Question about using randperm in a loop

Hi,

I'm using randperm in a loop that's meant to generate unique random three-digit numbers to fill a 5x5 matrix, r, like this:

for i = 1:length(r)
r(1:5,i) = (randperm(900,5) + 99);
end 

My question is:

Is there a possibility of repeating a number given the function is being called 5 times, once for each column? I want each number to be unique.

Thanks,

Veena

Answer 1

[John D'Errico]

So, you want to sample 5 sets of 5 numbers, all from the same set, but you want them to be unique? (The technical description would be to sample without replacement.)

The point is, you already know how to sample 5 such numbers. So just sample 25 numbers!

r = reshape(randperm(900,25) + 99,[5 5]);

There is no reason to need a loop.

[Veena Chatti]

Hi John,

Thanks, reshape worked!

Best,

Veena

[Veena Chatti]

And yes, I got rid of the loop!

Answer 2

[James Tursa]

If you want each number in the matrix to be unique, then don't call randperm( ) by row because that will not guarantee uniqueness between calls. Call randperm( ) only once for the entire matrix (25 elements) and then reshape to a 5x5. E.g., something like

r = reshape(randperm(900,25) + 99,5,5);

[Veena Chatti]

Hi James,

Thanks, reshape worked!

Best,

Veena