Symbolic vector/matrix problem

Hi everyone,

I cannot figure out a way to define symbolic matrices/vectors in the way I want. I know I can do something like

sym('A', [M N])

But the above solution has several problems:

1. A has to be of fixed size, but I don't want to specify anything other than A is a matrix
2. When I make a function involving A using matlabFunction, I have to supply each and every element of A as separate function parameters. This is very troublesome when A is large. Also, code below doesn't work:

A = sym('A', [2 2])
B = sym('B', [2 2])
C(A,B) = A*B

In short, I want to define a symbol for a matrix, not a matrix of symbols. How can I do this?

In Theano, a python symbolic package, there are scalar symbols, vector symbols and matrix symbols, so things work in exactly the way I expect. I wonder how to do that in Mablab?

Answer 1

[Christopher Creutzig]

matlabFunction happily handles matrices of parameters, with the meaning that the resulting function is called with a matrix in that place, but for reasons of MATLAB syntax, that requires you hand it a cell array. In the following, I've abbreviated the outputs for readability:

>> A = sym('A',[4,4]);
>> B = sym('B',[4,3]);
>> A*B

ans =

[ A1_1*B1_1 + A1_2*B2_1 + A1_3*B3_1 + A1_4*B4_1, ...
[ A2_1*B1_1 + A2_2*B2_1 + A2_3*B3_1 + A2_4*B4_1, ...
[ A3_1*B1_1 + A3_2*B2_1 + A3_3*B3_1 + A3_4*B4_1, ...
[ A4_1*B1_1 + A4_2*B2_1 + A4_3*B3_1 + A4_4*B4_1, ...

>> matmult = matlabFunction(A*B, 'Vars', {A, B})

matmult = 

    @(in1,in2)reshape([in1(1).*in2(1)+in1(5).*in2(2)+in1(9).*in2(3)+...

>> matmult(magic(4), reshape(1:12,[4,3]))

ans =

      81   217   353
      89   225   361
      89   225   361
      81   217   353

There is currently no way to say that A is supposed to denote an abstract matrix.