如何在test.day之前找到每个(loc.x,loc.y)对的最后一个值？
@H_502_2@dt <- data.table(
loc.x = as.integer(c(1,1,3,1)),loc.y = as.integer(c(1,2,2)),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")
所需输出：
@H_502_2@loc.x loc.y value 1: 1 1 a 2: 1 2 f 3: 3 1 c