卷起data.table

Question

我有一个数据表:

> (mydt <- data.table(id=c(1,1,1,1,2,2),
                      time=1:6,
                      v1=letters[1:6],
                      v2=LETTERS[1:6],
                      key=c("id","time")))
   id time v1 v2
1:  1    1  a  A
2:  1    2  b  B
3:  1    3  c  C
4:  1    4  d  D
5:  2    5  e  E
6:  2    6  f  F

我想"把它卷起"(这是正确的术语吗？),到"更改"表:对象1改变了3次(从时间戳1到2,2到3和3到4)对象2改变一次(时间) 5至6); 我对初始 v1和最终 感兴趣v2.所以,结果应该是:

> (res <- data.table(beg.time=c(1,2,3,5),
                     end.time=c(2,3,4,6),
                     v1=c('a','b','c','e'),
                     v2=c('B','C','D','F'),
                     key=c("beg.time","end.time")))
   beg.time end.time v1 v2
1:        1        2  a  B
2:        2        3  b  C
3:        3        4  c  D
4:        5        6  e  F

Answer 1

感谢可重复的例子!这是一个镜头.

首先,请注意您可以使用以下头尾惯用法将相隔一定距离的矢量条目放在一起:

x <- letters[1:5]
cbind(head(x, -1), tail(x, -1))
     # [,1] [,2]
# [1,] "a"  "b" 
# [2,] "b"  "c" 
# [3,] "c"  "d" 
# [4,] "d"  "e" 
cbind(head(x, -2), tail(x, -2))
     # [,1] [,2]
# [1,] "a"  "c" 
# [2,] "b"  "d" 
# [3,] "c"  "e" 

然后,我们可以使用按组执行此操作的by功能data.table.

mydt[,{
    ## if there's just one row in the group of ID's, return nothing
    if (.N == 1) return(NULL) 
    else {
        list(
            ## head and tail take the first and last parts of a vector
            ## this will place an element next to its subsequent element
            beg.time = head(time, -1),
            end.time = tail(time, -1),
            v1 = head(v1, -1),
            v2 = tail(v2, -1)
## group by ID
)}}, by = id]

#    id beg.time end.time v1 v2
# 1:  1        1        2  a  B
# 2:  1        2        3  b  C
# 3:  1        3        4  c  D
# 4:  2        5        6  e  F