在组

Question

我有data.table

X = data.table(x = c(1,1,1,1,1,2,2,2,2,2), y = c(3,2,1,-1,5,7,4,-2,3,5))

我想仅对一组中负值以上的行进行子集化：

res = data.table(x = c(1,1,1,2,2), y = c(3,2,1,7,4)

从第一组中的五个值开始，我只想获得前三个，因为第四个是负数，而第二个组是相同的。

Answer 1

以下是两个选项：

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或者（可能更有效，因为它避免了X[, .SD[seq_len(which.max(y<0)-1L)], by = x] ）：

.SD

Answer 2

我们也可能

X[, .SD[cummin(sign(y))>0], x]
#   x y
#1: 1 3
#2: 1 2
#3: 1 1
#4: 2 7
#5: 2 4