find zero of a custom function

Question

Paolo 2013-12-5

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https://ww2.mathworks.cn/matlabcentral/answers/108826-find-zero-of-a-custom-function

评论： Brett Cooper 2017-11-16

Hello, I would like to know if it is possible to use fzero for a custom function written using a script. For example: let us suppose I have defined a function f.m which takes 3 arguments f(x,y,z) I know the values of x and y and I want to find the value of z which returns the value of my function 0,

is it possible to use fzero ? if yes could you post a very simple example?

Thanks you Paolo

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Paolo 2013-12-7

Thanks Walter, just another question about fzero. I am trying to find the zero of the following inline function:

fun=inline('-Pt1+y+(zcb+(rate*tau2*y))')

where Pt1=0.999970474084335 zcb=0.005646500819616 rate=0.0056460 tau2=0.99722222222222

yres=fzero(@(y) fun(Pt1,tau2,rate,zcb,y),[0.2,1.0])

yres=0.994292181677045

but when I go to check the inline functionsubstituting the value of y with yres I get

val=-Pt1+yres+(zcb+(rate*tau2*yres))

val=0.005566388254360

while I was expecting a value quite close to 0

Any clue?

Thanks Paolo

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Answer 1

John D'Errico 2013-12-7

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在 MATLAB Online 中打开

First of all, do NOT use inline functions. They are slow, a hack given to us in the days before function handles were provided.

Pt1=0.999970474084335;
zcb=0.005646500819616;
rate=0.0056460;
tau2=0.99722222222222;
% a better fun. See that since all parameters but
% y are known, the function handle uses those values.
% I could also have written fun as a function of 5
% parameters, and then reduced it to a single variable
% function using a function handle in the call to zero.
fun = @(y) -Pt1+y+(zcb+(rate*tau2*y));

Next, plot the function!!!!!!!!!! Always do this.

ezplot(fun)
grid on

As you can see, there is clearly a root, just under 1.

ysol = fzero(fun,[0,2])
ysol =
         0.988756958482095
fun(ysol)
ans =
     -1.04083408558608e-17

Anyone who claims this was not a root should learn to read scientific notation. So I'm not at all sure what you did wrong, but it looks like you used some inaccurate value in the computation.