Undefined function or variable 'z' and error with ode45.

I need help, I don't know why this error occurred. Someone can explain me what I'm doing wrong and how can I solve it, please.

function dYfuncvecdz = ODEfun(z,Yfuncvec) 
X= Yfuncvec(1);
% Explicit equations
cao = 8;
k = .01;
z = 200 - lam;
lam = 200 - z;
ca = cao*(1-X);
E1 = 4.44658e-10*lam^4-1.1802e-7*lam^3+1.35358e-5*lam^2-.000865652*lam+.028004;
E2 = -2.64e-9*lam^3+1.3618e-6*lam^2-.00024069*lam+.015011;
F1 = 4.44658e-10/5*lam^5-1.1802e-7/4*lam^4+1.35358e-5/3*lam^3-.000865652/2*lam^2+.028004*lam;
F2 = -(-9.30769e-8*lam^3+5.02846e-5*lam^2-.00941*lam+.618231-1);
ra = -k*ca^2;
if lam<=70
    E=E1;
    F=F1;
else 
    E=E2;
    F=F2;
end
EF=E/(1-F);
% Differential equations
dXdz = -(ra/cao+EF*X);
dYfuncvecdz=[dXdz];

clc
zspan = [0 200]; % Range for the independent variable
y0 = [0]; % Initial values for the dependent variables
[t,y] = ode45(@ODEfun,tspan,y0);
z=size(y);
X=round(y(z(1,1),1),2);
a=['Conversion, X, predicted by Maximum Mixedness Model is ', num2str(X)];
disp(a)
plot(t,y,'LineWidth',1)
xlabel('time(mins)')
ylabel('X_m_m')
legend('X_m_m','Location','NorthWest')
title('Plot of conversion vs time predicted by Maximum Mixedness Model')
axis([0 200 0 1])

Answer 1

[Star Strider]

You defined:

zspan = [0 200]; % Range for the independent variable

so your ode45 call needs to be:

[t,y] = ode45(@ODEfun,zspan,y0);

Also, in ‘ODEfun’, these two statements should be in in this order:

lam = 200 - z;
z = 200 - lam;

[Walter Roberson]

I would suggest dropping the assignment to z as being unnecessary and confusing.

[Star Strider]

I would, as well, especially since those two lines together do nothing.

I assume something else may be intended, so I simply corrected their order for now.

[Bastian Alarcon Enriquez]

Thank you, Matlab gave me the conversion, but now I think that I need to fix the polynomial expression, because the graph gave me a weird curve.

About the 'z' I use it, because my guidebook explain with that substitution the metod. I Attach picture of the model.

[Star Strider]

I have absolutely no idea what you are doing with that.