Re: Indexing confusion

[Bill Denney]

John W. Eaton wrote:
> On  1-May-2008, Michael Grossbach wrote:
>
> | Assume a matrix
> | X = [1, 2; 3, 4];
> | To produce, say, the second element of the first row one would issue
> | X(1,2)
> | ans = 2
> | To obtain all combinations of elements in X I then use
> | fullfact(size(X))
> | ans =
> |     1   1
> |     1   2
> |     2   1
> |     2   2
> | 
> | To my confusion,
> | X(fullfact(size(X)))
> | 
> | produces
> | ans =
> |     1   1
> |     1   3
> |     3   1
> |     3   3
> | 
> | instead of the expected
> | ans =
> |     1
> |     2
> |     3
> |     4
> | Is this intended behaviour and am I missing something?
>
> It's expected.  Indexing a matrix with another single matrix produces
> a result the same size as the index.  Elements of the matrix and index
> are accessed in column-major order.  Note the difference between
> X(1,2) and X([1,2]).  To get the result you want, try
>
>   subs = fullfact (size (X));
>   X(sub2ind (size (X), subs(:,1), subs(:,2)))
>   
An alternative that I find more readable is:

subs = fullfact (size (X));
X(subs(:,1), subs(:,2))

I'm not positive that works (haven't tested it), but I think it does, 
and to me it is more readable.

Thanks,

Bill