[Rd] apply with zero-row matrix

[Emil Bode]

    Hi David,
    
    Besides Martins point, there is also the issue that for a lot of cases you would still like to have the right class returned.
    Right now these are returns:
    
    > apply(matrix(NA_integer_,0,5), 1, class)
    character(0)
    > apply(matrix(NA_integer_,0,5), 1, identity)
    integer(0)
    > apply(matrix(NA,0,5), 1, identity)
    logical(0)
    
    In your case, these would all return NULL, so I think there is value in running FUN at least once (Say if you'd want to check if FUN always returns the right class). 
    And from a philosophical point of view, R is mostly a functional programming language, I think if you want side-effects a for-loop would look better.
    
    
    Best regards, 
    Emil Bode
     
    Data-analyst
     
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    On 30/07/2018, 11:12, "R-devel on behalf of David Hugh-Jones" <r-devel-bounces using r-project.org on behalf of davidhughjones using gmail.com> wrote:
    
        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.
        
        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
        
        On Mon, 30 Jul 2018 at 08:41, Martin Maechler <maechler using stat.math.ethz.ch>
        wrote:
        
        > >>>>> David Hugh-Jones
        > >>>>>     on Mon, 30 Jul 2018 05:33:19 +0100 writes:
        >
        >     > Forgive me if this has been asked many times before, but I
        >     > couldn't find anything on the mailing lists.
        >
        >     > I'd expect apply(m, 1, foo) not to call `foo` if m is a
        >     > matrix with zero rows.  In fact:
        >
        >     > m <- matrix(NA, 0, 5)
        >     > apply(m, 1, function (x) {cat("Called...\n"); print(x)})
        >     > ## Called...
        >     > ## [1] FALSE FALSE FALSE FALSE FALSE
        >
        >
        >     > Similarly for apply(m, 2,...) if m has no columns.  Is
        >     > there a reason for this?
        >
        > Yes :
        >
        > The reverse is really true for almost all basic R functions:
        >
        >     They *are* called and give an "empty" result automatically
        >     when the main argument is empty.
        >
        > What you basicaly propose is to add an extra
        >
        >      if(<length 0 input>)
        >          return(<correspondingly formatted length-0 output>)
        >
        > to all R functions.  While that makes sense for high-level R
        > functions that do a lot of things, this would really be a bad
        > idea in general :
        >
        > This would make all of these basic functions larger {more to maintain} and
        > slightly slower for all non-zero cases just to make them
        > slightly faster for the rare zero-length case.
        >
        > Martin Maechler
        > ETH Zurich and R core Team
        >
        > --
        Sent from Gmail Mobile
        
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