[datatable-help] Running on variables in data.table

[Kostia]

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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