Find the maximum and minimum value of every column and then find the maximum and minimum value of every row

Question

I\'ve got this matrix:

a <- matrix(rnorm(1000 * 18, mean = 100, sd = sqrt(10)), 1000, 18)

I would like to find the maximum and minimum v

Answer 1

A faster alternative for row max/min would be using pmax() and pmin() even though you would first have to convert the matrix to a list (data.frame is a special case of a list):

apply(a,1,min)
apply(a,1,max)
# becomes
do.call(pmin, as.data.frame(a))
do.call(pmax, as.data.frame(a))

For the columns it will be less "competitive" because of having to transpose first:

apply(a,2,min)
apply(a,2,max)
# becomes
do.call(pmin, as.data.frame(t(a)))
do.call(pmin, as.data.frame(t(a)))

Benchmarking:

a <- matrix(rnorm(1000 * 18 *10, mean = 100, sd = sqrt(10)), 1000 * 10, 18 * 10)

microbenchmark::microbenchmark(
  do.call(pmin, as.data.frame(a)),
  apply(a,1,min),
  unit = "relative"
)
                            expr      min     lq     mean   median       uq       max neval
 do.call(pmin, as.data.frame(a)) 1.000000 1.0000 1.000000 1.000000 1.000000 1.0000000   100
                apply(a, 1, min) 2.281095 2.3576 2.096402 2.531092 2.618693 0.6284233   100