[R] Removing elements ..., NO bug in which() function

[Martin Maechler]

>>>>> "RK" == Rumen Kostadinov <rkostadi at gmail.com>
>>>>>     on Sun, 13 Feb 2011 12:46:52 -0500 writes:

    RK> Thanks Sarah,
    RK> Yes, the function behaves Exactly as documented:

    RK> check this out:
    >> a = c(1,2,3,4,5)
    >> a[which(a!=6)]
    RK> [1] 1 2 3 4 5
    >> a[!which(a==6)]
    RK> numeric(0)
    >> a[-which(a==6)]
    RK> numeric(0)
    >> a[!a==6]
    RK> [1] 1 2 3 4 5

    RK> I guess this is just a "gotcha", since
    RK> I often use !which and -which to remove elements,
    RK> So one should use
    RK> a[which(a!=stuff to remove)]
    RK> instead of
    RK> a[-which(a==stuff to remove)]
    RK> a[!which(a==stuff to remove)]

BTW: you have just seen why you should *NOT* use  
      a[ -which() ]
     
and probably should use which() much less than you currently do.
Some useRs seem overly fond of which() instead of working with
the logical vectors directly ...
{{Yes, I know that in some situations  which() "works with NAs"...}}

I think we (R-core) should get back to Bill Dunlap's suggestions
on the R-devel list and think of providing fast functions for
things like   "L && !is.na(L)"
...
in other words, versions of the utilities I have in (the R
package) Matrix/R/Auxiliaries.R,

  ## Need to consider NAs ;  "== 0" even works for logical & complex:
  ## Note that "!x" is faster than "x == 0", but does not (yet!) work for complex
  ## if we did these in C, would gain a factor 2 (or so):
  is0  <- function(x) !is.na(x) & x == 0
  isN0 <- function(x)  is.na(x) | x != 0
  is1  <- function(x) !is.na(x) & x   # also == "isTRUE componentwise"

  ##
  all0 <- function(x) !any(is.na(x)) && all(!x) ## ~= allFalse
  any0 <- function(x) isTRUE(any(x == 0))	      ## ~= anyFalse
  ## These work "identically" for  1 ('==' TRUE)  and 0 ('==' FALSE)
  ##	(but give a warning for "double"  1 or 0)
  ## TODO: C versions of these would be faster
  allTrue  <- function(x) all(x)  && !any(is.na(x))
  allFalse <- function(x) !any(x) && !any(is.na(x))## ~= all0
  anyFalse <- function(x) isTRUE(any(!x))		 ## ~= any0


Note that I comment twice that it would be nice to have fast
versions of these (via C).

Martin



    RK> On Sun, Feb 13, 2011 at 12:37 PM, Sarah Goslee
    <sarah.goslee at gmail.com> wrote:
    >> If by "bug" you mean "function behaving exactly as documented."
    >> 
    >> which() returns only the matches, the TRUE values. If there are
    >> no matches, it doesn't return anything.
    >> 
    >> If I understand what you are trying to do, and I may not,
    >> a[which(a != 5)] is really what you want, and it is precisely
    >> to preserve that behavior that which() does what it does.
    >> 
    >> Sarah
    >> 
    >> ---
    >> 
    >> which                   package:base                   R
    >> Documentation
    >> 
    >> Description:
    >> 
    >>     Give the ‘TRUE’ indices of a logical object, allowing for
    >> array     indices.
    >> 
    >> Value:
    >> 
    >>     If ‘arr.ind == FALSE’ (the default), an integer vector with
    >>     ‘length’ equal to ‘sum(x)’, i.e., to the number of ‘TRUE’s
    >> in ‘x’;     Basically, the result is ‘(1:length(x))[x]’.
    >> 
    >> ---
    >> On Sun, Feb 13, 2011 at 11:59 AM, Rumen Kostadinov
    >> <rkostadi at gmail.com> wrote:
    >>> Dear all,
    >>> 
    >>> I found a bug in the which() function.
    >>> 
    >>> When trying to remove elements with the which function, if the
    >>> criteria is not matched, numeric(0) is returned instead of the
    >>> array itself.
    >>> 
    >>> This is very weird.
    >>> 
    >>>> a = c(1,2,3,4,5)
    >>>> a[!a==6]
    >>> [1] 1 2 3 4 5
    >>>> a[-which(a==6)]
    >>> numeric(0)
    >>>> a[-which(a==5)]
    >>> [1] 1 2 3 4
    >>>> a[!a==5]
    >>> [1] 1 2 3 4
    >>> 
    >>> Is this correct? I believe this is a bug.
    >>> 
    >>> I have to rewrite a lot of my R code to use
    >>> a = a[!criteria]
    >>> and not
    >>> a = a[-which(criteria)]
    >>> 
    >>> R.
    >>> 
    >> 
    >> 
    >> --
    >> Sarah Goslee
    >> http://www.functionaldiversity.org
    >> 

    RK> ______________________________________________ RK>
    R-help at r-project.org mailing list RK>
    https://stat.ethz.ch/mailman/listinfo/r-help RK> PLEASE do read
    the posting guide http://www.R-project.org/posting-guide.html RK>
    and provide commented, minimal, self-contained, reproducible code.