Error in my FEM Matlab code. Please help

for i=1:2
    fepsn(i)=i;
end
%%Loop for epsilon values
for icounter=1:2
    epsn=fepsn(icounter)
end

%%Shape function for axial effect
N1(epsn)=(-1/2)*epsn*(1-epsn)
N2(epsn)=(1+epsn)*(1-epsn)
N3(epsn)=(1/2)*epsn*(1+epsn)

syms epsn
%%First Derivatives of Shape Function
N1=diff(N1(epsn),epsn)   (Error here)
N2=diff(N2(epsn),epsn)
N3=diff(N3(epsn),epsn)

%%%%Second Derivatives of Shape Function
N11=diff(N1(epsn));
N22=diff(N2(epsn));
N33=diff(N3(epsn));

[KSSV]

What error? How do you think without specifying error, error line and all variables you will get help?

[Virajan Verma]

Error using sym/subsindex (line 732) Invalid indexing or function definition. When defining a function, ensure that the arguments are symbolic variables and the body of the function is a SYM expression. When indexing, the input must be numeric, logical, or ':'.

Error in xz (line 21) N1=diff(N1(epsn),epsn)

[Virajan Verma]

error(message('symbolic:sym:subscript:InvalidIndexOrFunction'));

[madhan ravi]

Remove syms epsn line

[Virajan Verma]

I have removed that line and a new error has come. Attempted to access N1(2); index out of bounds because numel(N1)=0.

Error in xz (line 26) N11=diff(N1(epsn));

[KSSV]

Why you are using sym to code FEM?

[Virajan Verma]

I have removed it now. But error is there as stated in my previous comment

[Torsten]

syms epsn N1(epsn) N2(epsn) N3(epsn)
N1(epsn) = (-1/2)*epsn*(1-epsn);
N2(epsn) = (1+epsn)*(1-epsn);
N3(epsn) = (1/2)*epsn*(1+epsn);
DN1 = diff(N1,epsn);
DN2 = diff(N2,epsn);
DN3 = diff(N3,epsn);
D2N1 = diff(DN1,epsn);
D2N2 = diff(DN2,epsn);
D2N3 = diff(DN3,epsn);

Answer 1

[KSSV]

Refer this link: https://in.mathworks.com/discovery/finite-element-analysis.html

Refer PDE tool box.

Refer this book: https://www.amazon.in/Finite-Element-Mechanical-Aerospace-Engineering/dp/0849300967

The above book is the best for FEM using MATLAB.