[R] Using apply to get group means

[Alan Cohen]

Hi all,

I'm trying to improve my R skills and make my programming more efficient and succinct.  I can solve the following question, but wonder if there's a better way to do it:

I'm trying to calculate mean by several variables and then put this back into the original data set as a new variable.  For example, if I were measuring weight, I might want to have each individual's weight, and also the group mean by, say, race, sex, and geographic region.  The following code works:

> x1<-rep(c("A","B","C"),3)
> x2<-c(rep(1,3),rep(2,3),1,2,1)
> x3<-c(1,2,3,4,5,6,2,6,4)
> x<-as.data.frame(cbind(x1,x2,x3))
> x3.mean<-rep(0,nrow(x))
> for (i in 1:nrow(x)){
+   x3.mean[i]<-mean(as.numeric(x[,3][x[,1]==x[,1][i]&x[,2]==x[,2][i]]))
+   }                                      
> cbind(x,x3.mean)
  x1 x2 x3 x3.mean
1  A  1  1     1.5
2  B  1  2     2.0
3  C  1  3     3.5
4  A  2  4     4.0
5  B  2  5     5.5
6  C  2  6     6.0
7  A  1  2     1.5
8  B  2  6     5.5
9  C  1  4     3.5

However, I'd love to be able to do this with "apply" rather than a for-loop.  Or is there a built-in function? Any suggestions?

Also, any way to avoid the hassles with having to convert to a data frame and then again to numeric when one variable is character?

Cheers,
Alan Cohen