matrix with different randperm rows

Hi,

Say I have vector of N integers, 1 through N. Is there a quick way, without loops, to create a matrix where each row is a different randperm of the vector? My N can get large (up to 100).

Thanks

Answer 1

[Andrei Bobrov]

[~, out] = sort(rand(M,N),2);

[Jeremy]

Thanks. Now let me see if it speeds things up.

[Mikhail]

Apparently, not-))

[Jeremy]

Works orders of magnitude faster for large group sizes :)

Answer 2

[Guillaume]

cell2mat(arrayfun(@(dummy) randperm(n), 1:m, 'UniformOutput', false)');

Whether or not that can be said to be without a loop is debatable as arrayfun is effectively looping over the array 1:m

Answer 3

[Mikhail]

Yes, try http://www.mathworks.com/help/matlab/ref/perms.html

[Jeremy]

Thanks for the quick response, but my N can get large (up to 100) and rows with the same permutation are OK.

[Mikhail]

You originally wrote different permutation. Use randperm function for your programm. I don't know how do it without loop.

[Image Analyst]

In what universe, or decade, is 100 considered large? You actually mean 100, right, like ten times ten, not 100 million or 100 billion or something?

[Jeremy]

100 is large when you talk about some types of group coordination. I need to select k individuals from a group of N to interact at different time steps. What I've been doing is generating all combinations and randomly selecting ones to fill an interaction matrix, but that combination matrix gets big fast.

Answer 4

[Joseph Cheng]

Well there is a randperm() function that will give you a random permuation. How to get it as a MxN matrix without a loop will take some more time to think about it.

[Joseph Cheng]

ok well it is possible to do it without a loop in 2ish lines. Use the function arrayfun() or cellfun() on an array of N*ones(M,1).