On 04/12/2007, Duncan Murdoch <[EMAIL PROTECTED]> wrote: > On 12/4/2007 7:10 AM, [EMAIL PROTECTED] wrote: > > Full_Name: Rico Ihle > > Version: 2.6.1 > > OS: Windows XP Professional Version 2002 Pentium(R) 4 CPU 3.2 GHz > > Submission from: (NULL) (134.76.183.24) > > This is not a bug. See FAQ 7.31, "Why doesn't R think these numbers are > equal?" > > > > > > > # Bug in seq() function: > > x <- seq(-2,2,by=0.01) > > You've specified that the step size should be some number that's close > to 0.01, but not exactly 0.01, since R doesn't know how to represent that.
Here is another example adding to the clarification (confusion?): n <- 1e30; print(n*(1/n) < 1); ## [1] TRUE And another one: c <- 100; from <- -2; to <- 2; x <- seq(from=from, to=to, by=1/c); y <- seq(from=from*c, to=to*c, by=1)/c; print(identical(x,y)); ## [1] FALSE print(all.equal(x,y)); ## [1] TRUE Yet another example of problems with representation of certain values: by <- 1/c; z <- seq(from=from/by, to=to/by, by=1)*by; print(identical(x,z)); ## [1] FALSE print(identical(y,z)); # <<< NOTE ## [1] FALSE print(all.equal(x,z)); ## [1] TRUE print(all.equal(y,z)); ## [1] TRUE I guess the question is whether you prefer the 'x' or the 'y' sequence. Actually, if the representation of 'by=1/c' is, say, strictly smaller than 1/c, for some simple tests it looks like the deviation of d[k]=y[k]-x[k] increases as k grows. Here is a plot illustrating this: c <- 10; to <- 1e6; x <- seq(from=0, to=to, by=1/c); y <- seq(from=0, to=to*c, by=1)/c; d <- y-x; n <- length(d); # Plot (t,d), where t is uniform on the logaritmic scale, # because otherwise there are too many data points. t <- seq(from=0, to=log10(n), length.out=1e3); t <- unique(as.integer(10^t)); plot(t, d[t]); /Henrik > > > which(x==0.05)# How that?? > > # although: > > x# 0.05 seems to be at position 206 of x!!: > > x[206] > > # Why is this not equal to 0.05? > > > > # Reason: > > x2 <- as.character(x);x2 > > x2[206]# Ooooh... It's really not equal to 0.05!! How that? (compare lines > > 5 and > > 6!) > > > > # Remedy: > > x3 <- round(as.numeric(x),2) > > which(x3==0.05) > > > > # The (necessary) rounding is apparently and unfortunately NOT included in > > the > > seq() function!!! > > # But it should be!!! > > Why should it? You didn't specify a round number as the step size. > > Duncan Murdoch > > > # Because if one doesn't know about the demonstrated "nice" feature of the > > seq() > > function > > # (and it is not visible in lines 5 or 6!!!) > > # one gets mad that x[206] is not equal to 0.05 although x[206] is printed > > as > > 0.05!!! > > > > # Similarly: > > y <- seq(-0.5,.5,by=0.01) > > which(y == 0.05)# None? How that? Result should be 56!! > > y[56] > > > > # but: > > y2 <-as.character(y) > > y2[56] > > which(y2 == 0.05) > > > > # or rounding alternatively: > > which(round(y,2) == 0.05) > > > > ______________________________________________ > > R-devel@r-project.org mailing list > > https://stat.ethz.ch/mailman/listinfo/r-devel > > ______________________________________________ > R-devel@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-devel > ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
