compute means of a group by factor

Question

Is there a way that this can be improved, or done more simply?

means.by<-function(data,INDEX){
  b<-by(data,INDEX,function(d)apply(d,2,mean))
  return(         


        

Answer 1

Use only the generic function in R.

>d=data.frame(type=as.factor(rep(c("A","B","C"),each=3)),
x=rnorm(9),y=rgamma(9,2,1))
> d
type           x         y
1    A -1.18077326 3.1428680
2    A -0.91930418 4.4606603
3    A  0.88345422 1.0979301
4    B  0.06964133 1.1429911
5    B -1.15380345 2.7609049
6    B  1.13637202 0.6668986
7    C -1.12052765 1.7352306
8    C -1.34803630 2.3099202
9    C -2.23135374 0.7244689
>
> cbind(lm(x~-1+type,data=d)$coef,lm(y~-1+type,data=d)$coef)
         [,1]     [,2]
typeA -0.4055411 2.900486
typeB  0.0174033 1.523598
typeC -1.5666392 1.589873