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

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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
  5 Comments
Maia Paige de Beauchamp
Maia Paige de Beauchamp on 30 Nov 2019
It was a code provided to me for one of my classes, do I need to make it non-scalar?
KALYAN ACHARJYA
KALYAN ACHARJYA on 30 Nov 2019
Edited: KALYAN ACHARJYA on 30 Nov 2019
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

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Answers (1)

Dhananjay Kumar
Dhananjay Kumar on 3 Dec 2019
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.

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