error with index and array elements

Hello all,

I am asking for your help with an error I keep getting with my code. First, here is my current code:

clear all;
close all;
clc;

% Analytical solution
t=0:600;                        % Makes time go from 0 to 600 sec
Tcoffee=(130*exp(-t/200)+70);   % The function for the temp. of the coffee

% Numerical solution
X0=1;                           % Initial state
T0=0;                           % Initial time
T=600;                          % Final time of 600 sec
h=30;                           % Time step of 30 sec
g=70;                           % Forcing function (room temp)
N=T/h;                          % Number of time steps
Thist=[T0:h:T];                 % Creates a vector of the solution
Xhist=zeros(1,N);               % Creates a row vector of zeros
Xhist(1)=X0;                    % Assigns the first element

for i=2:1:N+1
    
    X=Xhist(i-1);               % "Old" X from step i-1
    T=Thist(i-1);               % "Old" time from step i-1
    f=(130*exp(-T/200)+g);      % Dynamics
    Xhist=X+f*h;                % Euler's method
    
end

and the error i keep getting:

Index exceeds the number of array elements (1).

Error in Untitled (line 22)
    X=Xhist(i-1);               % "Old" X from step i-1

Lastly, some things to note are that the value for Xhist when i type it into the command window is 6001 and not a table. What is also weird is that if i add a % infront of

 g=70;                           % Forcing function (room temp)   

becomes a 1x20 double table but then I get the error:

 Unrecognized function or variable 'g'.

Error in Untitled (line 24)
    f=(130*exp(-T/200)+g);      % Dynamics

but if I change g in

    f=(130*exp(-T/200)+g);      % Dynamics

to be +70, Xhist goes back to being the number 6001 again. So if anyone can shed so light on why I'm having this issue, I would be most appreciative.

Answer 1

[Star Strider]

In this line:

Xhist=X+f*h;                % Euler's method

‘XHist’ is re-defined as a scalar. That’s causing the problem. It probably needs to be subscripted. I leave that to you.