与data.table聚合时保持零计数组合 [英] Keeping zero count combinations when aggregating with data.table

问题描述

假设我有以下 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

每行代表以下事实：某人（由 id 和性别）吃了一个水果。我想计算每个性爱吃掉每个个水果的次数。我可以这样：

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")]

其中给出：

    fruit sex N
1:  apple   H 3
2: tomato   H 1
3: orange   H 1
4:  apple   F 2
5: tomato   F 2

问题是，通过这种方式，我失去了<$ sex == F 的c $ c> orange ，因为此计数为0。有没有办法进行这种聚合而不会丢失组合的零计数？

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?

要完全清楚，期望的结果如下：

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

非常感谢！

推荐答案

最简单的方法似乎是在传递给 i = ，将 by = .EACHI 设置为遍历它们：

Seems like the most straightforward approach is to explicitly supply all category combos in a data.table passed to i=, setting by=.EACHI to iterate over them:

setkey(dt, sex, fruit)
dt[CJ(sex, fruit, unique = TRUE), .N, by = .EACHI]
#    sex  fruit N
# 1:   F  apple 2
# 2:   F orange 0
# 3:   F tomato 2
# 4:   H  apple 3
# 5:   H orange 1
# 6:   H tomato 1

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