[R] For logical i and length(i) > length(x), x[i] <- value makes length(x) == length(i)

Sun Sep 13 13:44:54 CEST 2015

Thank you for looking.

If the last element of i is TRUE, the largest value of which(i) is length(i). So, even if x[i] <- value were translated to x[which(i)] <- value, the result would have length(x) == length(i).

Difference between x[i] <- value and x[which(i)] <- value in R would occur when the last element of i is FALSE, as in my original example:
x <- NA
x[c(FALSE,TRUE,FALSE)] <- 1
--------------------------------------------
On Sun, 13/9/15, William Dunlap <wdunlap at tibco.com> wrote:

 Subject: Re: [R] For logical i and length(i) > length(x), x[i] <- value makes length(x) == length(i)

 Cc: R-help at r-project.org
 Date: Sunday, 13 September, 2015, 9:22 AM

 Splus 8.2.0 (c. 2010) and
 Splus6.0 (c. 2001) act like R in this respect.
   x <- c(10.0, 11.0, 12.0)
   i <- c(FALSE,TRUE,FALSE,FALSE,TRUE)
   x[i]
   #[1] 11 NA
   x[i] <- 1:2
   x
   #[1] 10  1 12 NA  2
 I no
 longer have access to a running version of Splus 3.4, but it
 does not
 look like a feature we would have
 changed.

 Bill Dunlap
 TIBCO Software
 wdunlap
 tibco.com

 On Sat, Sep 12, 2015 at 9:17 AM, Suharto
 Anggono Suharto Anggono via
 R-help <r-help at r-project.org>
 wrote:
 > I'll make my points
 clearer.
 >
 > - The
 case of logical index vector longer than the original vector
 is not usual case for me. As I said, I encountered it in the
 code of function 'rank' in R.
 >
 > - Before, I have read
 "S-PLUS help" (version 3.4, it seems), http://www.uni-muenster.de/ZIV.BennoSueselbeck/s-html/helpfiles/Subscript.html.
 The following is the relevant part. There are missing
 pieces there that I fill by guessing.
 >
 > Vector subscripts are
 generated with when i and x are both vectors. (....) The
 result of the expression is to extract or replace elements
 of x corresponding to a vector of positive indices computed
 according to the value of i.
 >
 > .... If i is logical the indices are
 produced by starting at 1 and selecting the numbers for
 which the corresponding element is T. If is shorter than it
 is extended by cyclic repetition. It can be longer than as
 well, with no change in the computation of indices. ....
 >
 > ....
 >
 > For replacements,
 x[i] <- value the rule is that the length of will be set
 to the largest value in the indices, if that is bigger than
 the current length of x. ....
 >
 >
 > >From it, I infer
 that, if i is logical and length(i) >= length(x), x[i]
 <- value has the same effect to x[which(i)] <- value,
 where which(i) takes indices where i is T.
 >
 > - In R, if i is
 logical and length(i) > length(x), length(x) after x[i]
 <- value may be different from after x[which(i)] <-
 value.
 >
 > - So, I
 wonder if R inherits the behavior from S or not.
 >
 > - The behavior is not
 clearly documented in R. I just find "R Language
 Definition", "3.4.1 Indexing by vectors",
 that can be interpreted to imply the behavior.
 >
 > - However, for a
 particular case, function 'rank' in R relies on the
 behavior.
 >
 > However,
 it seems that relying on the behavior is not on purpose.
 Previously, at least until R 3.1.3, the code of function
 'rank' has the following before yy <- NA .
 >         yy <-
 integer(length(x))
 >     
    storage.mode(yy) <- storage.mode(y)
 >
 > It seems that yy[]
 <- NA is what is intended.
 >
 > - However, for me, the behavior is
 plausible. The assumption is that indices from 1 to
 length(i) exist.
 >
 >
 --------------------------------------
 >
 > I think this behavior
 is consistent with typical indexing behaviour in R... I
 would ask you what result you thought you should get? I, for
 one, can think of all sorts of uses for numeric indexes that
 have different lengths than the vector, but am stumped to
 think of any use for what you are proposing.
 >
 ---------------------------------------------------------------------------
 > Jeff Newmiller                 
       The     .....   
    .....  Go Live...
 >
 DCN:<[hidden email]>        Basics: ##.#.   
    ##.#.  Live Go...
 >   

    Live:   OO#.. Dead: OO#.. 
 Playing
 > Research Engineer
 (Solar/Batteries            O.O#.   
    #.O#.  with
 >
 /Software/Embedded Controllers)           
    .OO#.       .OO#.  rocks...1k
 >
 ---------------------------------------------------------------------------
 > Sent from my phone. Please excuse my
 brevity.
 >
 > On
 September 5, 2015 10:02:05 AM PDT, Suharto Anggono Suharto
 Anggono via R-help <[hidden email]> wrote:
 >
 >>I came across this
 behavior when I followed the code of function
 'rank'
 >>in R.
 >>
 >>It seems that
 subassignment of a vector by a logical index vector that
 >>is longer than the original vector
 always results in expanding the
 >>original vector to the length of the
 index vector.
 >>
 >>The resulting length may be different
 from the result of subassignment
 >>by
 the equivalent numeric vector. For subassignment of a vector
 by a
 >>numeric index vector, the
 original vector is expanded to the maximum
 >>index, if it is larger than the length
 of the original vector.
 >>
 >>This is an example.
 >>
 >>> x <-
 NA
 >>> x[c(FALSE,TRUE,FALSE)] <-
 1
 >>> x
 >>[1]
 NA  1 NA
 >>
 >>Compare to this.
 >>
 >>> x <-
 NA
 >>>
 x[which(c(FALSE,TRUE,FALSE))] <- 1
 >>> x
 >>[1] NA 
 1
 >>
 >>Does S
 exhibit the same behavior?
 >>
 >>Currently, if there is NA and na.last =
 "keep", function 'rank' in R
 >>relies on this behavior to give correct
 result length.
 >>
 >>In "R Language Definition",
 "3.4.1 Indexing by vectors" says:
 "Logical.
 >>The indexing i should
 generally have the same length as x. .... If it
 >>is longer, then x is conceptually
 extended with NAs. ...." The
 >>statement can be taught to support the
 observed behavior.
 >>
 >>> sessionInfo()
 >>R version 3.2.2 (2015-08-14)
 >>Platform: i386-w64-mingw32/i386
 (32-bit)
 >>Running under: Windows XP
 (build 2600) Service Pack 2
 >>
 >>locale:
 >>[1]
 LC_COLLATE=English_United States.1252
 >>[2] LC_CTYPE=English_United
 States.1252
 >>[3]
 LC_MONETARY=English_United States.1252
 >>[4] LC_NUMERIC=C
 >>[5] LC_TIME=English_United
 States.1252
 >>
 >>attached base packages:
 >>[1] stats     graphics 
 grDevices utils     datasets 
 methods   base
 >>
 >>______________________________________________
 >>[hidden email] 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.
 >
 >
 ______________________________________________
 > [hidden email] 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.
 >
 >
 ______________________________________________
 > R-help at 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.