r - Use a factor column in "by" and do not drop empty factors -


suppose have data.table:

x <- data.table(x=runif(3), group=factor(c('a','b','a'), levels=c('a','b','c')))

i want know how many rows in x exist each group:

x[, .n, by="group"] #    group n # 1:     2 # 2:     b 1

question: there way force above by="group" consider levels of factor group?

notice how since don't have rows of group 'c' in table, don't row c.

desired output:

x[, .n, by="group", ???] # somehow use levels in `group` #    group n # 1:     2 # 2:     b 1 # 3:     c 0

if willing run through factor levels enumerating them in i (rather setting by="group"), hoped results.

setkey(x, "group") x[levels(group), .n, by=.eachi] #    group n # 1:     2 # 2:     b 1 # 3:     c 0

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