目錄
SDcols is an useful but tricky method in data.table. The statement of it is hard to remember, so I wrote down some examples, copying and pasting when I need them.
Use lapply with .SDcols
cols %
.[, c(cols):= lapply(.SD, function(x) ifelse(is.na(x), 0, x))
, .SDcols = cols]
Aggregate with .SDcols
cols_by %
.[, lapply(.SD, sum), by = item_no_col, .SDcols = cols_sum]
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