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?
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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
1 个评论
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.
采纳的回答
John BG
2016-2-18
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:
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Atentamente
John
更多回答（1 个）
Walter Roberson
2016-2-18
0 个投票
It is not acceptable to have a space in the name of a function. "findbracket" not "find bracket".
Your code does not initialize dx or del. It appears to me from the PNG that dx and del should be the same value.
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