[R] Odp: Decimal Accuracy Loss?

From: Petr PIKAL <petr.pikal_at_precheza.cz>
Date: Wed, 06 Apr 2011 17:43:48 +0200

Hi

r-help-bounces_at_r-project.org napsal dne 06.04.2011 17:33:48:

> This is hopefully a quick question on decimal accuracy. Is any
> decimal accuracy lost when casting a numeric vector as a matrix? And
> then again casting the result back to a numeric?
>
> I'm finding that my calculation values are different when I run for
> loops that manually calculate matrix multiplication as compared to
> when I cast the vectors as matrices and multiply them using "%*%".
> (The errors are very small, but the process is run iteratively
> thousands of times, at which point the error between the two
> differences becomes noticeable.)
>
> I've read FAQ # 7.31 "Why doesn't R think these numbers are equal?",
> but just want to confirm that the differences in values are due to
> differences in the matrix multiplication operator and manual
> calculation via for loops, rather than information that is lost when
> casting a numeric as a matrix and back again.

Without some example it is difficult to see the possible sources of difference. Clever people may know how %*% operator really works, but only those who are able mind reading can know what you do inside your for loops.

Regards
Petr

>
> Thanks in advance for the help,
> Brigid
>
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