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

[jim holtman]

try this:

> dt[
+     , {
+         result <- list()
+         for (i in names(.SD)){
+             result[[i]] <- myFunction(unlist(.SD[, i, with = FALSE]))
+         }
+         result
+       }
+     , by = name
+     ]
   name var1 var2 var3
1:    a  2.0   22   42
2:    b  7.5   28   48
>



Jim Holtman
Data Munger Guru

What is the problem that you are trying to solve?
Tell me what you want to do, not how you want to do it.

On Tue, Jun 9, 2015 at 4:22 PM, Ramiro Barrantes <
ramiro at precisionbioassay.com> wrote:

> 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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>
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