Troubles on differentiation inside a function

2012 7 20
1 个回答
回答已采纳
20 次查看（30 天）

0 个投票

Do you guys know how to differentiate a function inside a function?
I want to input a function, ask the code to differentiate it and use both function and its derivative in a loop.
function [zero] = newton_raphson (x0,precisao,funcao)
%Estimates the root of a function
syms x
f=funcao(x);
derivada=diff(f,x);
x(1)=x0;
erro=realmax;
i=1;
while erro > precisao
x(i+1)=x(i)-f(x(i))/derivada(x(i));
erro = abs (x(i+1)-x(i));
i=i+1;
end
disp(['The estimate root is ' num2str(x(end)) ' with ' num2str(i) ' iteractions.'])
return
The error message when i enter "newton_raphson(2,10^-3, @(x) x+1): ??? Error using ==> mupadmex Error in MuPAD command: Index exceeds matrix dimensions.
Error in ==> sym.sym>sym.subsref at 1366
B = mupadmex('mllib::subsref',A.s,inds{:});
Error in ==> newton_raphson at 16
x(i+1)=x(i)-f(x(i))/derivada(x(i));
Thanks!

3 个评论
显示 1更早的评论 隐藏 1更早的评论

Sargondjani
Sargondjani 2012-7-21
you should give the error message
note that 'error' is a reserved matlab command, so you shouldnt use it.
Ryan
Ryan 2012-7-21
I'm guessing f isn't in a proper format for diff. What is "func"? What class is f? What does unique(f) return?
Arthur
Arthur 2012-7-22
Sorry, you guys. My variables are on portuguese, i tried to rename it on english to make it easier here, but it seems it just messed up. Code fixed as the original, with the message error.

请先登录，再进行评论。

请先登录，再回答此问题。

 采纳的回答

Walter Roberson
Walter Roberson 2012-7-22

0 个投票

deri=diff(f,x) %differentiate against a variable
Caution: what results will be a symbolic function, but later you try to invoke it as if it is a numeric function in the line
x(i+1)=x(i)-f(x(i))/deri(x(i));
I suggest you examine matlabFunction()

2 个评论
显示 无 隐藏 无

Arthur
Arthur 2012-7-22
I tried, but still couldn't do it.
The code on the while loop is ok. I tested it removing the variable funcao and putting a function and its derivative on the code, below the loop like this:
while erro > precisao
x(i+1)=x(i)-f(x(i))/derivada(x(i));
erro = abs (x(i+1)-x(i));
i=i+1;
end
disp(['The estimate root is ' num2str(x(end)) ' with ' num2str(i) ' iteractions.'])
return
function y=funcao(x);
y=x^2-2*x+1;
return
function y=derivada(x);
y=2*x-2;
return
Thank you for the attention.
Walter Roberson
Walter Roberson 2012-7-22
derivada = matlabFunction( diff(f,x), x);
Otherwise derivada would be a symbolic variable and derivada(x(i)) would be an attempt to index that variable at index x(i). You cannot pass a value to a symbolic expression using (VALUE) notation.

请先登录，再进行评论。

更多回答（0 个）

类别

在 帮助中心 和 File Exchange 中查找有关 Arithmetic Operations 的更多信息

产品

标签

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by