CODEWITHSUNDEEP
matlabfunctionminimum
Esther Rodriguez Piedracoba
Esther Rodriguez Piedracoba

Reputation: 11

Error using fzero

I need to know the minimum of the curve y= sin(r)+r.^2 using fzero. I created a function:

function w=myfun(r)
w=diff(sin(r)+r.^2);
end

Then at the command window:

r=[-pi:.01:pi];
fzero( @myfun,0)

But there is always an error:

Operands to the || and && operators must be convertible to logical scalar values.

Error in fzero (line 327)
elseif ~isfinite(fx) || ~isreal(fx)

fzero(@myfun,0)

What should I do?

Upvotes: 1

Views: 309

Answers (2)

Nicky Mattsson
Nicky Mattsson

Reputation: 3052

As stated by Cris, you cannot use diff, as it finds the difference between neighbouring points (with one value, this difference is undefined.).

Instead, you can do one of the following

Take the derivative yourself:

function w=myfun(r)
w=cos(r)+2*r;
end

Do a numerical differentiation

function dw=myfun(r)
h = sqrt(eps);
wh=(sin(r+h)+(r+h).^2;
w= (sin(r)+(r).^2;
dw = (wh - w)/(h);
end

Alternatively, use a numerical minimizer

w= @(r) (sin(r)+(r).^2;
xmin = fminsearch(w,0);

Upvotes: 1

Cris Luengo
Cris Luengo

Reputation: 60494

diff computed the finite difference derivative (i.e. the difference between neighbors). Here, r is scalar, so diff returns an empty array.

You will need to define your function myfun such that it returns the derivative given a single value. So you will have to manually compute the derivative function and write that into myfun.

Upvotes: 1

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