R: sample random element from each row of a matrix

Can anybody help me to optimize my way of sampling random values for each row of a matrix?

Thus far I have two ways of doing it:

volume <- matrix(rnorm(10000), 100, 100)
microbenchmark(
  
  test <- apply(volume, 1, function(x) {sample(x, 1)}),
  # make matrix into a vector, sample one time for all rows at the same time,
  # and then add 0, 100, 200 ...  to each sample
  test2 <- c(t(volume))[sample(x=1:ncol(volume), size=nrow(volume), replace=T) + seq(from=0, by=ncol(volume), length.out=nrow(volume))] 
)

Solution

You can index a matrix with a 2 column matrix so that the first column is the selector of the row and the second one is the selector for column.

For example:

index <- matrix(c(1, 4, 1, 3), ncol = 2)
index
     [,1] [,2]
[1,]    1    1
[2,]    4    3
matrix(c(1:15), ncol = 3)
     [,1] [,2] [,3]
[1,]    1    6   11
[2,]    2    7   12
[3,]    3    8   13
[4,]    4    9   14
[5,]    5   10   15
matrix(c(1:15), ncol = 3)[index]
[1]  1 14

Applying the same principle we can create a matrix of indexes with first column for each row (1:100) and second column with a random integer from 1 to 100 to select a random column.

test3 <- volume[matrix(c(1:100, sample.int(100, 100, replace = TRUE)), nrow = 100)]

It seems to be a little faster, probably because of vectorisation in R but I'm not strong enough to tell. I hope this helped you or gave you others ideas!