[R] Comparing "transform" to "with"

[Gabor Grothendieck]

Try this version of transform.  In the first test we show
it works on your example but we have used the head of the built in
anscombe data set.  The second and third show that
it necessarily is incompatible with transform because transform
always looks up variables in DF first whereas my.transform looks
up the computed ones first.

my.transform <- function(DF, ...) {
	f <- function(){}
	formals(f) <- eval(substitute(as.pairlist(c(alist(...), DF))))
	body(f) <- substitute(modifyList(DF, data.frame(...)))
	f()
}

# test
a <- head(anscombe)
# 1
my.transform(a, sum1 = x1+x2+x3+x4, sum2 = y1+y2+y3+y4, total = sum1+sum2)
# 2
my.transform(a, y2 = y1, y3 = y2)
# 3
transform(a, y2 = y1, y3 = y2) # different


On 9/1/07, Muenchen, Robert A (Bob) <muenchen at utk.edu> wrote:
> Hi All,
>
> I've been successfully using the with function for analyses and the
> transform function for multiple transformations. Then I thought, why not
> use "with" for both? I ran into problems & couldn't figure them out from
> help files or books. So I created a simplified version of what I'm
> doing:
>
> rm( list=ls() )
> x1<-c(1,3,3)
> x2<-c(3,2,1)
> x3<-c(2,5,2)
> x4<-c(5,6,9)
> myDF<-data.frame(x1,x2,x3,x4)
> rm(x1,x2,x3,x4)
> ls()
> myDF
>
> This creates two new variables just fine"
>
> transform(myDF,
>  sum1=x1+x2,
>  sum2=x3+x4
> )
>
> This next code does not see sum1, so it appears that "transform" cannot
> see the variables that it creates. Would I need to transform new
> variables in a second pass?
>
> transform(myDF,
>  sum1=x1+x2,
>  sum2=x3+x4,
>  total=sum1+sum2
> )
>
> Next I'm trying the same thing using "with". It doesn't not work but
> also does not generate error messages, giving me the impression that I'm
> doing something truly idiotic:
>
> with(myDF, {
>  sum1<-x1+x2
>  sum2<-x3+x4
>  total <- sum1+sum2
> } )
> myDF
> ls()
>
> Then I thought, perhaps one of the advantages of "transform" is that it
> works on the left side of the equation without using a longer name like
> myDF$sum1. "with" probably doesn't do that, so I use the longer form
> below. It also does not work and generates no error messages.
>
> # Try it again, writing vars to myDF explicitly.
> # It generates no errors, and no results.
> with(myDF, {
>  myDF$sum1<-x1+x2
>  myDF$sum2<-x3+x4
>  myDF$total <- myDF$sum1+myDF$sum2
> } )
> myDF
> ls()
>
> I would appreciate some advice about the relative roles of these two
> functions & why my attempts with "with" have failed.
>
> Thanks!
> Bob
>
> =========================================================
> Bob Muenchen (pronounced Min'-chen), Manager
> Statistical Consulting Center
> U of TN Office of Information Technology
> 200 Stokely Management Center, Knoxville, TN 37996-0520
> Voice: (865) 974-5230
> FAX: (865) 974-4810
> Email: muenchen at utk.edu
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>
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