comp.lang.idl-pvwave archive: archive » Re: Strange behavior of /cumulative keyword in total()

Home » Public Forums » archive » Re: Strange behavior of /cumulative keyword in total()

Show: Today's Messages :: Show Polls :: Message Navigator
E-mail to friend

Re: Strange behavior of /cumulative keyword in total() [message #63295 is a reply to message #63285]

Tue, 04 November 2008 05:38

David Fanning
Messages: 11724
Registered: August 2001

Senior Member

Chris writes:

> Can anybody explain this?
>
> arr = fltarr( 500000) + .1
> cumul = total(array, /cumulative)
>
> print, (cumul - shift(cumul, 1)) [1 : 10]
> print, (cumul - shift(cumul,1)) [499990:499999]
>
>> 0.100000 0.100000 0.100000 0.100000 0.100000 0.100000
> 0.100000 0.100000 0.100000 0.100000
>
>
>> 0.101562 0.101562 0.101562 0.101562 0.101562 0.101562
> 0.101562 0.101562 0.101562 0.101562
>
> Plotting cumul - shift(cumul,1) is even weirder. I can understand the
> net error of cumul growing over time, as floating point precision
> errors accumulate. However, shouldn't the error between any two
> entries in a cumulative sum not accumulate over the array?

Two words: double precision.

IDL> arr = fltarr( 500000) + .1D
IDL> cumul = total(arr, /cumulative, /double)
IDL>
IDL> print, (cumul - shift(cumul, 1)) [1 : 10]
0.10000000 0.10000000 0.10000000 0.10000000
0.10000000 0.10000000 0.10000000 0.10000000
0.10000000 0.10000000
IDL> print, (cumul - shift(cumul,1)) [499990:499999]
0.10000000 0.10000000 0.10000000 0.10000000
0.10000000 0.10000000 0.10000000 0.10000000
0.10000000 0.10000000

Cheers,

David

--
David Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Sepore ma de ni thui. ("Perhaps thou speakest truth.")

Report message to a moderator

[Message index]

		Re: Strange behavior of /cumulative keyword in total() By: jameskuyper on Tue, 04 November 2008 08:23
		Re: Strange behavior of /cumulative keyword in total() By: David Fanning on Tue, 04 November 2008 05:38
		Re: Strange behavior of /cumulative keyword in total() By: Chris[6] on Tue, 04 November 2008 12:40

Previous Topic:	Re: specialized color tables
Next Topic:	segment vector with missing data

-=] Back to Top [=-

[ Syndicate this forum (XML) ] [

] [

]

Current Time: Mon Oct 13 05:08:08 PDT 2025

Total time taken to generate the page: 1.75847 seconds