max.col with NA removal

Question

I\'m looking to find the columns of matrix row-maxima while ignoring NAs. E.g.,

set.seed(1)
a <- matrix(runif(15), ncol=3)
a[a<.3] <- NA
a[5,] <-         


        

Answer 1

We replace the 'NA' with -Inf in 'a' and apply the max.col on that.

v1 <- max.col(replace(a, is.na(a), -Inf), ties.method="first")

But, this will return 1 for the last row which have all NAs. To return NA, we can multiply it with the NA converted negated (!) rowSums of logical matrix (!is.na(a)).

v1 * NA^!rowSums(!is.na(a))
#[1]  2  2  3  1 NA

EDIT: Changed the replacement from 0 to -Inf based on @Frank's comment

---

As the OP was using apply, which.max can return the column index

apply(a, 1, function(x) which.max(x)[1])
#[1]  2  2  3  1 NA

Or

sapply(apply(a, 1, which.max), `length<-`, 1)
#[1]  2  2  3  1 NA