Convert syms matrix to double

How to convert the matrix with syms in double.

The use of double print error.

Answer 1

[Star Strider]

That is not possible. The reason is that the matrix contains symbolic variables. They have to be assigned numeric values to use the double function.

One possible approach is to use the matlabFunction funciton to turn it into a funiion —

syms a1 a2 a3 
c = [0 0 0 0 0; 0 a1 0 0 0; 0 0 a2 0 0; 0 0 0 0 0; 0 0 0 0 a3];
cfcn = matlabFunction(c)

cfcn = function_handle with value:
    @(a1,a2,a3)reshape([0.0,0.0,0.0,0.0,0.0,0.0,a1,0.0,0.0,0.0,0.0,0.0,a2,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,a3],[5,5])

c = cfcn(21,42,84)

c = 5×5

0 0 0 0 0 0 21 0 0 0 0 0 42 0 0 0 0 0 0 0 0 0 0 0 84

That ‘c’ matrix you can now use in other calculations outside the Symbolic Math Toolbox.

.

[Marianna]

I have to create the matrix c dynamic, because i have large dimension.

[Torsten]

The matlabFunction is one possibility. The other is

syms a1 a2 a3 
c = [0 0 0 0 0; 0 a1 0 0 0; 0 0 a2 0 0; 0 0 0 0 0; 0 0 0 0 a3];
cnum = double(subs(c,[a1 a2 a3],[21 42 84]))

cnum = 5×5

0 0 0 0 0 0 21 0 0 0 0 0 42 0 0 0 0 0 0 0 0 0 0 0 84

class(cnum)

ans = 'double'

[Marianna]

The problem is to create the c matrix dynamic, not manualy

[Torsten]

At the risk of outing me as an ignorant: What exactly do you mean by "dynamic" ? Dynamic in size ?

[Star Strider]

@Marianna — I have no idea what your requirements are for ‘c’, or how you create it.

Another option is to use the diag function. You have to determine what elements of the vector are non-zero, and either set the length using the zeros function and then fill the appropriate positions, or always have the (n,n) entry as the last element in the vector, where the dimensions of the matrix are (n,n).

Example —

c = diag([0 21 42 0 84])

c = 5×5

0 0 0 0 0 0 21 0 0 0 0 0 42 0 0 0 0 0 0 0 0 0 0 0 84

It might be possible to create that as a function as well, specifying the positions and their values as arguments, depending on what you want. (I have no idea that that is just now.)

.