Claudia Beleites wrote: >>> Wacek: >>> >>>> x[3:] >>>> instead of >>>> x[3:length(x)] >>>> x[3:end] >>>> >>> I don't think that would help: >>> what to use for end - 3 within the convention that negative values mean >>> exclusion? >>> >> might seem tricky, but not impossible: >> >> x[-2] >> # could mean 'all except for 2nd', as it is now >> >> x[1:-2] >> # could mean 'from start to the 2nd backwards from the end' >> > I know you get thus far. You might even think to decide whether exclusion or > 'from the end' is meant from ascending ./. descending order of the sequence, > but this messes around with returning the reverse order. > >
that's a design issue. one simple solution is to have this sort of indexing return always in ascending order. thus, x = 1:5 x[1:-1] # 1 2 3 4 5 x[5:-5] # NULL rather than 5 4 3 2 1 -- as in matlab or python x[seq(5,1)] # 5 4 3 2 1 that is, the ':'-based indexing can be made not to mess with the order. for reversing the order, why not use: x[5:-1:1] # 5 4 3 2 1 x[-3:-1:-5] # 3 2 1 rather than x[c(-3,-4,-5)], which would be 1 2 >> since r disallows mixing positive and negative indexing, the above would >> not be ambiguous. worse with >> >> x[-3:-1] >> >> which could mean both 'except for 3rd, 2nd, and 1st' and 'from the 3rd >> to the 1st from the end', and so would be ambiguous. in this context, >> indeed, having explicit 'end' could help avoid the ambiguity. >> > that's the problem. > also: how would 'except from the 5th last to the 3rd last' be expressed? > for exclusions you'd need to use negative indices anyway: x[seq(-5,-3)] now, neither x[-5:-3] nor x[-3:-5] would do the job they do now, but the above is not particularly longer, while selecting the 5th-to3rd-from-the-end columns is simply x[-5:-3] (which could be made to fail on out-of-range indices) instead of something like x[length(x) - 4:2] (which will silently do the wrong thing if length(x) < 4, and thus requires extra care). this is a rather loose idea, and unrealistic in the context of r, but i do not see much problem with it on the conceptual level. vQ ______________________________________________ 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.
