Whats wrong with my code?

When I run my script, that contains the following code:

% Clear workspace and command window
clear 
clc
 
dt=1;   % time step (minutes)
t=0:dt:120; % time vector
 
% Pregenerate vectors
z=zeros(1,length(t));
Glucose=z;
Glucose_Used=z;
Insulin=z;
Insulin_Breakdown=z;
 
% Modify Usuage_Fraction(t) by adding a values
a3 = -1.722E-14;
a2 = 4.666E-10;
a1 = 1.435E-8;
a0 = 8.262E-3
 
% Modify Insulin_Secretion(t) by adding b values
b3 = -2.626E-9;
b2 = 4.682E-5;
b1 = -0.1140;
b0 = 72.5;
 
% Constants and initial conditions
Glucose(1)=5940;    % mg initial glucose
Insulin(1==9000);    % mg initial insulin
Usage_Fraction(t) = a3* Insulin(t)^3 + a2* Insulin(t)^2 + a1* Insulin(t) + a0;
Blood_Volume=60; % dL
Glucose_Released=185.625*ones(1,length(t)); % mg/min
Insulin_Secretion(t) == b3* Glucose(t)^3 + b2* Glucose(t)^2 + b1* Glucose(t) == b0;     % mg/min
k = 26.5;
% calculate insulin and glucose amounts from given equations
for i=2:length(t);
    Glucose_Used(i-1)=Glucose(i-1)*Usage_Fraction(t);
    Glucose(i)=Glucose(i-1)+(Glucose_Released(i-1)-Glucose_Used(i-1))*dt;
    Insulin_Breakdown(i-1)=Insulin(i-1)/k*k;
    Insulin(i)=Insulin(i-1)+(Insulin_Secretion(i-1)-Insulin_Breakdown(i-1))*dt;
end
Glucose_Conc=Glucose/Blood_Volume; % glucose concentration mg/dL
% plot glucose release and blood glucose profiles
subplot(1,2,1)
plot(t,Glucose_Released)
title('Glucose Release Profile')
xlabel('Time (min)')
ylabel('Glucose released into blood (mg/min)')
subplot(1,2,2)
plot(t,Glucose_Conc)
title('Blood Glucose Profile')
xlabel('Time (min)')
ylabel('Blood glucose level (mg/dL)')

I get the error

Array indices must be positive integers or logical values.

Error in homeostasis_mod_1901465 (line 32)

Usage_Fraction(t) = a3* Insulin(t)^3 + a2* Insulin(t)^2 + a1* Insulin(t) + a0;

and im not sure how to correct it

[John D'Errico]

I'm not sure what you think this does:

Insulin(1==9000);    % mg initial insulin

Well, actually, I might giuess what you think it does. Regardless, whatever you may want it to do, is not what it does.

What else? We see this:

dt=1;   % time step (minutes)
t=0:dt:120; % time vector

The variable Insulin is a vector. It is NOT a function. Then you do this:

Usage_Fraction(t) = a3* Insulin(t)^3 + a2* Insulin(t)^2 + a1* Insulin(t) + a0;

We learend above, we learned the vector t starts out at zero.

However, MATLAB does not allow you to index a vector at element 0. What was the error message?

Array indices must be positive integers or logical values.

What should you have done?

I think you are trying to treat the vector Insulin as both a vector and a function. You cannot do that. Sorry.

[Walter Roberson]

Insulin_Secretion(t) == b3* Glucose(t)^3 + b2* Glucose(t)^2 + b1* Glucose(t) == b0;     % mg/min

That does not do whatever you might think it does. It contains no assignment operations, It contains two comparison operations, and after it is done with the calculation it throws the results of the calculations away.