更新时间:2021-12-28 21:07:09
这里有几个使用 data.table
的解决方案:
Here are couple of solutions using data.table
:
## Option 1 (cleaner solution, added 2016-11-29)
uDT <- unique(DT)
DT[, c("first","last"):=0L]
DT[uDT, first:=1L, mult="first"]
DT[uDT, last:=1L, mult="last"]
## Option 2 (original answer, retained for posterity)
DT <- cbind(DT, first=0L, last=0L)
DT[DT[unique(DT),,mult="first", which=TRUE], first:=1L]
DT[DT[unique(DT),,mult="last", which=TRUE], last:=1L]
head(DT)
# x y first last
# [1,] a A 1 1
# [2,] a B 1 1
# [3,] a C 1 0
# [4,] a C 0 1
# [5,] b A 1 1
# [6,] b B 1 1
显然,每一行都包含很多内容.但是,关键构造如下,它返回每个组中第一条记录的行索引:
There's obviously a lot packed into each of those lines. The key construct, though, is the following, which returns the row index of the first record in each group:
DT[unique(DT),,mult="first", which=TRUE]
# [1] 1 2 3 5 6 7 11 13 15