On 11/10/2007, Leeds, Mark (IED) <[EMAIL PROTECTED]> wrote: > I just noticed something by accident with R syntax that I'm sure is > correct but I don't understand it. If I have > a simple numeric vector x and I subscript it, it seems that I can then > subscript a second time with TRUE > or FALSE, sort of like a 2 dimensional array in C. Does someone know if > this is documented somewhere > Because it's neat but I never knew it existed. To me it seems like a 1 > dimensional vector should > have only one dimensional indexing ? > > x <- seq(1,10) > > x > [1] 1 2 3 4 5 6 7 8 9 10 > > x[2:4][c(TRUE,FALSE,TRUE)] > [1] 2 4 > > But, it only works for TRUE or FALSE and not numbers so I think it's not > really 2 dimensional indexing. > > x[1][2] > > [1] NA > > If someone could explain this mechanism or tell me what I should look > for in the archives, it would > be appreciated. Thanks.
I may be being naive, but it is much simply than you are trying to make it. Aren't the two indexing operations run in sequence? Therefore x[2:4][c(TRUE,FALSE,TRUE)] actually is interpreted in two steps. Try this: x <- seq(1,10) y <- x[2:4] z <- y[c(TRUE, FALSE, TRUE)] Indeed, while your code: x[1][2] doesn't work, if you try x[2:4][3] then you get the third index of the vector created by 2:4 ie y <- x[2:4] z <- y[3] Best wishes, Mark -- Dr. Mark Wardle Specialist registrar, Neurology Cardiff, UK ______________________________________________ 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.
