all possible permutations

Hi Guys, any chance someone know how to get all possible permutations from a set of numbers?

e.g. [1 -1 2 -2 3 -3 0] but I would need to use only 4 numbers at the time, so there should be 840 possible permutations,... problem is that "perms" takes all 7 numbers into account at the time,...

is there a way matlab can do this?

many thanks, ondrej

[Jan]

Are you talking about permutations or combinations?

Answer 1

[Dr. Seis]

Or...

a = perms([1, -1, 2, -2, 3, -3, 0]);
b = a(:,1:4);
c = unique(b,'rows');

[Fangjun Jiang]

+1! That should work and it's easier!

[Jonathan]

This definitely works. However, it uses a lot more time and space than it needs to. It uses 5040 rows for a and b and 7 columns for a.

Answer 2

[Jan]

If you search in the FEX, you will find this nice tool: FEX: combinator.

Answer 3

[Ondrej]

talking about permutations not combinations,...

nchoosek([1, -1, 2, -2, 3, -3, 0],4) will give you combinations mate, but thanks anyway,...

how no idae what is FEX, but I guess I need to give it a go,... many thanks,...

Answer 4

[Fangjun Jiang]

Would this give you the correct result?

a=[1 -1 2 -2 3 -3 0];
b=nchoosek(a,4);
M=size(b,1);
c=cell(M,1);
for k=1:M
    Temp=b(k,:);
    c{k}=Temp(perms(1:4));
end

d=cell2mat(c);

Answer 5

[Ondrej]

thank you very much! very nice! you done thinking I should have done,... thanks a lot,... I kind of hoped that matlab has a built-in function which I don't know about,... thanks a lot again!

Answer 6

[Ondrej]

thank you mate! really nice! apparently I'm not that great with matlab,... cheers,..

Answer 7

[Jonathan]

You can use this method. It does not use cell arrays, which are conceptually convenient but rather inefficient.

A = nchoosek([1 -1 2 -2 3 -3 0], 4);
P = perms(1:4);
B = zeros(size(A,1)*size(P,1),size(A,2));
for i = 1:size(P,1)
    B(1 + size(A,1)*(i-1):size(A,1)*i,:) = A(:,P(i,:));
end