R: ifelse loop across multiple dataframes

Question

I'd like to create an efficient ifelse statement such that if columns from df2 match columns from df1, then that row in df2 is coded a specific way. My code works but is very inefficient.

Example data:

Df1

A        B      C
111     2     1
111     5     2
111     7     3
112     2     4
112     8     5
113     2     6

Df2

A        B
112     2
111     2
113     2
111     5
111     7
112     8

Desired Outcome:

Df2

A       B       C
112     2     4
111     2     1
113     2     6
111     5     2
111     7     3
112     8     5

What I've done is this:
Df2$C<- ifelse(Df2$A == 111 & Df2$B == 2, 1, 0)
Df2$C<- ifelse(Df2$A == 111 & Df2$B == 5, 2, 0)
Df2$C<- ifelse(Df2$A == 111 & Df2$B == 7, 3, 0)...

This works, but is there a way such that df2 could reference the columns in df1 and create column df2$C, so that each combination doesn't have to be manually typed out?

Answer 1

This would typically be done with a join. left_join from dplyr will connect each of the rows in your first table with the each of the matching rows in the second table.

https://dplyr.tidyverse.org/reference/join.html

library(dplyr)
Df2 %>% left_join(Df1)

Joining, by = c("A", "B")
    A B C
1 112 2 4
2 111 2 1
3 113 2 6
4 111 5 2
5 111 7 3
6 112 8 5

---

merge from base R will give a similar result, but doesn't keep the original row order without some extra wrangling.

Merge two data frames while keeping the original row order

merge(Df2, Df1)
    A B C
1 111 2 1
2 111 5 2
3 111 7 3
4 112 2 4
5 112 8 5
6 113 2 6