r - 数据表按组滚动连接

Question

如何为每对(loc.x，loc.y)对找到test.day 之前的最后一个值？

dt <- data.table( 
 loc.x = as.integer(c(1, 1, 3, 1, 3, 1)),
 loc.y = as.integer(c(1, 2, 1, 2, 1, 2)),
 time = as.IDate(c("2015-03-11", "2015-05-10", "2015-09-27",
 "2015-11-25", "2014-09-13", "2015-08-19")), 
 value = letters[1:6]
)

setkey(dt, loc.x, loc.y, time)
test.day <- as.IDate("2015-10-01")

要求的输出:

   loc.x loc.y value
1:     1     1     a
2:     1     2     f
3:     3     1     c

Answer 1

另一种选择是使用last函数:

dt[, last(value[time < test.day]), by = .(loc.x, loc.y)]

这使:

   loc.x loc.y V1
1:     1     1  a
2:     1     2  f
3:     3     1  c