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

You want tapply or ave, depending on how you want your output:

> Data <- data.frame(grp=sample(letters[1:3],20,TRUE),x=rnorm(20))
> ave(Data$x, Data$grp)
 [1] -0.3258590 -0.5009832 -0.5009832 -0.2136670 -0.3258590 -0.5009832
 [7] -0.3258590 -0.2136670 -0.3258590 -0.2136670 -0.3258590 -0.3258590
[13] -0.3258590 -0.5009832 -0.2136670 -0.5009832 -0.3258590 -0.2136670
[19] -0.5009832 -0.2136670
> tapply(Data$x, Data$grp, mean)
         a          b          c 
-0.5009832 -0.2136670 -0.3258590 

# Example with more than one column:
> Data <- data.frame(grp=sample(letters[1:3],20,TRUE),x=rnorm(20),y=runif(20))
> do.call(rbind,lapply(split(Data[,-1], Data[,1]), mean))
             x         y
a -0.675195494 0.4772696
b  0.270891403 0.5091359
c  0.002756666 0.4053922