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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] Recommended Posts Learn Python And R In DataCamp. Start Your Data Science Career.