Regarding the mention of logical indexing, under ?Extract I see: For [-indexing only: i, j, ... can be logical vectors, indicating elements/slices to select. Such vectors are recycled if necessary to match the corresponding extent. i, j, ... can also be negative integers, indicating elements/slices to leave out of the selection.
On March 9, 2019 6:57:05 PM PST, Rolf Turner <r.tur...@auckland.ac.nz> wrote: >On 3/10/19 2:36 PM, David Goldsmith wrote: >> Hi! Newbie (self-)learning R using P. Dalgaard's "Intro Stats w/ R"; >not >> new to statistics (have had grad-level courses and work experience in >> statistics) or vectorized programming syntax (have extensive >experience >> with MatLab, Python/NumPy, and IDL, and even a smidgen--a long time >ago--of >> experience w/ S-plus). >> >> In exploring the use of is.na in the context of logical indexing, >I've come >> across the following puzzling-to-me result: >> >>> y; !is.na(y[1:3]); y[!is.na(y[1:3])] >> [1] 0.3534253 -1.6731597 NA -0.2079209 >> [1] TRUE TRUE FALSE >> [1] 0.3534253 -1.6731597 -0.2079209 >> >> As you can see, y is a four element vector, the third element of >which is >> NA; the next line gives what I would expect--T T F--because the first >two >> elements are not NA but the third element is. The third line is what >> confuses me: why is the result not the two element vector consisting >of >> simply the first two elements of the vector (or, if vectorized >indexing in >> R is implemented to return a vector the same length as the logical >index >> vector, which appears to be the case, at least the first two elements >and >> then either NA or NaN in the third slot, where the logical indexing >vector >> is FALSE): why does the implementation "go looking" for an element >whose >> index in the "original" vector, 4, is larger than BOTH the largest >index >> specified in the inner-most subsetting index AND the size of the >resulting >> indexing vector? (Note: at first I didn't even understand why the >result >> wasn't simply >> >> 0.3534253 -1.6731597 NA >> >> but then I realized that the third logical index being FALSE, there >was no >> reason for *any* element to be there; but if there is, due to some >> overriding rule regarding the length of the result relative to the >length >> of the indexer, shouldn't it revert back to *something* that >indicates the >> "FALSE"ness of that indexing element?) >> >> Thanks! > >It happens because R is eco-concious and re-cycles. :-) > >Try: > >ok <- c(TRUE,TRUE,FALSE) >(1:4)[ok] > >In general in R if there is an operation involving two vectors then >the shorter one gets recycled to provide sufficiently many entries to >match those of the longer vector. > >This in the foregoing example the first entry of "ok" gets used again, >to make a length 4 vector to match up with 1:4. The result is the same > >as (1:4)[c(TRUE,TRUE,FALSE,TRUE)]. > >If you did (1:7)[ok] you'd get the same result as that from >(1:7)[c(TRUE,TRUE,FALSE,TRUE,TRUE,FALSE,TRUE)] i.e. "ok" gets >recycled 2 and 1/3 times. > >Try 10*(1:3) + 1:4, 10*(1:3) + 1:5, 10*(1:3) + 1:6 . > >Note that in the first two instances you get warnings, but in the third >you don't, since 6 is an integer multiple of 3. > >Why aren't there warnings when logical indexing is used? I guess >because it would be annoying. Maybe. > >Note that integer indices get recycled too, but the recycling is >limited >so as not to produce redundancies. So > >(1:4)[1:3] just (sensibly) gives > >[1] 1 2 3 > >and *not* > >[1] 1 2 3 1 > >Perhaps a bit subtle, but it gives what you'd actually *want* rather >than being pedantic about rules with a result that you wouldn't want. > >cheers, > >Rolf Turner > >P.S. If you do > >y[1:3][!is.na(y[1:3])] > >i.e. if you're careful to match the length of the vector and the that >of >the indices, you get what you initially expected. > >R. T. > >P^2.S. To the younger and wiser heads on this list: the help on "[" >does not mention that the index vectors can be logical. I couldn't >find >anything about logical indexing in the R help files. Is something >missing here, or am I just not looking in the right place? > >R. T. -- Sent from my phone. Please excuse my brevity. ______________________________________________ R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see 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.
