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From: ecksor ansari on 23 May 2010 13:26
i have below program for matrix multiplication:
[m,n]=size(a);
[k,l]=size(b);
c=zeros(m,l);
for i=1:m,
for j=1:l,
for p=1:n,
c(i,j)=c(i,j)+a(i,p)*b(p,j);
end
end
end

i need to define a and b as a(x) and b(x), but then matlab returns this error:
---------------------------------------------------------------------------------
??? The following error occurred converting from sym to double:
Error using ==> mupadmex
Error in MuPAD command: DOUBLE cannot convert the input expression
into a double array.

If the input expression contains a symbolic variable, use the VPA
function instead.
---------------------------------------------------------------------------------

how can i change it to symbolic form?
From: Walter Roberson on 23 May 2010 14:13
ecksor ansari wrote:
> i have below program for matrix multiplication:
> [m,n]=size(a);
> [k,l]=size(b);
> c=zeros(m,l);
> for i=1:m,
> for j=1:l,
> for p=1:n,
> c(i,j)=c(i,j)+a(i,p)*b(p,j);
> end
> end
> end
>
> i need to define a and b as a(x) and b(x), but then matlab returns this
> error:
> ---------------------------------------------------------------------------------
>
> ??? The following error occurred converting from sym to double:
> Error using ==> mupadmex
> Error in MuPAD command: DOUBLE cannot convert the input expression
> into a double array.

Which line does it report that for?

It is not possible to receive that error unless there are key lines that
you have not shown us, such as declarations of something as a symbolic
variable.

> how can i change it to symbolic form?

You cannot code a symbolic multiplication with a 'for' loop: 'for'
requires numeric values to iterate over. If you want to code a symbolic
matrix multiplication, use the symbolic toolbox's matrix multiplication.

Watch out for rank problems. You mention defining "a" as a(x) but that
implies a vector and you are using it as a matrix with two subscripts.
If you happen to be multiplying two vectors together, remember that
matrix multiplication cannot do that unless one of the two is a row
vector and the other is a column vector of the same length.

You should be doing error checking to be sure the matrices are the
correct relative shape -- that n == k.

It is not recommended to use lower-case-L (l) as a variable name, as it
is too easy to misread it as the digit one (1). Indeed with the font I
am using right now, there is no visible difference.
From: Steven Lord on 24 May 2010 09:42

"ecksor ansari" <vvahidd(a)gmail.com> wrote in message
news:htbofc$dme$1(a)fred.mathworks.com...
>i have below program for matrix multiplication:
> [m,n]=size(a);
> [k,l]=size(b);
> c=zeros(m,l);
> for i=1:m,
> for j=1:l,
> for p=1:n,
> c(i,j)=c(i,j)+a(i,p)*b(p,j);
> end
> end
> end
>
> i need to define a and b as a(x) and b(x), but then matlab returns this
> error:
> ---------------------------------------------------------------------------------
> ??? The following error occurred converting from sym to double:
> Error using ==> mupadmex
> Error in MuPAD command: DOUBLE cannot convert the input expression
> into a double array.
>
> If the input expression contains a symbolic variable, use the VPA
> function instead.
> ---------------------------------------------------------------------------------
>
> how can i change it to symbolic form?

As you've defined c above, it's a double array. When you compute a(i,
p)*b(p, j) [assuming one or both of those matrices contain a symbolic
variable] the result is a sym object. Adding that sym object to a double
[c(i, j)] results in another sym object. In order to assign that sym object
into a double array, MATLAB would need to convert the sym object into a
double array using the DOUBLE function. Because the sym object contains a
symbolic variable, however, it cannot.

Either define c to be a sym object:

c = sym(zeros(m, l));

or just compute the product of a and b directly.

c = a*b;

--
Steve Lord
slord(a)mathworks.com
comp.soft-sys.matlab (CSSM) FAQ: http://matlabwiki.mathworks.com/MATLAB_FAQ
To contact Technical Support use the Contact Us link on
http://www.mathworks.com


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