Randperm is not returning all numbers and is duplicating some

Essentially, I need to produce a list of numbers. The first 24 are the numbers 1 through 24, in a random order. The next 24 (numbers 25-48 of the big list) are also the numbers 1 through 24, in a different random order. This gets repeated 15 times.

I have set up code that initially seemed to work using a for loop and randperm. I repeatedly ask randperm to randomize the numbers 1 through 24 and I add those into my list. However, when examining the list more closely, the first few sets of 24 are fine, but then randperm breaks down a bit. In the third iteration of randperm, I get the number 21 repeated twice and I've lost the number 3. The next time through I have two 6's, but no 22.

Any guesses about why this might be happening? Are there constraints I can enter for randperm to ensure this doesn't happen?

Thanks!

Answer 1

[Wayne King]

I don't see it repeating any elements.

   Y = zeros(24,15);
   for nn = 1:15

      tmp = randperm(24);
      Y(:,nn) = tmp;
      len(nn) = length(unique(Y(:,nn)));
   end

Look at the columns of Y. And look at len, that vector is all 24s.

Is it possible that you are making a mistake in the indexing?

[Suzanne]

I ran your code and it looks fine. It must be somewhere else in the code, then - and I thought I checked everything or that randperm had some specifications I wasn't aware of. Back to the drawing board!

Thanks, Wayne!

[Suzanne]

Drat! Just to finish this up, yes, it was an indexing problem. Good suggestion. Problem solved!