I would like to define a function that allows me to enter a symbolic expression in terms of x, raise that expression to the "n"th degree, then plot that expression from x=[-5,5]

Essentially, I would like to create a function that is very similar to fplot(), where I can enter an expression, and a power I would like to raise it to. The range of x-values it will be plotted for will be constant at x=[-5,5]. Here is my latest attempt that has not worked:

function [ ] = nthDegree( funcDef, n )
%Plots the the input function to the "n-th" degree.
%   
syms x ;
f = funcDef ;
y = matlabFunction(f)
Y = y.^n ;
fplot(Y,[-5,5])

end

Every attempt has resulted in the error message. Here is one such attempt:

nthDegree(x.^2,1)
Undefined function or variable 'x'.

Thanks for the help!

Answer 1

[Giorgos Papakonstantinou]

Justin you have to define outside your function what is x. This is the reason why Matlab complains about x. It does not know what x is and therefore it is undefined.

Furthermore, you should replace the

Y = y.^n ;

with

Y = @(y) y(x)^n;

in order to get the input function to the nth power.

[Justin S.]

Aha. So what you're saying is, there is no way to input a symbolic function using the term "x" like in the function fplot() without actually using that function or defining x values outside of my function? Is there any way to do this using apostrophes like in fplot()? Or something like an eval() which I believe is what fplot() is doing when you input a function? Thanks for getting back to me.

[Giorgos Papakonstantinou]

What I am saying is that if you have your function as it is , and you issue,

 syms x
 nthDegree(x.^2,1)

you get an error.

 Undefined function 'power' for input arguments of type 'function_handle'.

 Error in nthDegree (line 7)
 Y = y.^n;

Additionaly, after the statement,

y = matlabFunction(f);

y is not anymore a symbolic expression. It is a function handle. If you try to use fplot with a symbolic object, then Matlab would throw an error. fplot requires a function handle. If you want to plot a symbolic function you can use ezplot.

What do you mean with apostrophes, passing it as a string i.e. 'x.^2' in fplot? Yes it possible. I am quoting a part of the documentation:

fun must be:

• The name of a function
• A string with variable x that may be passed to eval, such as 'sin(x)', 'diric(x,10)', or '[sin(x),cos(x)]'
• A function handle

I hope these answer your question.