Passing data to a function with an unknown number of programmatically defined variables in data.table

Question

Let's assume I have the following code:

x = data.table(rowid=1:10, N1=2:11, N2=3:12, N3=4:13)
x[, sum(c(N1, N2, N3)),by=rowid]

Now assume I don't know the column names N1, N2, N3 in advance and they are saved in the vector colnames=c("N1", "N2", "N3"). If I know the number of variables to pass to sum(), I can plug in colnames as follows:

x[, sum(c(get(colnames[1]),get(colnames[2]),get(colnames[3]))),by=rowid]

Now assume I don't know the length of colnames. Is there any way I can rewrite the above so it works? Something like x[, sum(c(sapply(colnames, as.name))),by=rowid] (note this exact expression doesn't work).

G. Grothendieck · Accepted Answer · 2013-05-23 00:22:53Z

5

This sums all columns except rowid :

x[, sum(.SD), by = rowid]

This sums only those columns named in character vector colnames :

x[, sum(.SD), by = rowid, .SDcols = colnames]

In both cases the sums are by rowid .

edited May 23, 2013 at 0:22

answered May 22, 2013 at 15:49

G. Grothendieck

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