Create a array of 1 and -1

EDITED

Hi there, I want a 4 x 1 vector of all combinations of 1 and -1.

For instance [1 1 1 1] , [1 1 1 -1] , [1 1 -1 1]

etc..

[Walter Roberson]

[1 1 1 -1] appears to be the same combination as [1 1 -1 1]. Are you sure you want combinations and not permutations?

Answer 1

[Andrei Bobrov]

use function ff2n from Statistics Toolbox

out = ff2n(4)*2-1;

OR:

out = unique(nchoosek((-1).^(1:8),4),'rows');

[Dr. Seis]

I knew there was a more efficient way to do this!

[Andrei Bobrov]

Thank you Elige.

Answer 2

[Walter Roberson]

Taken how many at a time? If you say 2 at a time, then you have the problem that there are only 3 combinations: [-1 -1], [-1 1], [1 1]. The potential combination [1 -1] is the same combination as [-1 1] because by definition the order within a combination is irrelevant.

[Dr. Seis]

4x1 array... i.e., [1;1;1;1], [1;1;1;-1], [1;1;-1;-1], ...

[Mate 2u]

Wheres the input?

Answer 3

[Dr. Seis]

Not the most efficient, but:

>> a=nchoosek([1,2,3,4,5,6,7,8],4);
>> b = ones(size(a));
>> b(mod(a,2)==0) = b(mod(a,2)==0)*-1;
>> c = unique(b,'rows')'

c =

Columns 1 through 12

    -1    -1    -1    -1    -1    -1    -1    -1     1     1     1     1
    -1    -1    -1    -1     1     1     1     1    -1    -1    -1    -1
    -1    -1     1     1    -1    -1     1     1    -1    -1     1     1
    -1     1    -1     1    -1     1    -1     1    -1     1    -1     1

Columns 13 through 16

     1     1     1     1
     1     1     1     1
    -1    -1     1     1
    -1     1    -1     1