> On 17 Sep 2015, at 01:42, Dénes Tóth <toth.de...@ttk.mta.hu> wrote:
>
>
>
> On 09/16/2015 04:41 PM, Bert Gunter wrote:
>> Yes! Chuck's use of mapply is exactly the split/combine strategy I was
>> looking for. In retrospect, exactly how one should think about it.
>> Many thanks to all for a constructive discussion .
>>
>> -- Bert
>>
>>
>> Bert Gunter
>>
>>>>>
>>>>> Use mapply like this on large problems:
>>>>>
>>>>> unsplit(
>>>>> mapply(
>>>>> function(x,z) eval( x, list( y=z )),
>>>>> expression( A=y*2, B=y+3, C=sqrt(y) ),
>>>>> split( dat$Flow, dat$ASB ),
>>>>> SIMPLIFY=FALSE),
>>>>> dat$ASB)
>>>>>
>>>>> Chuck
>>>>>
>
>
> Is there any reason not to use data.table for this purpose, especially if
> efficiency is of concern?
>
> ---
>
> # load data.table and microbenchmark
> library(data.table)
> library(microbenchmark)
> #
> # prepare data
> DF <- data.frame(
> ASB = rep_len(factor(LETTERS[1:3]), 3e5),
> Flow = rnorm(3e5)^2)
> DT <- as.data.table(DF)
> DT[, ASB := as.character(ASB)]
> #
> # define functions
> #
> # Chuck's version
> fnSplit <- function(dat) {
> unsplit(
> mapply(
> function(x,z) eval( x, list( y=z )),
> expression( A=y*2, B=y+3, C=sqrt(y) ),
> split( dat$Flow, dat$ASB ),
> SIMPLIFY=FALSE),
> dat$ASB)
> }
> #
> # data.table-way (IMHO, much easier to read)
> fnDataTable <- function(dat) {
> dat[,
> result :=
> if (.BY == "A") {
> 2 * Flow
> } else if (.BY == "B") {
> 3 + Flow
> } else if (.BY == "C") {
> sqrt(Flow)
> },
> by = ASB]
> }
> #
> # benchmark
> #
> microbenchmark(fnSplit(DF), fnDataTable(DT))
> identical(fnSplit(DF), fnDataTable(DT)[, result])
>
> ---
>
> Actually, in Chuck's version the unsplit() part is slow. If the order is not
> of concern (e.g., DF is reordered before calling fnSplit), fnSplit is
> comparable to the DT-version.
>
But David’s version is faster than Chuck’s fnSplit. I modified David’s solution
slightly to get a result that is identical to fnSplit.
# David's version
# my modification to return a vector just like fnSplit
fnDavid <- function(dat) {
z <- mapply(
function(x,z) eval( x, list( y=z )),
expression(A= y*2, B=y+3, C=sqrt(y) ),
split( dat$Flow, dat$ASB ),
USE.NAMES=FALSE, SIMPLIFY=TRUE
)
as.vector(t(z))
}
Added this to Dénes's code.
Benchmarking with R package rbenchmark and testing result like this
library(rbenchmark)
benchmark(fnSplit(DF), fnDataTable(DT),fnDavid(DF))
identical(fnSplit(DF), fnDataTable(DT)[, result])
identical(fnSplit(DF), fnDavid(DF))
gave this:
test replications elapsed relative user.self sys.self user.child
2 fnDataTable(DT) 100 0.829 1.000 0.762 0.066 0
3 fnDavid(DF) 100 1.615 1.948 1.515 0.098 0
1 fnSplit(DF) 100 2.878 3.472 2.685 0.190 0
sys.child
2 0
3 0
1 0
> identical(fnSplit(DF), fnDataTable(DT)[, result])
[1] TRUE
> identical(fnSplit(DF), fnDavid(DF))
[1] TRUE
Berend
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