sample "n" random positions from a matrix

Hi there,

 I have a binary matrix (dim 100x100) filled with values 0 and 1. I need
 select a record "n" positions of that matrix when values are 1. How can
 I do that?

Is this what you want?

mat.100 <- diag(100) # just for instance
n <- 7  # supposing you want 7
which( mat.100==1, arr.ind=T )[ sample( 100, n ), ]

row col
[1,]  99  99
[2,]  93  93
[3,]  36  36
[4,]  74  74
[5,]  11  11
[6,]   3   3
[7,]  21  21

[...]

Charles C. Berry                        (858) 534-2098
                                          Dept of Family/Preventive Medicine
E mailto:cberry at tajo.ucsd.edu	         UC San Diego
http://biostat.ucsd.edu/~cberry/         La Jolla, San Diego 92093-0717