Advanced R Tutorial – Use SDcols In data.table

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('COL1', 'COL2', 'COL3')
main_tb <- main_tb %>% 
  .[,  c(cols):= lapply(.SD, function(x) ifelse(is.na(x), 0, x))
    , .SDcols = cols]

Aggregate with .SDcols

cols_by <- c('COL1', 'COL2')
cols_sum <- c('COL3', 'COL4')
dt <- dt %>%
  .[, lapply(.SD, sum), by = item_no_col, .SDcols = cols_sum]

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