On Tue, Jul 14, 2009 at 1:45 PM, Nair, Murlidharan T <mn...@iusb.edu> wrote:
> I am trying to calculate coordinate transformations and in the process of > debugging my code using debug I found the following > > Browse[1]> direction[i] > [1] -1.570796 > Browse[1]> cos(direction[i]) > [1] 6.123032e-17 > Browse[1]> cos(-1.570796) > [1] 3.267949e-07 > ... > I am not sure why I am getting one values when I am using a variable that > stores the value and another when I use the value directly. Am I missing > something here? > Because you are not using the same value. You say in a later message that your variable direction[i] was set to (0-90)*pi/180. So let's look at that: > x <- (0-90)*pi/180 > x - (-1.570796) [1] -3.267949e-07 That is, (0-90)*pi/180 is not exactly equal to -1.570796, but rather to -1.570796326794897: > print(x,digits=16) [1] -1.570796326794897 And that is equal to the calculated value. Well, almost: > print(x,digits=17) [1] -1.570796326794897 << the most digits R will print for a float > -1.570796326794897 - x [1] -4.440892e-16 << a very tiny difference See the R FAQ: http://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-doesn_0027t-R-think-these-numbers-are-equal_003f By the way, there is a bug in the R print routine which does not print out the full precision even if you specify it.... > -1.5707963267948965 - x << one more digit is actually needed [1] 0 [[alternative HTML version deleted]] ______________________________________________ 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.
