[Numpy-discussion] Different results from repeated calculation

[Stefan van der Walt]

On Sat, Jan 27, 2007 at 03:11:58PM -0700, Charles R Harris wrote:
>     Does anyone else see this happening?
> 
> 
> Yes,
> 
> test1:  0  differences
> test2:  51  differences
> test3:  0  differences
> 
>  Oddly, the relative error is always the same:
> 
> 98 z different 2.0494565872e-16
> 99 z different 2.0494565872e-16
> 
> Which is nearly the same as the double precision 2.2204460492503131e-16, the
> difference being due to the fact that the precision is defined relative to 1,
> and the error in the computation are in a number relatively larger (more bits
> set, but not yet 2).
> 
> So this looks like an error in the LSB of the floating number. Could be
> rounding, could be something not reset quite right. I'm thinking possibly
> hardware at this time, maybe compiler.

Interesting!  I don't see it on

Linux alpha 2.6.17-10-386 #2 Fri Oct 13 18:41:40 UTC 2006 i686 GNU/Linux
vendor_id       : AuthenticAMD
model name      : AMD Athlon(tm) XP 2400+
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 mmx fxsr sse syscall mmxext 3dnowext 3dnow up ts


but I do see it on


Linux voyager 2.6.17-10-generic #2 SMP Fri Oct 13 18:45:35 UTC 2006 i686 GNU/Linux
processor       : 0
vendor_id       : GenuineIntel
model name      : Genuine Intel(R) CPU           T2300  @ 1.66GHz
processor       : 1
vendor_id       : GenuineIntel
model name      : Genuine Intel(R) CPU           T2300  @ 1.66GHz
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe nx constant_tsc pni monitor vmx est tm2 xtpr

Both machines are running Ubuntu Edgy, exact same software versions.

Cheers
Stéfan