Hi,
Likely :
dt[,lapply(.SD,sum),by=type]
See the examples section of ?data.table for an example. `.SD` is
explained on that page too.
Matt
On 03/10/13 09:25, Kostia wrote:
Hi,
I have a data table with a number of variables and I wish to do some
function on each variable,
my data table looks like this:
type att1 att2 att3 att4
black 1 2 2 1
white 0 2 1 0
green 4 2 1 0
black 1 1 1 1
green 2 1 2 2
I would like to sum on each attribute by type, so my function will be:
dt[,att1type := sum(att1),by = type]
The problem is that I want to taht in a loop and don't know how to run on
all the columns.
dt[,att1type := sum(dt[[i]]),by = type]
or
dt[,att1type := sum(dt[i]),by = type]
doesn't work.
Thanks,
Kostia
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