FEVAL error in implementing Newton's method.
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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?
1 Kommentar
Paulo Silva
am 2 Mär. 2011
??? 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) ?
Akzeptierte Antwort
Paulo Silva
am 2 Mär. 2011
Here's one awesome resource you should view, it helped me a lot last year.
0 Kommentare
Weitere Antworten (3)
Sean de Wolski
am 2 Mär. 2011
fn(old)
dfn(old)
Are what you want. Not feval.
9 Kommentare
Paulo Silva
am 2 Mär. 2011
write doc inline in your matlab command line and see, same goes to other functions. You can also do:
help inline
Sean de Wolski
am 2 Mär. 2011
Ashley, It would be worth your time to review the syntax and methodology covered in these two documents:
doc function
doc function_handle
Matt Tearle
am 2 Mär. 2011
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.)
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