Evalulating a vector function with a vector with MATLAB's symbolic toolbox

Question

I have a function F that is a vector of functions in n variables. For example, consider the following:

syms x y z f(f1,f2,f3) f1(x,y,z) f2(x,y,z) f3(x,y,z)

X0 = [1 0 1 0]';

f1(x,y,z) =  x+y+2;
f2(x,y,z) =  w-5*x+z^2-4;
f3(x,y,z) =  2*x+y^2-z-12;


F = vertcat(f1, f2, f3)

Here, F is a vector of three functions in three variables. In the case I am considering, I want F to be considered n equations in n variables.

I want to take a vector of scalars the length of F, like the following:

X = [1 2 3]';

and evaluate F at this vector. So something like:

F(X(1),X(2),X(3))

However, I need a way to write that without knowing n. I could find n by writing

dims = size(X);

But that doesn't solve how I parametrically evaluate F at X. Is there a way to do this?

Any specific reason you’re using symbolic math here? The example you give does not require it. This would be much simpler using normal numeric syntax. — Cris Luengo
– Cris Luengo, Commented Oct 14, 2019 at 1:22

David · Accepted Answer · 2019-10-14 00:14:21Z

1

If you don't mind using cells instead of arrays, you can replace X = [1 2 3]' by X = {1 2 3} (the transpose is not necessary) and F(x(1),x(2),x(3)) by F(X{:}).

answered Oct 14, 2019 at 0:14

David

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2 Comments

Unique Worldline Over a year ago

I'll look into this solution and let you know how it goes. Thanks!

Unique Worldline Over a year ago

It works! I had to also define a numerical array x to store iterative solutions for what X contains in my larger problem, but this solution allowed the indexing to work. Thanks for your help.

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