MATLAB Central Newsreader - Is this function available?

Is this function available?

LeanMean82 — Sat, 06 Dec 2008 21:49:00 -0500

Hey Guys,

Quick Question.
I just want to know whether matlab has a function that can create a matrix which is an intersection of column spaces of two original matrices. Such that:

A has 5 column vectors, B has 3 column vectors. A and B share at least two column vectors and therefore creates a matrix C with these two column vectors.

Thanks.

Re: Is this function available?

Roger Stafford — Sat, 06 Dec 2008 22:13:02 -0500

LeanMean82 <nndhawan@ufl.edu> wrote in message <852406.1228600170649.JavaMail.jakarta@nitrogen.mathforum.org>...
> Hey Guys,
>
> Quick Question.
> I just want to know whether matlab has a function that can create a matrix which is an intersection of column spaces of two original matrices. Such that:
>
> A has 5 column vectors, B has 3 column vectors. A and B share at least two column vectors and therefore creates a matrix C with these two column vectors.
>
> Thanks.

Yes. Use the 'intersect' function with the 'rows' option. (You'll have to do some transposing to make it work.)

Roger Stafford

Re: Is this function available?

David Doria — Sat, 06 Dec 2008 23:03:02 -0500

Roger, I feel like he is talking more about the intersection of the column spaces, not a discrete kind of intersection. Intersect() will just say that two things intersect if they are exactly the same, right? Where the vector spaces intersect in a much more complicated way.

I could be way off...

Dave

Re: Is this function available?

Roger Stafford — Sun, 07 Dec 2008 00:35:03 -0500

"David Doria" <daviddoria@gmail.com> wrote in message <ghf0b6$l55$1@fred.mathworks.com>...
> Roger, I feel like he is talking more about the intersection of the column spaces, not a discrete kind of intersection. Intersect() will just say that two things intersect if they are exactly the same, right? Where the vector spaces intersect in a much more complicated way.
> I could be way off...
> Dave

Here is a quote from the Matlab 7 Function Reference, Vol. 2: "c = intersect(A, B, 'rows') when A and B are matrices with the same number of columns returns the rows common to both A and B." With a transpose understood, you can read that with the words 'rows' and 'columns' interchanged. Nothing is said about the matrices being the same size, only their respective numbers of columns (which translates to rows with the transpose).

For example, if

A = [1 2 3 4 5;6 7 8 9 10;11 12 13 14 15]

and

B = [5 16 2;10 17 7;15 18 12]

then C could be calculated as

C = intersect(A',B','rows')' = [2 5;7 10;12 15].

I presume from the OP's description, since A and B share two columns in common, that C should consist of just these two columns.

Roger Stafford

Re: Is this function available?

John D'Errico — Sun, 07 Dec 2008 02:47:01 -0500