获取data.table中前一个组的最后一行

Question

This is what my data table looks like:

library(data.table)
dt <- fread('
    Product  Group    LastProductOfPriorGroup
    A          1          NA
    B          1          NA
    C          2          B
    D          2          B
    E          2          B
    F          3          E
    G          3          E
')

The LastProductOfPriorGroup column is my desired column. I am trying to fetch the product from last row of the prior group. So in the first two rows, there are no prior groups and therefore it is NA . In the third row, the product in the last row of the prior group 1 is B . I am trying to accomplish this by

dt[,LastGroupProduct:= shift(Product,1), by=shift(Group,1)]

to no avail.

Answer 1

You could do

dt[, newcol := shift(dt[, last(Product), by = Group]$V1)[.GRP], by = Group]

This results in the following updated dt , where newcol matches your desired column with the unnecessarily long name. ;)

   Product Group LastProductOfPriorGroup newcol
1:       A     1                      NA     NA
2:       B     1                      NA     NA
3:       C     2                       B      B
4:       D     2                       B      B
5:       E     2                       B      B
6:       F     3                       E      E
7:       G     3                       E      E

Let's break the code down from the inside out. I will use ... to denote the accumulated code:

• dt[, last(Product), by = Group]$V1 is getting the last values from each group as a character vector.
• shift(...) shifts the character vector in the previous call
• dt[, newcol := ...[.GRP], by = Group] groups by Group and uses the internal .GRP values for indexing

Update: Frank brings up a good point about my code above calculating the shift for every group over and over again. To avoid that, we can use either

shifted <- shift(dt[, last(Product), Group]$V1)
dt[, newcol := shifted[.GRP], by = Group]

so that we don't calculate the shift for every group. Or, we can take Frank's nice suggestion in the comments and do the following.

dt[dt[, last(Product), by = Group][, v := shift(V1)], on="Group", newcol := i.v] 

Answer 2

Another way is to save the last group's value in a variable.

this = NA_character_    # initialize
dt[, LastProductOfPriorGroup:={ last<-this; this<-last(Product); last }, by=Group]
dt
   Product Group LastProductOfPriorGroup
1:       A     1                      NA
2:       B     1                      NA
3:       C     2                       B
4:       D     2                       B
5:       E     2                       B
6:       F     3                       E
7:       G     3                       E

NB: last() is a data.table function which returns the last item of a vector (of the Product column in this case).

This should also be fast since no logic is being invoked to fetch the last group's value; it just relies on the groups running in order (which they do).