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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]

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Justin S.
Justin S. on 10 Mar 2015
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!

Answers (1)

Giorgos Papakonstantinou
Giorgos Papakonstantinou on 10 Mar 2015
Edited: Giorgos Papakonstantinou on 10 Mar 2015
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.
  2 Comments
Giorgos Papakonstantinou
Giorgos Papakonstantinou on 11 Mar 2015
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.

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