Bisection method- code stops after one iteration

Question

MG 2015-12-4

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/258834-bisection-method-code-stops-after-one-iteration

评论： MG 2015-12-4

Hi, my code doesn't seem to continue beyond the first iteration of the bisection method in my loop. I also am not getting the right answer. I'm not sure where the mistake is.

function c = findroot(f,a,b,e)
% a is the initial value
% b is the final value
% e is the tolerance
if f(a)*f(b)>0
    error('f(a) and f(b) must have opposite signs')
end
c = (a+b)/2;
while abs(b-c) <= e;
    f = f(c);
    break
    if f(a)*f(c) <= 0;
      c = b;
        f(c) = f(b);
    else 
      c = a;
        f(c) = f(a);
    end
end
end

I am calling the following in the command window:

>> f = @(x) x.^2-x-1;

>> findroot(f, 1, 2, 0.001)

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Torsten 2015-12-4

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/258834-bisection-method-code-stops-after-one-iteration#answer_202164

在 MATLAB Online 中打开

function c = findroot(f,a,b,e)
% a is the initial value
% b is the final value
% e is the tolerance
left = a;
right = b;
middle = (left+right)/2;
fleft = f(left);
fright = f(right);
fmiddle = f(middle);
if fleft*fright > 0
    error('f(a) and f(b) must have opposite signs')
end
while right-left >= e && abs(fmiddle) >= e
    if fmiddle*fleft <= 0
      right = middle;
        fright = fmiddle;
    else 
      left = middle;
        fleft = fmiddle;
    end
    middle = (left+right)/2;
    fmiddle = f(middle);
end
c = middle;
end