Error: "Conversion from logical to sym is not possible"

I keep getting that error and I am not sure what I am supposed to do to fix it, can someone lead me in the right direction?

alpha=40*pi/180;
func=5/3*cos(alpha)-5/2*cos(phi)+11/6-cos(alpha-phi);
dfunc=diff(func);
xr=30*pi/180;
es=0.000001;
maxit=50;
iter=0;
while (1)
    xrold=xr;
    xr=xr-func(xr)/dfunc(xr);
    iter=iter+1;
    if xr~=0, ea=abs((xr-xrold)/xr)*100; end
    if ea <= es || iter>=maxit, break, end
end

Warning: Integer operands are required for colon operator when used as index.

Warning: Integer operands are required for colon operator when used as index.

Conversion to logical from sym is not possible.

root=xr

[KALYAN ACHARJYA]

Please share the code in text (cody) format, not screenshot.

[Maia Paige de Beauchamp]

Okay, I fixed it.

[KALYAN ACHARJYA]

phi?

What you are trying do here?

xr=xr-func(xr)/dfunc(xr);

Please note that xr and func are scalar (single values) here.

[Maia Paige de Beauchamp]

It was a code provided to me for one of my classes, do I need to make it non-scalar?

[KALYAN ACHARJYA]

func(xr)>> Represents array indexing in Matlab, if say example

func=[2 3 6 10] and xr=2

then func(xr)>> represents >> func(2)=3 OK in Matlab

But

if xr=0.2 or any decimal number/ negative/zero which is not allowed

func(xr) % Not Allowed

Answer 1

[Dhananjay Kumar]

I am assuing that you have declared phi as a symbolic variable by writing

>> syms phi

So func and dfunc are symbolic expressions.

If you want to evaluate the func at a particular phi ( here phi = xr = 30*pi/180), you should use the subs function like:

xr = xr-subs(func,phi,xr)/subs(dfunc,phi,xr);

You are getting first and second warning because of this wrong syntax only.

About the error "Conversion to logical from sym is not possible." :

The expression "ea <= es" (in if statement) just a symbolic expression not a constant value, that's why you are getting this error.