Combine two columns in R

Question

How do I combine two columns and replace missing values in R?

Let's say I have a data.frame:

D = data.frame(a = c(1,NA,3), b = c(1,2,NA))

Looks like this

   a  b
1  1  1
2 NA  2
3  3 NA

How would I get it to look like this:

  c
1 1
2 2
3 3

Answer 1

Assuming the two columns will either have the same values or one of them will be NA

D = data.frame(a = c(1,NA,3), b = c(1,2,NA))
D[['c']] <- rowMeans(D, na.rm = TRUE)
D

   a  b c
1  1  1 1
2 NA  2 2
3  3 NA 3

Answer 2

If Column a is the master column and you want to copy values in b that are NA in a then:

D = data.frame(a = c(1,NA,3), b = c(1,2,NA))
D2=data.frame(c = D$a)
D2$c[is.na(D2$c)] <- D$b[is.na(D$a)]
D2
  c
1 1
2 2
3 3