[R] Specifying parameters for use in "plyr" / "ddply"

[David Winsemius]

On Apr 7, 2010, at 11:41 AM, Peter Ehlers wrote:

> On 2010-04-07 8:37, Dimitri Liakhovitski wrote:
>> Dear R-ers!
>>
>> # I have a data frame with one factor and 2 numeric variables:
>> x<-data.frame(group=c("b","b","d","d","e","e"),a=c(1,NA, 
>> 10,20,100,200),b=c(5,15,20,NA,10,30))
>> x
>>
>> # I want to divide each value of each variable by its group mean -
>> using plyr and ddply. It works fine, for example, for variable "a":
>> library(plyr)
>> x2<-ddply(x, "group", transform, a = a / mean(a, na.rm = T))
>> x2
>>
>> # Because I want to do the same for both variables (a and b) I want  
>> to
>> put it into a function.
>> # So, I am parametrising the grouping variable and the variable to  
>> transform.
>> # However, my code below is not working - I know that x[[variable]]  
>> is
>> not correct - but what is the right way of doing it?
>> grouping.factor<-"group"
>> variable<-"a"
>> x2<-ddply(x, grouping.factor, transform, x[[variable]] =  
>> x[[variable]]
>> / mean(x[[variable]], na.rm = T))
>>
>>
>> Or is there a more effective way of using ddply on a bunch of  
>> variables?
>> Thank you very much for your advise!
>>
> Yes, there is: colwise()
>
>  f <- function(x) x / mean(x, na.rm = TRUE)
>  ddply(x, "group", colwise(f, c("a", "b")))
>

Which might also be productive strategy for yesterday's question (for  
which we have yet to be offered reproducible data example, however):

https://stat.ethz.ch/pipermail/r-help/2010-April/234363.html
>
-- 

David Winsemius, MD
West Hartford, CT