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Bug in IDL or, may be in my head ? [message #88645]

Fri, 23 May 2014 01:01

franciscoaiglesias
Messages: 2
Registered: May 2014

Junior Member

Hi all,

Any clue what's going on here ?

a = randomn(seed, 248, 256, 9972) * 100. + 4.e3

print, total(a) ; gives 1.42030e+12
print, total(a, /double) ;gives 2.5324144e+12
print, total(total(total(a, 1),1));gives 2.53250e+12

Thanks !

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Re: Bug in IDL or, may be in my head ? [message #88650 is a reply to message #88645]

Fri, 23 May 2014 15:39

Michael Galloy
Messages: 1114
Registered: April 2006

Senior Member

On 5/23/14, 2:01 AM, franciscoaiglesias@gmail.com wrote:
> Hi all,
>
> Any clue what's going on here ?
>
> a = randomn(seed, 248, 256, 9972) * 100. + 4.e3
>
> print, total(a) ; gives 1.42030e+12
> print, total(a, /double) ;gives 2.5324144e+12
> print, total(total(total(a, 1),1));gives 2.53250e+12
>
> Thanks !
>

Check out Kahan summation:

http://en.wikipedia.org/wiki/Kahan_summation_algorithm

I have an implementation in mglib:

IDL> print, mg_total(a)
% Loaded DLM: MG_ANALYSIS.
2.53241e+12

Get mglib at:

github.com/mgalloy/mglib

-Mike

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