Merge and replace values ​​in two data.tables

Question

I am new to data.table package and have a simple problem. I have two data.tables that are compared using keys. In data table 1, the value of column C changes from "NO" to "OK" if the key columns A and B are equally found in data.table B. This step is inevitable and must be performed.

library(data.table)
df_1 <- data.frame(A=c(1,1,3,5,6,7), B = c("x","y","z","q","w","e"), C = rep("NO",6))
df_2 <- data.frame(A=c(3,5,1), B = c("z","q","x"), D=c(3,5,99))
keys <- c("A","B")
dt_1 <- data.table(df_1, key = keys)
dt_2 <- data.table(df_2, key = keys)
dt_1[dt_2, C := "OK"]

Now I get data.table:

   A     B     C
1: 1     x     OK
2: 1     y     NO
3: 3     z     OK
4: 5     q     OK
5: 6     w     NO
6: 7     e     NO

I would like to include a second operation. If in data.table 2 the value of column A is not equal to column D, the value of column D should be used after the first operation. The value of column D exceeds A. This should work no matter how many values ​​in D differ. The desired data table is as follows:

   A     B     C
1: 99    x     OK
2: 1     y     NO
3: 3     z     OK
4: 5     q     OK
5: 6     w     NO
6: 7     e     NO

I'm tired of something without success.

dt_1[dt_2, A != D, A := D]

Thank you for your help!

Answer 1

Try:

dt_1[C == "OK", A:= dt_2[,D]]

#   A B  C
# 1: 99 x OK
# 2:  1 y NO
# 3:  3 z OK
# 4:  5 q OK
# 5:  6 w NO
# 6:  7 e NO

---

, .

data.table ( setDT)

dt_1 <- data.table(A=c(1,1,3,5,6,7), B = c("x","y","z","q","w","e"), C = rep("NO",6))
dt_2 <- data.table(A=c(3,5,1), B = c("z","q","x"), D=c(3,5,99))

setkeyv <-

keys <- c("A","B")
setkeyv(dt_1, keys)
setkeyv(dt_2, keys)

dt_1[dt_2, `:=`(C = "OK", A = i.D)]
#     A B  C
# 1: 99 x OK
# 2:  1 y NO
# 3:  3 z OK
# 4:  5 q OK
# 5:  6 w NO
# 6:  7 e NO

df_1$A != df_2$D