Using ifelse() to replace NAs in one data frame by referencing another data frame of different length

Question

I already reviewed the following two posts and think they might answer my question, although I\'m struggling to see how:

1) Conditional replacement of value

Answer 1

You may also use:

df1$B[is.na(df1$B)] <- df2$B[match(df1$A[is.na(df1$B)],df2$A)]
df1

#             B          C  A
# 1   1.7169811 2012-10-01  0
# 2   0.3396226 2012-10-01  5
# 3   4.0000000 2012-10-01 10
# 4   0.1509434 2012-10-01 15
# 5   0.0754717 2012-10-01 20
# 6  20.0000000 2012-10-01 25
# 7   1.7169811 2012-10-01  0
# 8   0.3396226 2012-10-01  5
# 9   5.0000000 2012-10-01 10
# 10  5.0000000 2012-10-01 15