I used the -which() construct initially to try to show "deleting" cases. I believe it hung around longer than it should have. That said, I have also had David's experience with NAs. What about a vectorized version of identical(TRUE, x)? This avoids the which() problem Bill pointed out, and the NA issue David mentioned. Does it introduce new problems?
x <- 1:10 y <- log(x-5) VisTRUE <- Vectorize(isTRUE) x[VisTRUE(y > -Inf)] Josh On Sun, Oct 17, 2010 at 4:38 PM, David Winsemius <dwinsem...@comcast.net> wrote: > > On Oct 17, 2010, at 3:56 PM, William Dunlap wrote: > >> >>> I had been thinking of: >>>> >>>> x <- c(1, (2^(0.5))^2 , 3, 5, (2^(0.5))^2 , 3, 1) >>>> y <- 2 >>>> x[-which(zapsmall(x-y) == 0)] >>> >>> [1] 1 3 5 3 1 >> >> Using which() to convert logicals into integer >> subscripts is almost always unnecessary and often wrong. > > At one time I believed that too. However, in the situation where the test > produces NA rather than a numeric value when one is indexing in the first > argument. I have had the unpleasant experience of pages if useless and > frustrating to understand output because of this "feature". > > I learned to either use which() in the first argument to "[" or to use > subset to avoid inadvertent "returns" from logical indexing. > >> x <- 1:10 >> y <- log(x-5) > Warning message: > In log(x - 5) : NaNs produced >> x[y>-Inf] > [1] NA NA NA NA 6 7 8 9 10 > >> x[which(y>-Inf)] > [1] 6 7 8 9 10 > > If that test were used in a dataframe indexing, the entire line might come > back as a "result". > > > >> In this case it fails when no x is close to y, >> because integer(0) is the same thing as -integer(0): >> >>> x[-which(zapsmall(x-10) == 0)] >> >> numeric(0) >> >> The whichless version, using logical subscripts, >> works (in this case we want all of x): >> >>> x[zapsmall(x-10)!=0] >> >> [1] 1 2 3 5 2 3 1 > > Maybe the rule should be don't use the -which construction: > >> x <- c(1, (2^(0.5))^2 , 3, 5, (2^(0.5))^2 , 3, 1) >> y <- 2 >> x[which(zapsmall(x-10) != 0)] > [1] 1 2 3 5 2 3 1 > > -- > David. >> >> When using logicals as subscripts, read the "[" >> as "such that". >> >> Bill Dunlap >> Spotfire, TIBCO Software >> wdunlap tibco.com > > -- Joshua Wiley Ph.D. Student, Health Psychology University of California, Los Angeles http://www.joshuawiley.com/ ______________________________________________ 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.
