --build=x86_64-pc-linux-gnu --disable-altivec --enable-nls --without-included-gettext --with-system-zlib --disable-checking --disable-werror --disable-libunwind-exceptions --enable-multilib --disable-libmudflap --disable-libgcj --enable-languages=c,c++,f77 --enable-shared --enable-threads=posix --enable-__cxa_atexit --enable-clocale=gnu
OS:                Linux

when try to calculate integral of function f(x)=x from -8 to 8 value 0.163 is reported, but exact value iz 0. when increase integration interval error increases. even simple numerical integration for such a simple function should give proper value...
Comment 1 Fredrik Edemar 2006-02-09 01:05:12 UTC 
Have you tried to decrease the step width in the Configure->Configure Kmplot dialog? I get the result 0 if I set the stepping to 0.5.
Comment 2 Zeliboba 2006-02-09 12:04:40 UTC 
you mean "point per pixel"? it is not obvious from the name that it is integration step... anyway, you are right, when this value set up to 0.5 result is correct. if you increase limits to -80 and 80 result incorrect again! for this limits I've got correct result using step of 0.4, but this step gives incorrect result in case of -8 to 8 limits! additionally, decreasing of the step do not approach me to the correct result (essentially non-linear behavior)
how integral is computed? from the image? if so, it is totally wrong way!!!
Comment 3 David Saxton 2006-04-12 18:54:07 UTC 
Now fixed.

The bug was caused by the intervals used to calculate the integral numerically not perfectly tiling the range [-8,8]. (e.g. in your example, the step size was approximately (8-(-8))/(800 pixels) = 0.02, and the numerical integration missed out a chunk at the end of the plot of value 8, giving an inaccuracy of 0.02 * 8 = 0.16).
Comment 4 Bram Schoenmakers 2007-05-06 20:21:20 UTC 
Most probably David's fix caused bug 136479.