Thread Subject: left division, engEvalString() does not work?

Hi,

I am trying to use matlab engine. I want it do a left division as
following lines

A = mxCreateDoubleMatrix(ROWS,3,mxREAL);//creat and fill in A, y
y = mxCreateDoubleMatrix(ROWS,1,mxREAL);
                ......//initializes A, y
                ......//from matlab prompt-line I can see they are all
properly initialized
engEvalString(ep, "x=A\y"); //left division
                      //from promptline, I cannot see variable 'x', as
a result of the above execution!
                x = engGetVariable(ep, "x"); //as a result, this step
returns x=0 !!!

Can anyone tell me what's wrong with this simple use?


Thanks,
Shufei

On May 4, 10:57 am, Vanderbilt <shufei....@gmail.com> wrote:
> Hi,
>
> I am trying to use matlab engine. I want it do a left division as
> following lines
>
> A = mxCreateDoubleMatrix(ROWS,3,mxREAL);//creat and fill in A, y
> y = mxCreateDoubleMatrix(ROWS,1,mxREAL);
> ......//initializes A, y
> ......//from matlab prompt-line I can see they are all
> properly initialized
> engEvalString(ep, "x=A\y"); //left division
> //from promptline, I cannot see variable 'x', as
> a result of the above execution!
> x = engGetVariable(ep, "x"); //as a result, this step
> returns x=0 !!!
>
> Can anyone tell me what's wrong with this simple use?
>
> Thanks,
> Shufei

any ideas?
Still waiting for your input.
Thanks

In article <1178739237.353666.212190@n59g2000hsh.googlegroups.com>,
Vanderbilt <shufei.fan@gmail.com> wrote:

> On May 4, 10:57 am, Vanderbilt <shufei....@gmail.com> wrote:
> > Hi,
> >
> > I am trying to use matlab engine. I want it do a left division as
> > following lines
> >
> > A = mxCreateDoubleMatrix(ROWS,3,mxREAL);//creat and fill in A, y
> > y = mxCreateDoubleMatrix(ROWS,1,mxREAL);
> > ......//initializes A, y
> > ......//from matlab prompt-line I can see they are all
> > properly initialized
> > engEvalString(ep, "x=A\y"); //left division
> > //from promptline, I cannot see variable 'x', as
> > a result of the above execution!
> > x = engGetVariable(ep, "x"); //as a result, this step
> > returns x=0 !!!
> >
> > Can anyone tell me what's wrong with this simple use?
> >
> > Thanks,
> > Shufei
>
> any ideas?
> Still waiting for your input.
> Thanks
-------------------
  What value is 'ROWS' set to? You can get zeros in some of the three
elements of x = A\y if 'ROWS' is less than 3. If 'ROWS' is equal to 3, in
general there will be a single exact answer. For larger values of 'ROWS',
you will not in general get exact solutions but only those with least
square error.

Roger Stafford

>
> > any ideas?
> > Still waiting for your input.
> > Thanks
>
> -------------------
> What value is 'ROWS' set to? You can get zeros in some of the three
> elements of x = A\y if 'ROWS' is less than 3. If 'ROWS' is equal to 3, in
> general there will be a single exact answer. For larger values of 'ROWS',
> you will not in general get exact solutions but only those with least
> square error.
>
> Roger Stafford

My ROWS is much larger than 3.
You are right, what I want is an approximate solution, with least
square error.
My problem is not in the correctness of solution. It is that no
solution was returned at all!

( During my debug, After executing the line "engEvalString(ep, "x=A
\y"); //left division ", I try to test it in the matlab workspace,
there were no variable 'x' produced as a result. But, when I input the
command x=A\y at matlab prompt, x was correctly computed! )

I suspect there are some error in the way I use the engine?

Thanks
Shufei

I guess it's a little to late for that question to be
answered, but just in case some one else encounters the
same problem.

Vanderbilt <shufei.fan@gmail.com> wrote in message
<1178290663.323760.120190@y80g2000hsf.googlegroups.com>...
> engEvalString(ep, "x=A\y"); //left division
>
> Can anyone tell me what's wrong with this simple use?

Try "x=A\\y". The backslash is an escape character in
C/C++, you'll have to use '\\' to get an ordinary '\'.

best wishes
Sven

"Sven Fischer" <fisher311-matlab@yahoo.de> wrote in
message <ful499$csk$1@fred.mathworks.com>...
> I guess it's a little to late for that question to be
> answered, but just in case some one else encounters the
> same problem.
>
> Vanderbilt <shufei.fan@gmail.com> wrote in message
>
<1178290663.323760.120190@y80g2000hsf.googlegroups.com>...
> > engEvalString(ep, "x=A\y"); //left division
> >
> > Can anyone tell me what's wrong with this simple use?
>
> Try "x=A\\y". The backslash is an escape character in
> C/C++, you'll have to use '\\' to get an ordinary '\'.
>
> best wishes
> Sven

And an even later suggested option:

engEvalString(ep, "x=mldivide(A,y)");

James Tursa