FEVAL error in implementing Newton's method.

Question

Ashley Dunn 2011-3-2

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2357-feval-error-in-implementing-newton-s-method

This is M-File 1

function sol=newtonian(fn,dfn,x0,tol)
tol=0.000001;
old=x0+1; 
while abs (x0 - old) > tol; old=x0; 
  x0=old - feval(fn, old)/feval(dfn, old);
end; 
sol=x0;
end

M-File 2

function y=fn(x0,y)
    x0=2;
    y=(x0).^2-exp(x0)+2;
end

M-File 3

function dy=dfn(x0)
  x0=-2;
  dy=2*(x0)-exp(x0);
end

But I keep getting this ERROR:

??? Error using ==> feval
Argument must contain a string or function_handle.
Error in ==> newtonian at 5
x0=old - feval(fn, old)/feval(dfn, old);

What does this error mean?

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Paulo Silva 2011-3-2

??? Error using ==> feval

Argument must contain a string or function_handle.

means that you are not providing the arguments that the function is expecting, maybe feval(@fn, old) or feval('fn', old) ?

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Answer 1

Paulo Silva 2011-3-2

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Here's one awesome resource you should view, it helped me a lot last year.

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Answer 2

Sean de Wolski 2011-3-2

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在 MATLAB Online 中打开

 fn(old) 
 dfn(old)

Are what you want. Not feval.

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Ashley Dunn 2011-3-2

what does doc inline mean?

Paulo Silva 2011-3-2

write doc inline in your matlab command line and see, same goes to other functions. You can also do:

help inline

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Answer 3

Sean de Wolski 2011-3-2

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在 MATLAB Online 中打开

Ashley, It would be worth your time to review the syntax and methodology covered in these two documents:

 doc function
 doc function_handle

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Ashley Dunn 2011-3-2

what is this??

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Answer 4

Matt Tearle 2011-3-2

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在 MATLAB Online 中打开

The accepted answer from Paulo looks like a great resource for the theory behind gradient methods, but I'm not sure if that's actually solved your MATLAB issues or not, so...

The issue is that you need to give the names of two functions to newtonian (the first two arguments, fn and dfn). Values like x0 and tol are easy to pass to a function as either literal values or variables that hold numeric values. For example

sqrt(pi)

or

x = pi;
sqrt(x)

But how do you pass a reference to (or name of) a function as an input to another function? The answer in MATLAB is to use a function handle. If you have a function file (like, say, fn.m) you can create a function handle to it using the @ symbol: @fn

Then, inside newtonian the variable fn will be a function handle to the function file fn.m. You can then use it as if it was a direct use of the fn function, as in Sean's answer: fn(old).

(Sorry, I realize it's a bit confusing, with the same name being used for everything.)