[R] standardizing one variable by dividing each value by the mean - but within levels of a factor

[hadley wickham]

On Wed, Jan 20, 2010 at 4:37 PM, Dimitri Liakhovitski <ld7631 at gmail.com> wrote:
> Hello!
>
> I have a data frame with a factor and a numeric variable:
>
> x<-data.frame(factor=c("b","b","d","d","e","e"),values=c(1,2,10,20,100,200))
>
> For each level of "factor" - I would like to divide each value of
> "values" by the mean of "values" that corresponds to the level of
> "factor"
> In other words, I would like to get a new variable that is equal to:
> 1/1.5
> 2/1.5
> 10/15
> 20/15
> 100/150
> 200/150
>
> I realize I could do it through tapply starting with:
> factor.level.means<-tapply(x$values,x$factor,mean) ... etc.
>
>
> But it seems clunky to me.
> Is there a more elegant way of doing it?

Here's one way with the plyr package:

library(plyr)
ddply(x, "factor", transform, scaled = values / mean(values, na.rm = T))

Hadley

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