R devel - Sep 2010 - value returned by by()

Hi,

I noticed that by() returns an object of class 'by', regardless of what
its argument 'simplify' is.  ?by says that it always returns a list if
simplify=FALSE, yet by.data.frame shows:

---<--------------------cut
here---------------start------------------->---
function (data, INDICES, FUN, ..., simplify = TRUE) 
{
    if (!is.list(INDICES)) {
        IND <- vector("list", 1L)
        IND[[1L]] <- INDICES
        names(IND) <- deparse(substitute(INDICES))[1L]
    }
    else IND <- INDICES
    FUNx <- function(x) FUN(data[x, , drop = FALSE], ...)
    nd <- nrow(data)
    ans <- eval(substitute(tapply(1L:nd, IND, FUNx, simplify = simplify)), 
        data)
    attr(ans, "call") <- match.call()
    class(ans) <- "by"
    ans
}
<environment: namespace:base>
---<--------------------cut
here---------------end--------------------->---

One could force a list by wrapping it around an lapply(by.object,
"["),
but this is not possible if the object contains S4 objects.  How does
one force a list in those cases?


Cheers,

-- 
Seb

It returns a list with athe class attribut set to "by", just use:

x <- by(.....)
unclass(x)


Uwe Ligges


On 14.09.2010 00:11, Seb wrote:
> Hi,
>
> I noticed that by() returns an object of class 'by', regardless of
what
> its argument 'simplify' is.  ?by says that it always returns a list
if
> simplify=FALSE, yet by.data.frame shows:
>
> ---<--------------------cut
here---------------start------------------->---
> function (data, INDICES, FUN, ..., simplify = TRUE)
> {
>      if (!is.list(INDICES)) {
>          IND<- vector("list", 1L)
>          IND[[1L]]<- INDICES
>          names(IND)<- deparse(substitute(INDICES))[1L]
>      }
>      else IND<- INDICES
>      FUNx<- function(x) FUN(data[x, , drop = FALSE], ...)
>      nd<- nrow(data)
>      ans<- eval(substitute(tapply(1L:nd, IND, FUNx, simplify =
simplify)),
>          data)
>      attr(ans, "call")<- match.call()
>      class(ans)<- "by"
>      ans
> }
> <environment: namespace:base>
> ---<--------------------cut
here---------------end--------------------->---
>
> One could force a list by wrapping it around an lapply(by.object,
"["),
> but this is not possible if the object contains S4 objects.  How does
> one force a list in those cases?
>
>
> Cheers,
>

On Sep 15, 2010, at 10:55 , Uwe Ligges wrote:
> 
> 
> On 14.09.2010 20:50, Seb wrote:
>> On Tue, 14 Sep 2010 12:02:04 +0200,
>> Uwe Ligges<ligges at statistik.tu-dortmund.de>  wrote:
>> 
>>> It returns a list with athe class attribut set to "by",
just use: x<-
>>> by(.....)  unclass(x)
>> 
>> Thanks Uwe, however, that still returns an array when using the
>> data.frame method for by():
>> 
>> R>  class(unclass(with(warpbreaks, by(warpbreaks[, 1:2], tension,
summary))))
>> [1] "array"
>> 
>> It seems as if the only way to really ensure a list:
>> 
>> R>  class(lapply(unclass(with(warpbreaks, by(warpbreaks[, 1:2],
tension, summary))), function(x) x))
>> [1] "list"
>> 
>> but it seems like a waste to call another function just to do this.
>> 
>> 
> 
> Then you could still do
> 
> x <- by(.....)
> attributes(x) <- NULL
> 
Or just use c() instead of unclass(). (The root cause is that even with
simplify=FALSE, tapply() will always create an array, in this case a 1d array
with dim=3. The _contents_ of the array will be a list, though.)

Notice that in the relevant cases, what you get really _is_ a list, and both
walks and quacks like one.  E.g.
> L <- with(warpbreaks, by(warpbreaks[, 1], tension, mean,
simplify=FALSE))
> is.list(L)
[1] TRUE
> L$M
[1] 26.38889






-- 
Peter Dalgaard
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: (+45)38153501
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com