[R] more complex by with data.table???

[Ramiro Barrantes]

Hello,

I am trying to do something that I am able to do with the "by" function within data.frame but can't figure out how to achieve with data.table.

Consider

dt<-data.table(name=c(rep("a",5),rep("b",6)),var1=0:10,var2=20:30,var3=40:50)
myFunction <- function(x) { mean(x) }

I am aware that I can do something like:

dt[, .(meanVar1=myFunction(var1)) ,by=.(name)]

but how could I do the equivalent of:

df<-data.frame(name=c(rep("a",5),rep("b",6)),var1=0:10,var2=20:30,var3=40:50)
myFunction <- function(x) { mean(x) }

columnNames <- c("var1","var2","var3")
result <- by(df, df$name, function(x) {
   output <- c()
   for(col in columnNames) {
     output[col] <- myFunction(x[,col])
   }
  output
})
do.call(rbind,result)

Thanks in advance,
Ramiro

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