[Rd] apply with zero-row matrix
wdunl@p @ending from tibco@com
Mon Aug 13 23:41:36 CEST 2018
vapply has a mandatory FUN.VALUE argument which specifies the type and size
of FUN's return value. This helps when you want to cover the 0-length case
without 'if' statements. You can change your apply calls to vapply calls,
but they will be a bit more complicated. E.g., change
apply(X=myMatrix, MARGIN=2, FUN=quantile)
The latter will always return a 5-row by ncol(myMatrix) matrix.
On Mon, Jul 30, 2018 at 5:38 AM, Martin Maechler <maechler using stat.math.ethz.ch
> >>>>> David Hugh-Jones
> >>>>> on Mon, 30 Jul 2018 10:12:24 +0100 writes:
> > Hi Martin, Fair enough for R functions in general. But the
> > behaviour of apply violates the expectation that apply(m,
> > 1, fun) calls fun n times when m has n rows. That seems
> > pretty basic.
> Well, that expectation is obviously wrong ;-) see below
> > Also, I understand from your argument why it makes sense
> > to call apply and return a special result (presumably
> > NULL) for an empty argument; but why should apply call fun?
> > Cheers David
> The reason is seen e.g. in
> > apply(matrix(,0,3), 2, quantile)
> [,1] [,2] [,3]
> 0% NA NA NA
> 25% NA NA NA
> 50% NA NA NA
> 75% NA NA NA
> 100% NA NA NA
> and that is documented (+/-) in the first paragraph of the
> 'Value:' section of help(apply) :
> > Value:
> > If each call to ‘FUN’ returns a vector of length ‘n’, then ‘apply’
> > returns an array of dimension ‘c(n, dim(X)[MARGIN])’ if ‘n > 1’.
> > If ‘n’ equals ‘1’, ‘apply’ returns a vector if ‘MARGIN’ has length
> > 1 and an array of dimension ‘dim(X)[MARGIN]’ otherwise. If ‘n’ is
> > ‘0’, the result has length 0 but not necessarily the ‘correct’
> > dimension.
> To determine 'n', the function *is* called once even when
> length(X) == 0
> It may indeed be would helpful to add this explicitly to the
> help page ( <R>/src/library/base/man/apply.Rd ).
> Can you propose a wording (in *.Rd if possible) ?
> With regards,
> R-devel using r-project.org mailing list
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