random permutation with no matches to the position

Hello everyone,

I have a question. When using randperm function I would like to get only those permutations where no i number appears on the position i in the vector. Does anybody have a suggestion on how to do so?

Example: 1 2 3 4 5

acceptable permutation: 3 1 5 2 4

non-acceptable permutation: 1 3 5 2 4

Thanks a lot in advance!

[Stephen23]

Brute force derangement:

N = 9;
out = randperm(N);
while any(out==(1:N))
    out = randperm(N);
end

[Valeria]

great, thanks!

Answer 1

[Torsten]

An answer is provided under

http://stackoverflow.com/questions/28051975/generating-random-derangements-in-matlab-permutations-without-any-fixed-Point

Best wishes

Torsten.

Answer 2

[Jos (10584)]

This specific type of permutation is called a derangement, and there are many methods to generate one. However, it is no so easy to come up with an elegant and fast algorithm.

Take a look at this old thread: http://www.mathworks.com/matlabcentral/newsreader/view_thread/302075#820677

There are several implementations on the file exchange. I will promote my own ;-) http://www.mathworks.com/matlabcentral/fileexchange/30189-randpermfull

[Valeria]

thank you Jos!