[R] unexpected 'else' in " else"

[Martin Maechler]

>>>>> Ebert,Timothy Aaron 
>>>>>     on Fri, 21 Oct 2022 12:05:58 +0000 writes:

    > I can get it to work with
    > ifelse(is.matrix(r), r[w!=0, , drop=FALSE], r[w!=0])

Note that this is *not* good advice:

  if(Cnd) A else B    is very much more efficient  than
  ifelse(Cnd, A, B)

whenever it is appropriate, i.e.,
the condition Cnd is a simple TRUE or FALSE.

ifelse() is very much over-used!

{For the more sophisticated reader:
 In R, these both are function calls:
 `if` is a function of 3 argument with a "peculiar" syntax and
 the third argument with default NULL.
}

Martin Maechler
ETH Zurich  and  R Core team



    > With w and r as defined r is not a  matrix, so the first part will never execute. The test is for w not equal to zero so it is always true for these vectors. It is usually good to have test code such that all possible cases are activated.

    > r<--1:8
    > w<- -1:5
    > if(is.matrix(r)){
    > r[w!=0, , drop=FALSE]
    > } else r[w != 0]

    > I think this also works. The "else" must follow the } and you put in a carriage return after the }

    > I think that is the answer, but not 100% sure.

    > Tim

    > -----Original Message-----
    > From: R-help <r-help-bounces using r-project.org> On Behalf Of Andrew Simmons
    > Sent: Friday, October 21, 2022 5:37 AM
    > To: Jinsong Zhao <jszhao using yeah.net>
    > Cc: R-help Mailing List <r-help using r-project.org>
    > Subject: Re: [R] unexpected 'else' in " else"

    > [External Email]

    > The error comes from the expression not being wrapped with braces. You could change it to

    > if (is.matrix(r)) {
    > r[w != 0, , drop = FALSE]
    > } else r[w != 0]

    > or

    > {
    > if (is.matrix(r))
    > r[w != 0, , drop = FALSE]
    > else r[w != 0]
    > }

    > or

    > if (is.matrix(r)) r[w != 0, , drop = FALSE] else r[w != 0]


    > On Fri., Oct. 21, 2022, 05:29 Jinsong Zhao, <jszhao using yeah.net> wrote:

    >> Hi there,
    >> 
    >> The following code would cause R error:
    >> 
    >> > w <- 1:5
    >> > r <- 1:5
    >> >         if (is.matrix(r))
    >> +             r[w != 0, , drop = FALSE]
    >> >         else r[w != 0]
    >> Error: unexpected 'else' in "        else"
    >> 
    >> However, the code:
    >> if (is.matrix(r))
    >> r[w != 0, , drop = FALSE]
    >> else r[w != 0]
    >> is extracted from stats::weighted.residuals.
    >> 
    >> My question is why the code in the function does not cause error?
    >> 
    >> Best,
    >> Jinsong
    >> 
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