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I have the following dataframes

df1:

 round mun1 mun2
 1 SP PA
 1 RJ PR
 1 BH BA
 2 BA SP
 2 PR BH
 2 PA RJ
 3 RJ BH
 3 PA PR
 3 SP BA

df2:

mun p01 p02 p03
 SP 3 4 7
 RJ 0 3 4
 BH 3 6 9
 BA 0 1 1
 PA 1 2 3 
 PR 1 4 5

I need a collumn in df1, P, that equals 0 if round==1, maxvalue of p01 if round ==2 and the maxvalue of p02 if round==3.
In the real data, in df1 i have 38 rounds, and 380 lines, and in df2 i have 20 lins(each for unique mun). I tried the following loop:

p <-matrix(0, nrow=380,ncol=1)

for(i in 2:38) 
 p <- if(round==i) max(p[[i-1]] %in% df2)

But this dont work. is there a way to do that?

Using dplyr. It will work if you can merge df1 and df2 based on mun1 or mun2 and mun.

df %>% 
 left_join(df2, by = c("mun1" = "mun")) %>% #Merging the data
 mutate(P = ifelse(round == 1, 0, #Applying the condition
 ifelse(round == 2, max(p01),
 ifelse(round == 3, max(p02), NA))))

 round mun1 mun2 p01 p02 p03 P
1 1 SP PA 3 4 7 0
2 1 RJ PR 0 3 4 0
3 1 BH BA 3 6 9 0
4 2 BA SP 0 1 1 3
5 2 PR BH 1 4 5 3
6 2 PA RJ 1 2 3 3
7 3 RJ BH 0 3 4 6
8 3 PA PR 1 2 3 6
9 3 SP BA 3 4 7 6