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Suppose I've got the following data.table : dt <- data.table(id = c(rep(1, 5), rep(2, 4)), sex = c(rep("H", 5), rep("F", 4)), fruit = c("apple", "tomato", "apple", "apple", "orange", "apple", "apple", "tomato", "tomato"), key = "id") id sex fruit 1: 1 H apple 2: 1 H tomato 3: 1 H apple 4: 1 H apple 5: 1 H orange 6: 2 F apple 7: 2 F apple 8: 2 F tomato 9: 2 F tomato Each row represents the fact that someone (identified by it's id and sex ) ate a fruit . I want to count the number of times each fruit has been eaten by sex . I can do it with : dt[ , .N, by = c("fruit", "sex")] Which gives: fruit sex N 1: apple H 3 2: tomato H 1 3: orange H 1 4: apple F 2 5: tomato F 2 The problem is, by doing it this way I'm losing the count of orange for sex == "F" , because this count is 0. Is there a way to do this aggregation without loosing combinations of zero counts? To be perfectly clear, the desired result would be the following: fruit sex N 1: apple H 3 2: tomato H 1 3: orange H 1 4: apple F 2 5: tomato F 2 6: orange F 0 Thanks a lot !

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