How do I implement a Matlab function, findtbracket.m, that, when a random function is inputed, the file will find the initial interval of the function?

Question

Monica Medina 2016-2-18

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/268721-how-do-i-implement-a-matlab-function-findtbracket-m-that-when-a-random-function-is-inputed-the-f

评论： Monica Medina 2016-2-18

采纳的回答： John BG

Screen Shot 2016-02-17 at 3.10.10 PM.png

在 MATLAB Online 中打开

The assignment asks us to implement a Matlab file, findtracket.m, of the form

function [a,b]=findbracket(f,X0)
% f: function handle f(x) to find a zero for
% x0: starting point/center of interval containing zero

to try to find an initial interval [a,b] containing the input x_0 and bracketing the zero of f(x)

Also, the function should begin with a=b=x_0 and a step size del=2^(-k) choosen so that

fl(x_0-del)<fl(x_0-del/2)=x_0

While sgn f(a)=sgn f(b), decrease a by del, increase b by del, evaluate f(a) and f(b) and double del.

This is what I have so far (I am a Matlab newbie):

function [a,b]=find bracket(f,x0)
a=x0;
b=x0;
while 1
    a=a-dx;
    if f(a)*f(b)<0,break; 
    b=b+del;
    if f(a)*f(b)<0, break;
end

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Walter Roberson 2016-2-18

The PNG image asks for "findbracket" not "findtracket".

You are not showing us how you invoke the code. And for sure you will need del and/or dx to be defined to use your code.

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Answer 1

John BG 2016-2-18

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/268721-how-do-i-implement-a-matlab-function-findtbracket-m-that-when-a-random-function-is-inputed-the-f#answer_210267

在 MATLAB Online 中打开

Mónica

prueba lo siguiente:

x0=4.333  
a=x0
b=x0
k=1
f=@(x) sin(x)
k3=1
while ~(floor(x0-2^(-k3-1))==floor(x0))
k3=k3+1
end
k4=1
while ~(floor(x0-2^-k4)==floor(x0))
k4=k4+1
end
k=max(k3,k4)
d=2^-k
while (sign(f(a))==sign(f(b)))
  a=a-d
  b=b+d
  d=2*d
end

da

a =   2.583333000000000
b =   6.083333000000000

me parece que el enunciado que has adjuntado, implementado tal y como se lee, se cuelga para ciertos valores de x0, por ejemplo, tanto

k2=1
while ~(floor(x0-2^(-k2-1))==x0 && floor(x0-2^-k2)<x0) 
  k2=k2+1
end

como

k2=1
while ~(floor(x0-2^(-k2-1))==floor(x0) && floor(x0-2^-k2)<floor(x0))  
  k2=k2+1
end

se cuelgan para x0=4.9

por lo que quizás sea posible modificar el enunciado así:

Si esta respuesta te resulta de utilidad para solucionar tu pregunta, te agradeceria me votes haciendo click en el link que tiene el pulgar arriba al lado de esta respuesta.

Atentamente

John

jgb2012@sky.com