I cannot figure out how to fix this error "Objective function is returning undefined values at initial point. fsolve cannot continue."

function dummyhw8 
clc

x0=[1,1,1,0.02,0,02,0.02];
x=fsolve(@fcn,x0);
disp ('Velocities are the first three values. Whereas friction is last 3.');
disp(x);

function z=fcn(x)

z(1)=x(1)-(0.64*x(2))-(0.64*x(3)); % where x1=v1 x2=v2 and x3=v3
z(2)=40-(102*x(4)*x(1)^2)-(127*x(5)*x(2)^2); % where x4=f1 x5=f2 x6=f3
z(3)=60-(102*x(4)*x(1)^2)-(127*x(6)*x(3)^2);
z(4)=1/sqrt(x(4))+2.0*log10((4.5e-4/3.7)+2.81e-5/x(1)/sqrt(x(4)));
z(5)=1/sqrt(x(5))+2.0*log10((5.625e-4/3.7)+3.52e-5/x(2)/sqrt(x(5)));
z(6)=1/sqrt(x(6))+2.0*log10((5.625e-4/3.7)+3.52e-5/x(3)/sqrt(x(6)));

[DGM]

If you want anyone to toubleshoot code, you have to post it as something other than an image.

That said, 1/sqrt(0) isn't a good starting value. You might just have to adjust your x0.

[Dominic Riepenhoff]

function dummyhw8

clc

x0=[1,1,1,0.02,0,02,0.02];

x=fsolve(@fcn,x0);

disp ('Velocities are the first three values. Whereas friction is last 3.');

disp(x);

function z=fcn(x)

z(1)=x(1)-(0.64*x(2))-(0.64*x(3)); % where x1=v1 x2=v2 and x3=v3

z(2)=40-(102*x(4)*x(1)^2)-(127*x(5)*x(2)^2); % where x4=f1 x5=f2 x6=f3

z(3)=60-(102*x(4)*x(1)^2)-(127*x(6)*x(3)^2);

z(4)=1/sqrt(x(4))+2.0*log10((4.5e-4/3.7)+2.81e-5/x(1)/sqrt(x(4)));

z(5)=1/sqrt(x(5))+2.0*log10((5.625e-4/3.7)+3.52e-5/x(2)/sqrt(x(5)));

z(6)=1/sqrt(x(6))+2.0*log10((5.625e-4/3.7)+3.52e-5/x(3)/sqrt(x(6)));

[Stephen23]

Even though you made it hard for us to help you (by uploading a rather useless screenshot, rather than helpfully pasting your code as actual text), it is clear that your objective function indeed has at least one undefined value at the initial point. I did not check all of the element of x0, but several of the x values are used as the denominators in several division operations and these are an obvious source of undefined values. For example, x0(5) == 0 and yet you divide some value by sqrt(x(5)).

How do you expect FSOLVE to continue when you provide it with NaN or Inf values?

[Dominic Riepenhoff]

Yup that did it, thanks! Prof gave us those values to guess with, so it still does not make the most sense but it works now.

Thanks again!

[Dominic Riepenhoff]

Are there any ways (besides changing those values) to avoid this fsolve error, so that I could still use my initial guess values that are close to zero?

Answer 1

[Matt J]

I suspect you meant to have 6 values in x0 rather than 7.

x0=[1,1,1,0.02,0.02,0.02];