Problem with a function: "??? Undefined function or variable 'x'"

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Alex Carnevalli
Alex Carnevalli 2016-10-20
编辑： dpb 2016-10-21
I'm trying to write a generic algorithm to use the Newton-Raphson method of finding the roots of a function, but my algorithm is having troubles with the call function system, giving the error: ??? Undefined function or variable 'x'.
Here's my algorithm, it doesn't have mistakes, because when I try to substitute the f, x and tol variables without calling the function it works perfectly.
function [x,y,err]=raphson(f,a,tol)
%syms x
%f=2*x.^3+log(x)-5
%a=2
%tol=10^-7
syms x
f1=diff(f);
x=a;
err=1;
while err>tol;
y=subs(f,x);
y1=subs(f1,x);
xk1=x-(y/y1);
err=y;
x=xk1;
end
end
The comentaries in the beginning of the algorythm are my attempts to atribute a value to the variables without using the function. I've tried everything and I can't get it to work.

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回答（1 个）

Chaya N
Chaya N 2016-10-20
The input argument f is a symbolic expression. You are probably seeing the error because the variable x has not been defined as a symbolic variable before calling your function. Simply declare syms x before calling your function and it should work.

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