Index exceeds number of array elements (1)?

I got Index exceeds the number of array elements (1), and error in ph3 (line 90) [t, xsim] = ode45(@(t,x) g(t,x), [0 100], X_0);

Please help me :(

%Define experimental conditions
pH = 3;
I = 30;    %Current density mA
F = 96500;  %Faraday constant
Vwork = 12;  %Work potential of the cell Volts
V = 365;    %Volume mL
A = 57.17;  %Surface area cm2
km = 1.2;  %Mass transfer coefficient 
kj = 0.9; %
Sred = 0; %0

%Define initial conditions
initial_paraquat = 10; %mg/L
    
    %pH < 3
  
    %Define stochiometry coefficients for each chemical reaction
    %OC/TPi/TPi'/TPj/TPj'/CO2/M(OH)/M(HClO)/Cl2
    psi = [0 0 0 0 0 0 1 0 0;
           0 0 0 0 0 0 1 1 0;
           0 0 0 0 0 0 0 1 1;
           1 1 0 0 0 0 0 0 1;
           1 0 1 0 0 0 0 0 1;
           0 1 0 1 0 0 0 0 1;
           0 0 1 0 1 0 0 0 1;
           0 0 0 1 0 1 0 0 1;
           0 0 0 0 1 1 0 0 1;
           0 0 1 0 0 1 0 0 1;
           0 0 0 0 0 0 1 0 0];
    
    %Transpose psi matrix
    psi = transpose(psi);
    
    %Kinetic rate coefficients (base zone)
    k = [0;
         0;
         0;
         0;
         1;
         0;
         1;
         0;
         1;
         1;
         0];
    
    %Potential coffecients 
    V_reactions = [1;
                   1;
                   1;
                   1;
                   NaN;
                   1;
                   NaN;
                   1;
                   NaN;
                   NaN;
                   1];
    
    %Computation of electrons proportions
    DiffWork = Vwork - V_reactions;
    TotalWork = sum(Vwork - V_reactions, 'omitnan');
    
    alpha = DiffWork/TotalWork;
    
    %Initial conditions [anode; cathode; base]
    X_0 = [0; 0; initial_paraquat; %Paraquat
           0; 0; 0; %TPi
           0; 0; 0; %TPi'
           0; 0; 0; %TPj
           0; 0; 0; %TPj'
           0; 0; 0; %CO2
           0; 0; 0; %M(OH)
           0; 0; 0; %M(HClO)
           0; 0; 0]; %Cl2 
             
    %Define function that models the system of ODEs
    syms t x
    
    g= @(t,x) [A/V*(km(x(3) - x(1)) + psi(1,4)*I/F*alpha(4) + kj*(x(25)*x(1))); A/V*(km*(x(3) - x(2)) + kj*(Sred*x(2))); A*(km*(x(1) - x(3)) + km*(x(2) - x(3)) + psi(1,5)*k(5)*x(27)*x(3)); %Paraquat
               A/V*(km(x(6) - x(4)) + psi(2,4)*I/F*alpha(4) + psi(2,6)*I/F*alpha(6) + kj*(x(25)*x(4))); A/V*(km*(x(6) - x(5)) + kj*(Sred*x(5))); A*(km*(x(4) - x(6)) + km*(x(5) - x(6))); %TPi
               A/V*(km(x(9) - x(7)) + kj*(x(25)*x(7))); A*(km*(x(9) - x(8)) + kj*(Sred*x(8))); A/V*(km*(x(7) - x(9)) + km*(x(8) - x(9)) + psi(3,5)*k(5)*x(27)*x(3) + psi(3,7)*k(7)*x(27)*x(9) + psi(3,10)*k(10)*x(27)*x(9)); %TPi'
               A/V*(km(x(12) - x(10)) + psi(4,6)*I/F*alpha(6) + psi(4,8)*I/F*alpha(8) + kj*(x(25)*x(10))); A/V*(km*(x(12) - x(11)) + kj*(Sred*x(11))); A*(km*(x(10) - x(12)) + km*(x(11) - x(12))); %TPj
               A/V*(km(x(15) - x(13)) + kj*(x(25)*x(13))); A*(km*(x(15) - x(14)) + kj*(Sred*x(14))); A/V*(km*(x(13) - x(15)) + km*(x(14) - x(15)) + psi(5,7)*k(7)*x(27)*x(9) + psi(5,9)*k(9)*x(27)*x(15)); %TPj'
               A/V*(km(x(18)-x(16)) + psi(6,8)*I/F*alpha(8) + kj*(x(25)*x(16))); A/V*(km*(x(18) - x(17)) + kj*(Sred*x(17))); A*(km*(x(16) - x(18)) + km*(x(17) - x(18)) + psi(6,9)*k(9)*x(27)*x(15) + psi(6,10)*k(10)*x(27)*x(9)); %CO2
               A/V*(psi(7,1)*I/F*alpha(1) + psi(7,2)*I/F*alpha(2) + psi(7,11)*I/F*alpha(11) + kj*(x(25)*x(19)));0;0; %M(OH)
               A/V*(psi(8,2)*I/F*alpha(2) + psi(8,3)*I/F*alpha(3) + kj*(x(25)*x(22)));0;0; %M(HClO)
               A/V*(km(x(27)-x(25)) + psi(9,3)*I/F*alpha(3) + psi(9,4)*I/F*alpha(4) + psi(9,6)*I/F*alpha(6) + psi(9,8)*I/F*alpha(8)); A*(km*(x(27) - x(26))); A*(km*(x(25) - x(27)) + km*(x(26) - x(27)) + psi(9,5)*k(5)*x(27)*x(3) + psi(9,7)*k(7)*x(27)*x(9)) + psi(9,9)*k(9)*x(27)*x(15) + psi(9,10)*k(10)*x(27)*x(9)]; %Cloro
               
   [t, xsim] = ode45(@(t,x) g(t,x), [0 100], X_0);

   figure
   plot(t, xsim(:,1))

[Daniel Pollard]

I suspect that it's trying to solve the ODE for some values of x, and you calling x(3) (for example) in the function makes it confused.

What does the entire error message say?

Answer 1

[Chidvi Modala]

When defining g, you are trying to use km(x(3) - x(1)). Here MATLAB assumes '()' is used for accessing elements in an array. You might need to replace km(x(3) - x(1)) with km*(x(3) - x(1)) wherever applicable.