Problem with creating a random permutation with seed

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Alexandra
Alexandra 2012-2-19
评论： Steven Lord 2019-9-25
Hello,
I need to create a random permutation of numbers. I also need to be able to create the same random permutation several times (for tests).
I have read in the documentation that this can be achieved either with "rng(Seed)" or by setting RandStream. I tried both in the matlab command line and they both don't work for me.
I get an eror message "??? Undefined function or method 'rng' for input arguments of type 'double'." for the example "rng(54321)".
And I get an error message "??? The class RandStream has no property or method named 'setGlobalStream'." while trying out the example "s = RandStream('mt19937ar','Seed',0) RandStream.setGlobalStream(s);". When I simply try "rand(s,10,1)" after defining s I get different results each time. The matlab version I use is R2010a.
Can you give me hint how to solve this problem, please?
Alexandra
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MOLLAH KHOKON ALI
MOLLAH KHOKON ALI 2017-10-7
编辑：MOLLAH KHOKON ALI 2017-10-7
Hello,
I am also facing this problem and I am using Matlab R2010a version so What kind of work can I do for this solution.
Please Give me information very quickly.
Mollah Khokon Ali

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回答（3 个）

Image Analyst
Image Analyst 2012-2-19
rand() does not give random permutations - it gives random numbers. If, as you say, you need random permutations, you need to use randperm().
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Fadime Bekmambetova
Fadime Bekmambetova 2019-9-25
Image Analyst, same here - works for me.
However, if I run rng(10) followed by multiple randperm's then I get different results every time.
Steven Lord
Steven Lord 2019-9-25
That's expected.
rng(10); % "Shuffle" a "deck of cards"
theHands = zeros(4, 5);
theHands(1, :) = randperm(52, 5);
theHands(2, :) = randperm(52, 5);
theHands(3, :) = randperm(52, 5);
theHands(4, :) = randperm(52, 5)
Each of those hands is different. [Though with this approach the same card can come up in multiple hands (and does.) For this application you'd probably want to draw 20 cards at once with randperm(52, 4*5) and reshape it afterwards into four 5-card hands to avoid duplicating cards.]
If you "shuffle the deck" once and "remember" exactly how you shuffled it, you can generate the same hands by shuffling it the same way again. For most people, this would be difficult; for stage magicians and for MATLAB, it's easier.
rng(10); % Shuffle
hand1 = randperm(52, 5)
rng(10); % Stack the deck back the same way as before drawing hand1
hand2 = randperm(52, 5)
Both those hands are the same.
You may be tempted to reseed the generator every time you need random numbers. Resist that temptation; see the Note on this documentation page for some explanation.

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Image Analyst
Image Analyst 2012-2-19
for k = 1:10
% Seed with the same seed every time.
rng(10);
% Generate the same permutation every time.
randperm(15)
end
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Alexandra
Alexandra 2012-2-19
I doesn't work for me unfortunately, because apparently my version does not allow the use of "rng".

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Walter Roberson
Walter Roberson 2012-2-19
I see no evidence in what you have written that you are resetting the seed each time. Setting the default stream does NOT reset the seed.
http://www.mathworks.com/matlabcentral/answers/29369-exponential-random-draws-and-the-seed
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Alexandra
Alexandra 2012-2-19
I didn't realize I had to set default sream every time before using it. Thanks, now it works the way I wanted!

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