[R] Decimal Accuracy Loss?

Brigid Mooney bkmooney at gmail.com
Wed Apr 6 17:50:14 CEST 2011

Thanks, Bert.  That's a big help.


On Wed, Apr 6, 2011 at 11:45 AM, Bert Gunter <gunter.berton at gene.com> wrote:
> Confirmed. "Casting" just adds/removes the dim attribute to the
> numeric vector/matrix.
> -- Bert
> On Wed, Apr 6, 2011 at 8:33 AM, Brigid Mooney <bkmooney at gmail.com> wrote:
>> 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.
>> Thanks in advance for the help,
>> Brigid
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>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
> --
> "Men by nature long to get on to the ultimate truths, and will often
> be impatient with elementary studies or fight shy of them. If it were
> possible to reach the ultimate truths without the elementary studies
> usually prefixed to them, these would not be preparatory studies but
> superfluous diversions."
> -- Maimonides (1135-1204)
> Bert Gunter
> Genentech Nonclinical Biostatistics

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