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

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Eman Ahmed Elsayed on 31 May 2011
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 on 31 May 2011
(And if so, how are you doing the differentiation if what you are handed is a function handle?)

John D'Errico on 31 May 2011
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 on 31 May 2011
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 on 31 May 2011
What data type is your equation? diff() only differentiates if the data type is "symbol" from the symbolic toolbox.

Todd Flanagan on 31 May 2011
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 on 31 May 2011
Um, if the input is symbolic, then diff acts "diff"erently.

Eman Ahmed Elsayed on 31 May 2011
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 on 31 May 2011
If you are going to use diff() to differentiate then your input needs to be a sym() object created by the symbolic toolkit.

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