aggregating multiple columns in data.table

r dataframe aggregate data.table
Alex picture Alex · Jul 27, 2012 · Viewed 10.3k times · Source

I have the following sample data.table:

dtb <- data.table(a=sample(1:100,100), b=sample(1:100,100), id=rep(1:10,10))

I would like to aggregate all columns (a and b, though they should be kept separate) by id using colSums, for example. What is the correct way to do this? The following does not work:

 dtb[,colSums, by="id"]

This is just a sample and my table has many columns so I want to avoid specifying all of them in the function name

Answer

Alex picture Alex · Jul 30, 2012

this is actually what i was looking for and is mentioned in the FAQ:

dtb[,lapply(.SD,mean),by="id"]

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