Question

I would like to randomly re-order the rows of matrix A to generate another new matrix. How to do that in R?

Answer 1

Use sample() to generate row-indices in a (pseudo-)random order and reorder the matrix using [.

## create a matrix A for illustration
A <- matrix(1:25, ncol = 5)

Giving

> A
     [,1] [,2] [,3] [,4] [,5]
[1,]    1    6   11   16   21
[2,]    2    7   12   17   22
[3,]    3    8   13   18   23
[4,]    4    9   14   19   24
[5,]    5   10   15   20   25

Next, generate a random order for the rows

## generate a random ordering
set.seed(1) ## make reproducible here, but not if generating many random samples
rand <- sample(nrow(A))
rand

This gives gives

> rand
[1] 2 5 4 3 1

Now use that to reorder A

> A
     [,1] [,2] [,3] [,4] [,5]
[1,]    1    6   11   16   21
[2,]    2    7   12   17   22
[3,]    3    8   13   18   23
[4,]    4    9   14   19   24
[5,]    5   10   15   20   25
> A[rand, ]
     [,1] [,2] [,3] [,4] [,5]
[1,]    2    7   12   17   22
[2,]    5   10   15   20   25
[3,]    4    9   14   19   24
[4,]    3    8   13   18   23
[5,]    1    6   11   16   21