计算 data.table 中满足条件的行数

Question

我有以下table

DT = data.table(x=rep(c("a","b","c"),each=3), y=c(1,3,6), v=rep(4:6, 3))

我想统计有多少行满足条件(y==3 & v==5)。

我可以得到满足条件的行，所以我可以保存它们然后统计行数。但是，我知道使用 .N 可以更有效地完成它，我只是不知道该怎么做。我的代码：

require(data.table)
keycols = c("y","v")
setkeyv(DT,keycols) 

DT[J(3,5)] # This gets the subset I am interested in

DT[ , `:=` (count = .N), by = J(3,5)] # This is one of the multiple unsuccessful ways I have been trying to count the rows.

有人知道如何使最后一行起作用吗？

Answer 1

怎么样

DT[.(3,5), .N]
# [1] 3

## These are also equivalent
## DT[J(3,5), .N]
## DT[list(3,5), .N]

Answer 2

这应该有效

DT[ y == 3 & v == 5, count := .N]

计算 data.table 中满足条件的行数

Counting rows in data.table that meet a condition

r

count

data.table