filling in columns with matching IDs from two dataframes in R

Question

I have two dataframes (df1, df2). I want to fill in the AGE and SEX values from df1 to df2 conditioned on having the same ID between the two. I tried several ways using for-loop

Answer 1

You could use match with lapply for this. If we iterate [[ with matching on the ID column of each of the original data sets over a vector of names, we can get the desired result.

nm <- c("AGE", "SEX")
df2[nm] <- lapply(nm, function(x) df1[[x]][match(df2$ID, df1$ID)])
df2
#      ID AGE SEX  Conc
# 1 90901  39   0  5.00
# 2 90901  39   0 10.00
# 3 90901  39   0 15.00
# 4 90903  40   1 30.00
# 5 90903  40   1  5.00
# 6 90902  28   0  2.45
# 7 90902  28   0 51.00
# 8 90902  28   0  1.00
# 9 70905  NA  NA  0.50

Note that this is also quite a bit faster than merge.