Hadley,
As far as I can tell from a quick look, it is because implicit printing
uses a different mechanism which does a fair bit more work.
>From comments in print.c in the R sources:
* print.default() -> do_printdefault (with call tree below)
*
* auto-printing -> PrintValueEnv
* -> PrintValueRec
* -> call print() for objects
* Note that auto-printing does not call print.default.
* PrintValue, R_PV are similar to auto-printing.
PrintValueEnv includes, among other things, checks for functions, S4
objects, and s3 objects before constructing (in C code) an R call to print
for S3 objects and show for S4 objects and evaluating it using Rf_eval. So
there is an extra trip to the R evaluator.
I imagine that extra work is where the hangup is but that is a
slightly-informed guess as I haven't done any detailed timings or checks.
Basically my understanding of the processes is as follows:
print(df)
print call is evaluated, S3 dispatch happens, print.default in C is called,
result printed to terminal, print call returns
df
expression "df" evaluated, auto-print initiated, type of object returned by
expression is determined, print call is constructed in C code, print call
is evaluated in C code, THEN all the stuff above happens.
I dunno if that helps or not as I can't speak to how to change/fix it atm.
~G
On Wed, Oct 30, 2013 at 3:22 PM, Hadley Wickham <h.wick...@gmail.com> wrote:
> Hi all,
>
> Can anyone help me understand why an implicit print (i.e. just typing
> df at the console), is so much slower than an explicit print (i.e.
> print(df)) in the example below? I see the difference in both Rstudio
> and in a terminal.
>
> # Construct large df as quickly as possible
> dummy <- 1:18e6
> df <- lapply(1:10, function(x) dummy)
> names(df) <- letters[1:10]
> class(df) <- c("myobj", "data.frame")
> attr(df, "row.names") <- .set_row_names(18e6)
>
> print.myobj <- function(x, ...) {
> print.data.frame(head(x, 2))
> }
>
> start <- proc.time(); df; flush.console(); proc.time() - start
> # user system elapsed
> # 0.408 0.557 0.965
> start <- proc.time(); print(df); flush.console(); proc.time() - start
> # user system elapsed
> # 0.019 0.002 0.020
>
> sessionInfo()
> # R version 3.0.2 (2013-09-25)
> # Platform: x86_64-apple-darwin10.8.0 (64-bit)
> #
> # locale:
> # [1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8
> #
> # attached base packages:
> # [1] stats graphics grDevices utils datasets methods base
>
> Thanks!
>
> Hadley
>
> --
> Chief Scientist, RStudio
> http://had.co.nz/
>
> ______________________________________________
> R-devel@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>
--
Gabriel Becker
Graduate Student
Statistics Department
University of California, Davis
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