Extract the first TRUE per row in logical matrix efficiently in R [duplicate]

Answer 1

We can use max.col

colnames(m)[max.col(m, "first")]
#[1] "A" "B" "C" "B" "B" "A"

If there are no TRUE in a row, then we can change it to NA (if needed)

colnames(m)[max.col(m, "first")*NA^!rowSums(m)]

Or with ifelse

colnames(m)[ifelse(rowSums(m)==0, NA, max.col(m, "first"))]