Conditional merge/replacement in R

by

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]
}

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