how to select random rows from a matrix?

i have a matrix of size 10037 by 9.

please tell me how to select 1000 random rows from the matrix and save it as a mnew matrix in another variable.

thanks in advance.

Answer 1

[Andrei Bobrov]

m = rand(10037,9);
k = randperm(10037);
mnew = m(k(1:1000),:);

[N]

thanks, but i want to select 1000 random rows from the matrix 'f' that i already have in hand. please help me.

[Andrei Bobrov]

k = randperm(size(f,1));
mnew = f(k(1:1000),:);

so?

[Jan]

And since Matlab 2011b:

mnew = f(randperm(size(f, 1), 1000), :);

[abishek dixit]

what if i want to keep that matrix as well from which the rows were extracted. i mean the matrix with remaining 37 rows.

[KL]

Along with Andrei's code, use

 mnew_rest = f(k(1001:end),:);

[Jan]

Or with the original order:

M = rand(10037,9);
k = randperm(10037, 1000);
Selected  = M(k, :);
r         = true(1,10037);
r(k)      = false;
Remaining = M(r, :);

[Alejandro Reyes]

This works. Thank you!

Answer 2

[Richard Willey]

Statistics Toolbox includes a nice function called randsample

% Generate a matrix named foo
foo = randn(10000,2);

% draw 1000 random samples without replacement
index = randsample(1:length(foo), 1000);
bar = foo(index,:);

% draw 1000 random samples with replacement
index = randsample(1:length(foo), 1000, true);
bar = foo(index,:);

[Peter Perkins]

If you happen to be using R2011b, and have access to the Statistics Toolbox, you can also use the new datasample function:

m = rand(10037,9);

mnew = datasample(m,1000);

This also allows you to sample with replacement, or with weights.

[Ege]

And when we selected 1000 of them, how do we delete the selected rows from the original matrix which in this case foo ? And what does replacement mean in here?