[Rd] Light-weight data.frame class: was: how to add method to .Primitive function

[Vadim Ogranovich]

Hi,

Encouraged by a tip from Simon Urbanek I tried to use the S3 machinery
to write a faster version of the data.frame class.
This quickly hits a snag: the "[.default"(x, i) for some reason cares
about the dimensionality of x.
In the end there is a full transcript of my R session. It includes the
motivation for writing the class and the problems I have encountered.

As a result I see three issues here:
* why "[.default"(x, i) doesn't work if dim(x) is 2? After all a single
subscript into a vector works regardless of whether it's a matrix or
not. Is there an alternative way to access "[.default"?
* why does unclass() make deep copy? This is a facet of the global
over-conservatism of R with respect to copying.
* is it possible to add some sort copy profiling to R? Something like
copyProfiling(TRUE), which should cause R to log sizes of each copied
object (just raw sizes w/o any attempt to identify the object). This
feature should at least help assess the magnitude of the problem.

Thanks,
Vadim

Now the transcript itself:
> # the motivation: subscription of a data.frame is *much* (almost 20
times) slower than that of a list
> # compare
> n = 1e6
> i = seq(n)
> 
> x = data.frame(a=seq(n), b=seq(n))
> system.time(x[i,], gcFirst=TRUE)
[1] 1.01 0.14 1.14 0.00 0.00
> 
> x = list(a=seq(n), b=seq(n))
> system.time(lapply(x, function(col) col[i]), gcFirst=TRUE)
[1] 0.06 0.00 0.06 0.00 0.00
> 
> 
> # the solution: define methods for the light-weight data.frame class
> lwdf = function(...) structure(list(...), class = "lwdf")
> 
> # dim
> dim.lwdf = function(x) c(length(x[[1]]), length(x))
> 
> # for pretty printing we define print.lwdf via a conversion to
data.frame
> # as.data.frame.lwdf
> as.data.frame.lwdf = function(x) structure(unclass(x),
class="data.frame", row.names=as.character(seq(nrow(x))))
> 
> # print
> print.lwdf = function(x) print.data.frame(as.data.frame.lwdf(x))
> 
> # now the real stuff
> 
> # "["
> # the naive "[.lwdf" = function (x, i, j) lapply(x[j], function(col)
col[i])
> # won't work because evaluation of x[j] calls "[.lwdf" again and not
"[.default"
> # so we switch by the number of arguments
> "[.lwdf" = function (x, i, j) {
+   if (nargs() == 2)
+     NextMethod("[", x, i)
+   else
+     structure(lapply(x[j], function(col) col[i]),  class = "lwdf")
+ }
> 
> x = lwdf(a=seq(3), b=letters[seq(3)], c=as.factor(letters[seq(3)]))
> i = c(1,3); j = c(1,3)
> 
> # unfortunately, for some reasons "[.default" cares about
dimensionality of its argument
> x[i,j]
Error in "[.default"(x, j) : incorrect number of dimensions
> 
> 
> # we could use unclass to get it right
> "[.lwdf" = function (x, i, j) {
+   structure(lapply(unclass(x)[j], function(col) col[i]),  class =
"lwdf")
+ }
> 
> x[i,j]
  a c
1 1 a
2 3 c
> 
> # *but* unclass creates a deep copy of its argument as indirectly
evidenced by the following timing
> x = lwdf(a=seq(1e6)); system.time(unclass(x))
[1] 0.01 0.00 0.01 0.00 0.00
> x = lwdf(a=seq(1e8)); system.time(unclass(x))
[1] 0.44 0.39 0.82 0.00 0.00

> version
         _                       
platform x86_64-unknown-linux-gnu
arch     x86_64                  
os       linux-gnu               
system   x86_64, linux-gnu       
status                           
major    2                       
minor    0.1                     
year     2004                    
month    11                      
day      15                      
language R