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Re: Friday Afternoon Puzzler [message #66840 is a reply to message #66834]

Fri, 12 June 2009 14:07

pgrigis
Messages: 436
Registered: September 2007

Senior Member

David Fanning wrote:
> Folks,
>
> If you've already had a beer, you may want to work
> on this one. Should we be concerned? Or is this just
> one more example for the Sky is Falling article?
> I've voiced my opinion, but it didn't satisfy the
> complainant. What say you?
>
> curly=[-1.912,-1.852,-1.386,-0.818,-0.343,-0.343,-1.233]
> larry=[-1.912,-1.852,-1.386,-0.818,-0.343,-0.343,-1.233]
> curly = curly*!PI/180.
> larry *= !PI/180.

I'd argue that you are multiplying curly bi pi and dividing the result
by 180
in the first line, and dividing pi by 180 first and multiplying larry
by the result
in the second line. Therefore, you are performing two different set of
floating
point operations, therefore I am not surprised they give slightly
different results.

Ciao,
Paolo

> diff=curly-larry
> print,diff
>
> curly=[-1.912,-1.852,-1.386,-0.818,-0.343,-0.343,-1.233]
> larry=[-1.912,-1.852,-1.386,-0.818,-0.343,-0.343,-1.233]
> curly = curly*!PI/180.
> larry = larry*!PI/180.
> diff=curly-larry
> print,diff
>
> curly=[-1.912,-1.852,-1.386,-0.818,-0.343,-0.343,-1.233]
> larry=[-1.912,-1.852,-1.386,-0.818,-0.343,-0.343,-1.233]
> curly *= !PI/180.
> larry *= !PI/180.
> diff=curly-larry
> print,diff
>
> The results:
>
> -3.72529e-009 0.000000 0.000000-9.31323e-010 0.000000
> 0.000000 0.000000
> 0.000000 0.000000 0.000000 0.000000 0.000000
> 0.000000 0.000000
> 0.000000 0.000000 0.000000 0.000000 0.000000
> 0.000000 0.000000
>
>
> Why aren't the first set of differences (where the assignment
> is made with the *= operator) *exactly* zero?
>
> 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.")

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[Message index]

		Re: Friday Afternoon Puzzler By: Michael Galloy on Fri, 12 June 2009 15:40
		Re: Friday Afternoon Puzzler By: R.G. Stockwell on Fri, 12 June 2009 14:40
		Re: Friday Afternoon Puzzler By: Kenneth P. Bowman on Fri, 12 June 2009 14:09
		Re: Friday Afternoon Puzzler By: pgrigis on Fri, 12 June 2009 14:32
		Re: Friday Afternoon Puzzler By: R.G. Stockwell on Fri, 12 June 2009 14:30
		Re: Friday Afternoon Puzzler By: R.G. Stockwell on Fri, 12 June 2009 14:23
		Re: Friday Afternoon Puzzler By: pgrigis on Fri, 12 June 2009 14:15
		Re: Friday Afternoon Puzzler By: pgrigis on Fri, 12 June 2009 14:09
		Re: Friday Afternoon Puzzler By: pgrigis on Fri, 12 June 2009 14:07

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