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Re: WRONG RESULTS WITH IDL [message #11710 is a reply to message #11708]

Tue, 05 May 1998 00:00

Thomas A. McGlynn
Messages: 23
Registered: March 1996

Junior Member

Frank Loewenthal wrote:
>
> Hi Folks
>
> For calculation of beam-propagation I use IDL. But now I realize that
> for large arguments, even with double-precision, IDL gives wrong
> results:
> For example:
>
> sin(12345678901.12345678901) = -03727 9004 9960 8007
>
> where IDL get:
>
> IDL> x = double(12345678901.12345678901)
> IDL> print, sin(x)
> 0.098761418
>
> Can somebody confirm this result, and does anybody know the solution
> to overcome this problem?
>
> Best regards
>
> Frank

You're misunderstanding how to create a double precision literal in
IDL. E.g.:

IDL> x=12345678901.12345678901
IDL> print,sin(x)
0.0987614
IDL> x=12345678901.12345678901d0
IDL> print,sin(x)
-0.37327885

In the first case the literal value is interpreted in single
precision and then assigned to x. To preserve
the precision you need to indicate that it's a double
precision value.

Regards,
Tom McGlynn
tam@silk.gsfc.nasa.gov

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[Message index]

		Re: WRONG RESULTS WITH IDL By: Richard G. French on Tue, 05 May 1998 00:00
		Re: WRONG RESULTS WITH IDL By: Thomas A. McGlynn on Tue, 05 May 1998 00:00
		Re: WRONG RESULTS WITH IDL By: Andy Loughe on Tue, 05 May 1998 00:00
		Re: WRONG RESULTS WITH IDL By: Caesar E. Ordonez on Tue, 05 May 1998 00:00

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