[R] Help on aggregate method

[Erik Iverson]

Stella Pachidi wrote:
> Dear Erik and R experts,
> 
> Thank you for the fast response!
> 
> I include an example with the ChickWeight dataset:
> 
> ap.dat <- ChickWeight
> 
> matchMeanEx <- function(ind,dataTable,aggrTable)
> {
>    index <- which((aggrTable[,1]==dataTable[["Diet"]][ind]) & 
> (aggrTable[,2]==dataTable[["Chick"]][ind]))
>    as.numeric(aggrTable[index,3])
> }
> 
> avgW <- aggregate(ap.dat[["weight"]], by = list(ap.dat[["Diet"]], 
> ap.dat[["Chick"]]), FUN="mean")
> meanW <- sapply((1:length(ap.dat[,1])), FUN=matchMeanEx, ap.dat, avgW)
> ap.dat <- cbind (ap.dat, meanW)
> 


How about simply using ave.

ap.dat$meanW <- ave(ap.dat$weight, list(ap.dat$Diet, ap.dat$Chick))