Question about using randperm in a loop

Question

Veena Chatti 2020-3-9

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评论： Veena Chatti 2020-3-10

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

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Answer 1

John D'Errico 2020-3-10

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编辑：John D'Errico 2020-3-10

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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.

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Veena Chatti 2020-3-10

Hi John,

Thanks, reshape worked!

Best,

Veena

Veena Chatti 2020-3-10

And yes, I got rid of the loop!

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Answer 2

James Tursa 2020-3-10

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编辑：James Tursa 2020-3-10

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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);