how to create a matrix with variables in it?

for eg., to create following matrix and work on it,

A=[x1 x1*x2 ; x1^2 x2^2]

Answer 1

[Kye Taylor]

The best answer to your question will likely depend on what you want to do with A. Sometimes, I find the following commands useful, which will create an anonymous function, A, that takes two inputs, x1 and x2 and returns the matrix you describe. Of course, this assumes you have values for the variables x1 and x2.

A = @(x1,x2)[x1,x1*x2;x1^2,x2^2]

And compute, for example, with

det(A(1,2))

If you want A to be symbolic, you can begin using the following command

syms x1 x2
A = [x1,x1*x2;x1^2,x2^2]

Then compute, for example, with

det(A)

[Sergio E. Obando]

As of R2021b, you may also be interested in symmatrix: https://www.mathworks.com/help/symbolic/use-symbolic-matrix-variables.html

Answer 2

[Aman Kumar]

syms x1 x2

A = [x1,x1*x2;x1^2,x2^2]

[Michael Wolfe]

How would you make a matrix with both numbers and variables in it?

[Walter Roberson]

syms x1 x2
A = [x1,x1*pi;.8,x2^2]

[Nic]

What if I want to create a matrix with expressions?

syms x

A=[4*x^2+8*x+5 -8*x -5;8x -4*x^2-16*x 4*x;s 4*s -9]

Why doesn't it work?

[Steven Lord]

What does "doesn't it work" mean in this context?

• Do you receive warning and/or error messages? If so the full and exact text of those messages (all the text displayed in orange and/or red in the Command Window) may be useful in determining what's going on and how to avoid the warning and/or error.
• Does it do something different than what you expected? If so, what did it do and what did you expect it to do?
• Did MATLAB crash? If so please send the crash log file (with a description of what you were running or doing in MATLAB when the crash occured) to Technical Support so we can investigate.

[John D'Errico]

@Nic

Let me guess why it does not work. In what you wrote, what is s?

syms x 
A=[4*x^2+8*x+5 -8*x -5;8x -4*x^2-16*x 4*x;s 4*s -9]

Do you see that you defined x as a sym, but how about s?

If you define all of the variables properly, of course it will work.