On Wed, Apr 06, 2011 at 11:33:48AM -0400, Brigid Mooney 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.
Others already confirmed that casting a numeric as a matrix and back again does not change the numbers. It is likely that the library operator "%*%" is more accurate than a straightforward for loop. For example, sum(x) uses a more accurate algorithm than iteration of s <- s + x[i] in double precision. Petr Savicky. ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
