MATLAB: Symbolic matrix with symbolic size

symbolic indexsymbolic matrix

I am trying to define a symbolic matrix with symbolic indices

Normally, I know that a symbolic matrix A which has 2 rows and 4 columns is defined as :

A = sym('A', [2 4])

But I want to define it with m rows and n columns, when I use the following code, I have an error message: Error using sym (line 253) Second argument must be assumption or size vector:

syms m n

assume(m,'integer')

assumeAlso(m>=0)

assumeAlso(m<=M-1)

assume(n,'integer')

assumeAlso(n>=1)

assumeAlso(n<=2^(k-1))

A = sym('A', [m n])

So, how can I accomplish this?

Thank you in advance.

Best Answer

Hi,

As mentioned in this question , Matrices with unknown size is not supported in MATLAB and thus the correct error message is displayed. You can try the workaround given in the answer by defining a large matrix and then cropping it based on the actual values of ‘m’ and ‘n’.

Related Solutions

MATLAB: Does the Symbolic Toolbox support symbolic matrix operations

To work with matrices in the Symbolic Toolbox, a variable must have each of its elements explicitly defined to be recognized as a matrix. Otherwise, variables are treated as scalars.

For example, this code snippet will define two symbolic matrices:

M = sym(zeros(2,2));
M1 = sym(zeros(2,2));
for row = 1:2;
   for col=1:2; 
     M(row,col) = sym(['a' num2str(row) num2str(col)]);
     M1(row,col) = sym(['b' num2str(row) num2str(col)]);
   end
end

These two matrices can now be multiplied together:

M*M1
 ans =
 [ a11*b11+a12*b21, a11*b12+a12*b22]
 [ a21*b11+a22*b21, a21*b12+a22*b22]
M1*M
 ans =
 [ a11*b11+a21*b12, b11*a12+b12*a22]
 [ b21*a11+b22*a21, a12*b21+a22*b22]

If your matrix elements are defined by an equation, use MESHGRID to help in defining that matrix. For example, the following code will create a symbolic matrix whose elements are described by the equation 1/(i+j-t), where "i" is the row index and "j" is the column index:

syms t
n = 3;
[J,I] = meshgrid(1:n);
A = sym(1./(I + J - t));

This produces

A
 A =
 [ 1/(2-t), 1/(3-t), 1/(4-t)]
 [ 1/(3-t), 1/(4-t), 1/(5-t)]
 [ 1/(4-t), 1/(5-t), 1/(6-t)]

MATLAB: Symbolic computation

e.g. (R2011b):

A = sym('A',[10 10])
A1 = sym('A1_',[100 1])

[x y] = meshgrid(1:10);
for i1 = 1:numel(x)
    A2(y(i1),x(i1)) = sym(sprintf('A2_%d_%d',y(i1),x(i1)));
end

[x y] = meshgrid(1:10);
k = arrayfun(@(i1)sym(sprintf('A3_%d_%d',y(i1),x(i1))),1:numel(x),'un',0);
A3 = reshape([k{:}],10,[]);

Best Answer

Related Solutions

MATLAB: Does the Symbolic Toolbox support symbolic matrix operations

MATLAB: Symbolic computation

Related Question