[R] transform() on selective names. Is it possible?

[Juan Carlos Borrás]

Wonderful, or the closest to heaven I've been the whole afternoon, but
not quite there:

# begin code
N <- 300
M <- 2
x <- matrix(data=rnorm(N*M, 0, 3)-10, ncol=M, nrow=N)
y <- matrix(c(1,-2,-2,1), ncol=M, nrow=M)
z <- data.frame(x %*% y)
colnames(z) <- c('x','y')
par(mfrow=c(1,3))
plot(z, pch=5, col="blue")

whiten <- scale  # no need to re-invent the wheel, I agree
fc <- function(dfrm, coln) transform(dfrm, coln=whiten(dfrm[coln]))
colxy <- "x"
z1 <- fc(z, colxy)  # the "[" function will interpret colxy
colnames(z1)
> [1] "x"   "y"   "x.1"

fc <- function(dfrm, coln) transform(dfrm, coln=whiten(dfrm[,coln]))
colxy <- "x"
z2 <- fc(z, colxy)  # the "[" function will interpret colxy
colnames(z2)
> [1] "x"    "y"    "coln"

#end code

What I want is to know whether I can customize the column name of the
result of the transform() call.
Your hint is fantastic, thanks there, but I keep getting into that
particular pattern of computation over and over and I wonder if it's
possible to skip a column clean-up after applying your trick.

2011/4/7 David Winsemius <dwinsemius at comcast.net>:
>
> On Apr 7, 2011, at 9:56 AM, Juan Carlos Borrás wrote:
>
>> Hi all,
>> I am whitening my data:
>>
>> # code begins
>> N <- 300
>> M <- 2
>> x <- matrix(data=rnorm(N*M, 0, 3)-10, ncol=M, nrow=N)
>> y <- matrix(c(1,-2,-2,1), ncol=M, nrow=M)
>> z <- data.frame(x %*% y)
>> colnames(z) <- c('x','y')
>> par(mfrow=c(1,3))
>> plot(z, pch=5, col="blue")
>>
>> whiten <- function(x) { (x-mean(x))/sd(x) }
>
> Consider:
>
>  whiten <- scale  # no need to re-invent the wheel
>  fc <- function(dfrm, coln) transform(dfrm, coln=whiten(dfrm[coln]))
>  colxy <- "x"
>  z <- fc(z, colxy)  # the "[" function will interpret colxy
>  z
>
>>
>> zz <- transform(z, x=whiten(x), y=whiten(y))
>> plot(zz, pch=3, col="red")
>>
>> #code ends
>>
>> And everything looks fine enough.
>> But now I want to withen just one of the columns and I won't know
>> which one until my script is running, hence I can't hard code it in
>> the script.
>> Then I though, well maybe if I define some convenient f...
>>
>> #begin code
>>
>> f <- function(a) { paste(a,"=withen(",a,")", sep='') }
>> a <- 'x' # or a <- 'y' depending on user input.....
>> f(a)
>>>
>>> [1] "x=withen(x)"
>>
>> # so I could try....
>> zzz <- transform(z, eval(f('x')))
>> # which of course doesn't work
>> plot(zz, pch=3, col="green")
>>
>> head(z, n=2)
>>>
>>>        x         y
>>> 1 17.167380  6.884402
>>> 2  8.234507 13.940932
>>
>> head(zzz, n=2)
>>>
>>>        x         y
>>> 1 17.167380  6.884402
>>> 2  8.234507 13.940932
>>
>> #end code
>>
>> Could someone provide me with some hint on whether the attempted trick
>> above is possible and how to proceed further?
>> Thanks in advance.
>> jcb!
>>
>> ______________________________________________
>> R-help at r-project.org mailing list
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>> PLEASE do read the posting guide
>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>
> David Winsemius, MD
> West Hartford, CT
>
>


Cheers,
jcb!