Row-wise variance of a matrix in R

Question

I\'d like to compute the variance for each row in a matrix. For the following matrix A

     [,1] [,2] [,3]
[1,]    1    5    9
[2,]    5    6            


        

Answer 1

This is one of the main reasons why apply() is useful. It is meant to operate on the margins of an array or matrix.

set.seed(100)
m <- matrix(sample(1e5L), 1e4L)
library(microbenchmark)
microbenchmark(apply(m, 1, var))
# Unit: milliseconds
#              expr      min       lq   median       uq      max neval
#  apply(m, 1, var) 270.3746 283.9009 292.2933 298.1297 343.9531   100 

Is 300 milliseconds too long to make 10,000 calculations?