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

Answer 2

Try the following code which takes your original statement and makes a small tweak in the TRUE argument of the ifelse function:

> df1$B <- ifelse(is.na(df1$B) == TRUE, df2$B[df2$A %in% df1$A], df1$B)   
#                         Switched '==' to '%in%' ---^
> 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

Answer 3

df1[is.na(df1$B), c('A','C','B')] <- merge(df1[is.na(df1$B), -1], df2, by = 'A')