Why do I get "Array indices must be positive integers or logical values." error?

Hi, I keep getting this error while computing Newton's method for systems for 3 nonlinear system of equations. Why do I keep this error? Checking for a hour but still couldn't find.

The error is;

Array indices must be positive integers or logical values.

Error in indexing (line 1075)
            R_tilde = builtin('subsref',L_tilde,Idx);

Error in q6 (line 36)
    F = [f(a,b,c); g(a,b,c); h(a,b,c)];

And my code is;

    clc;
clear;
close all;

syms x y z
f(x,y,z)=2*x+y+2*z^2-5;
g(x,y,z)=y^3+4*z-4;
h(x,y,z)=x*y+z-exp(z);

fdx(x,y,z) = diff(f,x); %derivative of f wrt x
fdy(x,y,z) = diff(f,y); %derivative of f wrt y
fdz(x,y,z) = diff(f,z); %derivative of f wrt z
gdx(x,y,z) = diff(g,x); %derivative of g wrt x
gdy(x,y,z) = diff(g,y); %derivative of g wrt y
gdz(x,y,z) = diff(g,z); %derivative of g wrt z
hdx(x,y,z) = diff(h,x); %derivative of h wrt x
hdy(x,y,z) = diff(h,y); %derivative of h wrt y
hdz(x,y,z) = diff(h,z); %derivative of h wrt z

%Initial guesses
a=2;
b=2;
c=2;

ap=0;
bp=0;
cp=0;
iteration=1;

double(a);
double(b);
double(c);

eps=0.01;
for i=1:100
    F = [f(a,b,c); g(a,b,c); h(a,b,c)];
    jac=[fdx(a,b,c) fdy(a,b,c) fdz(a,b,c);gdx(a,b,c) gdy(a,b,c) gdz(a,b,c);hdx(a,b,c) hdy(a,b,c) hdz(a,b,c)]; %jacobian
    h=-1*inv(jac)*F;
    ap=a;
    bp=b;
    cp=c;
    a=a+h(1);
    b=b+h(2);
    c=c+h(3);
    iteration=iteration+1;
    if (abs(a-ap)<eps && abs(b-bp)<eps && abs(c-cp)<eps)%check for error
        break
        
    end
end
fprintf('iteration=%d\ti=%f\tj=%f\tk=%f',iteration,a,b,c);

Answer 1

[Torsten]

syms x y z
f=2*x+y+2*z^2-5;
g=y^3+4*z-4;
h=x*y+z-exp(z);
fdx = diff(f,x); %derivative of f wrt x
fdy = diff(f,y); %derivative of f wrt y
fdz = diff(f,z); %derivative of f wrt z
gdx = diff(g,x); %derivative of g wrt x
gdy = diff(g,y); %derivative of g wrt y
gdz = diff(g,z); %derivative of g wrt z
hdx = diff(h,x); %derivative of h wrt x
hdy = diff(h,y); %derivative of h wrt y
hdz = diff(h,z); %derivative of h wrt z
F=[f;g;h];
jac = [fdx fdy fdz;gdx gdy gdz;hdx hdy hdz]
F = matlabFunction(F);
jac = matlabFunction(jac);
%Initial guesses
a=2;
b=2;
c=2;
ap=0;
bp=0;
cp=0;
iteration=1;
eps=0.01;
for i=1:100
    h=-jac(a,b,c)\F(a,b,c);
    ap=a;
    bp=b;
    cp=c;
    a=a+h(1);
    b=b+h(2);
    c=c+h(3);
    iteration=iteration+1;
    if (abs(a-ap)<eps && abs(b-bp)<eps && abs(c-cp)<eps)%check for error
        break
        
    end
end
fprintf('iteration=%d\ti=%f\tj=%f\tk=%f',iteration,a,b,c);

[Faruk Yagiz]

Thank you so much, so only needed thing was this code then :)

F = matlabFunction(F);
jac = matlabFunction(jac);

Answer 2

[Image Analyst]

This is asked every day. So, see the FAQ for a thorough discussion of the error.

https://matlab.fandom.com/wiki/FAQ#.22Subscript_indices_must_either_be_real_positive_integers_or_logicals..22

I don't have the Symbolic Toolbox but you probably can't have a symbolic variable as an index. You need an actual, specific number.