How do I select a sample of rows at random with re

Question

How do I select a sample of rows at random with repetition from a matrix in R?

So do be clear, I would start with a matrix of, for example, 100 rows and I would be able to select 5 of those rows and make a new matrix. I would want the option of doing this either with or without replacement.

Answer 1

This seems to work better with data frames:

sample_df<-x[sample.int(nrow(x),size=100,replace=TRUE),]

Answer 2

use the sample function:

x <- matrix(1:1000, nrow=100)

With replacement:

x[sample(1:100, 5, replace=TRUE), ]
     [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
[1,]   19  119  219  319  419  519  619  719  819   919
[2,]   51  151  251  351  451  551  651  751  851   951
[3,]   42  142  242  342  442  542  642  742  842   942
[4,]   48  148  248  348  448  548  648  748  848   948
[5,]   73  173  273  373  473  573  673  773  873   973

Wihtout replacement:

x[sample(1:100, 5, replace=FALSE), ]
     [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
[1,]   64  164  264  364  464  564  664  764  864   964
[2,]   67  167  267  367  467  567  667  767  867   967
[3,]   20  120  220  320  420  520  620  720  820   920
[4,]   17  117  217  317  417  517  617  717  817   917
[5,]    6  106  206  306  406  506  606  706  806   906

Answer 3

Use sample on the rows with replace=TRUE or replace=FALSE.

If X is your original matrix then

X[sample(nrow(X),size=5,replace=TRUE),]

or

X[sample(nrow(X),size=5,replace=FALSE),]

should work. (It may be more readable if you choose the sample first: s <- sample(...) and then subset: newmat <- X[s,])