R数据表: How to sum variables by group based on a condition?

Question

假设我有以下 R data.table(尽管我也很高兴使用 base R，data.frame)

library(data.table)

dt = data.table(Category=c("First","First","First","Second","Third", "Third", "Second"), Frequency=c(10,15,5,2,14,20,3), times = c(0, 0, 0, 3, 3, 1))

> dt
   Category Frequency times
1:    First        10     0
2:    First        15     0
3:    First         5     0
4:   Second         2     3
5:    Third        14     3
6:    Third        20     1
7:   Second         3     0

如果我想按类别对频率求和，我会使用以下内容:

data[, sum(Frequency), by = Category]

但是，如果且仅当 times 非零且不等于 时，假设我想按 Category 对 Frequency 求和>不适用?

如何根据单独列的值使此总和成为条件？

编辑:为显而易见的问题道歉。快速补充:如果某列的元素是字符串怎么办？

例如

> dt
   Category Frequency times
1:    First        ten    0
2:    First        ten    0
3:    First        five   0
4:   Second        five   3
5:    Third        five   3
6:    Third        five   1
7:   Second        ten    0

Sum() 不会计算 10 与 5

的频率

Answer 1

记住data.table的逻辑:dt[i, j, by]，即取dt，子集行使用i，然后计算按by分组的j。

dt[times != 0 & !is.na(times), sum(Frequency), by = Category]
   Category V1
1:   Second  2
2:    Third 34