Average across Columns in R, excluding NAs

Question

I can't imagine I'm the first person with this question, but I haven't found a solution yet (here or elsewhere).

I have a few columns, which I want to average in R. The only minimally tricky aspect is that some columns contain NAs.

For example:

Trait Col1 Col2 Col3
DF    23   NA   23
DG    2    2    2
DH    NA   9    9

I want to create a Col4 that averages the entries in the first 3 columns, ignoring the NAs. So:

 Trait Col1 Col2 Col3 Col4
 DF    23   NA   23   23
 DG    2    2    2    2
 DH    NA   9    9    9 

Ideally something like this would work:

data$Col4 <- mean(data$Chr1, data$Chr2, data$Chr3, na.rm=TRUE)

but it doesn't.

Answer 1

You want rowMeans() but importantly note it has a na.rm argument that you want to set to TRUE. E.g.:

> mat <- matrix(c(23,2,NA,NA,2,9,23,2,9), ncol = 3)
> mat
     [,1] [,2] [,3]
[1,]   23   NA   23
[2,]    2    2    2
[3,]   NA    9    9
> rowMeans(mat)
[1] NA  2 NA
> rowMeans(mat, na.rm = TRUE)
[1] 23  2  9

To match your example:

> dat <- data.frame(Trait = c("DF","DG","DH"), mat)
> names(dat) <- c("Trait", paste0("Col", 1:3))
> dat
  Trait Col1 Col2 Col3
1    DF   23   NA   23
2    DG    2    2    2
3    DH   NA    9    9
> dat <- transform(dat, Col4 = rowMeans(dat[,-1], na.rm = TRUE))
> dat
  Trait Col1 Col2 Col3 Col4
1    DF   23   NA   23   23
2    DG    2    2    2    2
3    DH   NA    9    9    9