select randomly "N" rows of a matrix?

how i can select randomly "N" rows of a matrix? for example

M =

     [ a12, a12]

     [ a21, a22]

     [ a31, a32]

     [ a41, a42]

For N=2

B =

     [ a12, a12] 
     [ a31, a32] 
     [ a21, a22]

Answer 1

[Thomas]

try:

N=2; % no. of rows needed
M=rand(5)  % generate a 5x5 matrix random
c=randperm(length(M),N)  

B=M(c,:)  % output matrix

[Walter Roberson]

Right. The above does require a relatively new version of MATLAB, though; older versions of MATLAB did not support the second argument to randperm()

[Noe alvarado]

i have the following error

c=randperm(length(M),N)

??? Error using ==> randperm

Too many input arguments.

[Jan]

@Noe, Then use: c = randperm(size(M, 1)); c = c(1:N);

[Noe alvarado]

thanks