Conditional merge/replacement in R

Question

I have two data frames:

df1
x1  x2
1   a
2   b
3   c
4   d

and

df2
x1  x2
2   zz
3   qq

I want to replace some of the values in df1$x2 with values in df2$x2 based on the conditional match between df1$x1 and df2$x2 to produce:

df1
x1  x2
1   a
2   zz
3   qq
4   d

Answer 1

use match(), assuming values in df1 are unique.

df1 <- data.frame(x1=1:4,x2=letters[1:4],stringsAsFactors=FALSE)
df2 <- data.frame(x1=2:3,x2=c("zz","qq"),stringsAsFactors=FALSE)

df1$x2[match(df2$x1,df1$x1)] <- df2$x2
> df1
  x1 x2
1  1  a
2  2 zz
3  3 qq
4  4  d

If the values aren't unique, use :

for(id in 1:nrow(df2)){
  df1$x2[df1$x1 %in% df2$x1[id]] <- df2$x2[id]
}