create new database from two dataframes, with different rows

Question

I have two dataframes, df1 of 19600 rows and df2 of 18830 rows, df2$id finds correspondence in the df1$id, while the remaining 770 have no correspondence, I would like to merge the two dataframes creating df3 with column df3$id equal to df1$id , if the row matches df2 it gives me values ​​df2$value, otherwise it gives me 0, so in the 770 unmatched places I will have 0 as value

Answer 1

Using data.table():

library(data.table)

DT1 =  data.table(ID = c(1:15),
                  Value = c(3:10))

DT2 = data.table(ID = c(1:10),
                 Value = c(7:13))

Join the tables and then recode the variable:

DT3 = DT2[DT1, on = .(ID)][, .(ID,Value)]

DT3[!ID %in% DT2$ID, Value := 0] 

tail(DT3)

Output

 ID Value
1: 10     9
2: 11     0
3: 12     0
4: 13     0
5: 14     0
6: 15     0