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

Answer 1

Here is another way that has a better performance w.r.t. @Cath solutions:

a <- which(m, arr.ind = T)
colnames(m)[aggregate(col~row,a[order(a[,1]),],min)$col]

# [1] "A" "B" "C" "B" "B" "A"

Benchmarking given the matrix used by @Cath:

m0h3n <- function(m){
   a <- which(m, arr.ind = T)
   colnames(m)[aggregate(col~row,a[order(a[,1]),],min)$col]
}

all.equal(akrun(n), cath(n), joe(n), m0h3n(n))
# [1] TRUE

microbenchmark(akrun(n), cath(n), joe(n), m0h3n(n))

# Unit: microseconds
     # expr      min       lq      mean    median        uq       max neval
 # akrun(n) 2291.981 2395.793 2871.7156 2482.7790 3561.9150  4205.370   100
  # cath(n) 8263.210 8554.665 9695.9375 8782.8710 9947.9415 58239.983   100
   # joe(n)  274.029  298.517  526.6722  312.0375  342.5355  2366.798   100
 # m0h3n(n) 3890.178 3974.309 4280.6677 4073.1635 4227.7550  6337.501   100

Therefore, here is the ranked solutions (in terms of efficiency):

1. joe
2. akrun
3. m0h3n
4. Cath