CODEWITHSUNDEEP
rjoindata.tableupdates
Sven
Sven

Reputation: 465

Update value in a table with value from other table and based on condition in other table in data.table way

I have two data.table 

library(data.table)

DT1 <- data.table(
                  Part = c("A", "B", "C"),
                  Code = c(1, 3, 1),
                  PartTo = c(NA, "D", NA)
                  )

DT2 <- data.table(
                  Part = c("A", "B", "C"),
                  Info = c(1, 3, 1)
                  )

and want to update a value in the first table with a value coming from the second table based on a condition from the second table.

I have tried the following, but it updates not to "D", but to NA:

DT2[DT1$Code == 3, Part := DT1$PartTo]

with the warning message:

Warning message:
In `[.data.table`(DT2, DT1$Code == 3, `:=`(Part, DT1$PartTo)) :
Supplied 3 items to be assigned to 1 items of column 'Part' (2 unused)

Result:

    Part Info
1:    A    1
2:   NA    3
3:    C    1

Thanks for any help!

Upvotes: 2

Views: 91

Answers (1)

akrun
akrun

Reputation: 887691

We can subset the first dataset where 'Code' is 3, do a join on the 'Part' with 'DT2', and assign (:=) the 'PartTo' from DT1 to 'Part'

DT2[DT1[Code==3], Part := PartTo, on = .(Part)]
DT2
#   Part Info
#1:    A    1
#2:    D    3
#3:    C    1

Upvotes: 2

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