How do I pass an equation as a parameter to a class constructor?

I want to make method takes an equation as input and then differentiate it and substitute with x=0,1 in the output equation

When I try to run my code I get this error:

 >> aa=f(2x+1) 
  ??? aa=f(2x+1) 
  | 
  Error: Unexpected MATLAB expression.

Is this because the constructor doesn't accept any equation?

Here is my code:

 classdef SAA
    %UNTITLED5 Summary of this class goes here
    %   Detailed explanation goes here

    properties
        x
        equation
    end

    methods
        function f=SAA(e)
            f.equation=e;
        end 

        function [value_1,value_2]=differntiate(~)
            dydx=diff(equation,x)
            % I can't complete the problem and substitute
        end
    end
  end

any help will be appreciated.

[Walter Roberson]

Was this issue solved for you by your other question, http://www.mathworks.com/matlabcentral/answers/8528-how-can-i-pass-an-equation-as-input-parameter-to-function ?

(And if so, how are you doing the differentiation if what you are handed is a function handle?)

Answer 1

[John D'Errico]

What can't you complete? Is part of your problem that you spelled it incorrectly as differntiate in the classdef? Then when you spell it properly when you call it, it must fail.

Or is the problem that you don't know how to use subs?

help subs

Or is the problem that you are trying to differentiate equation, rather than f.equation?

Why did you feel the need to make x a property anyway?

[Eman Ahmed Elsayed]

my problem is I can't run the constructor on the command window

when I wirte

>> aa=f(2x+1)

the output is

??? aa=f(2x+1)

|

Error: Unexpected MATLAB expression.

is this because the constructor doesn't accept any equation??

at any way thanks for your answer

[Paulo Silva]

aa=f(2*x+1)

[John D'Errico]

As Paulo says, matlab requires * when you wish to multiply. Why are you writing this as a class anyway? That is truly using a Mack truck to carry a pea to Boston. And when you write f(2*x+1), where did f come from???

[Eman Ahmed Elsayed]

when I write

f=SAA(2)// it's works well

but when I write

f=SAA(2*x+1)

the compiler said

??? Undefined function or variable 'x'.

Please, How can I fix this problem

Answer 2

[Walter Roberson]

What data type is your equation? diff() only differentiates if the data type is "symbol" from the symbolic toolbox.

Answer 3

[Todd Flanagan]

You wouldn't pass something like 2x + 1 to DIFF. DIFF expects a vector or matrix of coefficients. In your case something like:

>> aa=f([2 1])

[John D'Errico]

Um, if the input is symbolic, then diff acts "diff"erently.

Answer 4

[Eman Ahmed Elsayed]

when I write f=SAA(2)// it's works well but when I write f=SAA(2*x+1) the compiler said ??? Undefined function or variable 'x'. Please, How can I fix this problem

[Walter Roberson]

If you are going to use diff() to differentiate then your input needs to be a sym() object created by the symbolic toolkit.