# Error While running a while loop (index exceeds number of array elements)

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Mia Loposser on 26 Oct 2020
Edited: Cris LaPierre on 26 Oct 2020
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Mia Loposser on 26 Oct 2020
clear
close
%givens
E0 = 3; %input voltage
ls = 7*10^-14; %value of ls
R = 200; %value of resistor
C = .026; %value of KT/q
V_vec = [];
Vdiode(1) = 1; %initial guess for Vdiode
Vdiode(2)= 2;
i=1;
%iterate until difference between last 2 Vdiodes < 10^-6
while abs(Vdiode(i + 1) - Vdiode(i)) > 10^-6
% derivative of f
fprime = -1-ls*R/C * exp(Vdiode(i)/C);
% f at Vdiode
f = E0 - Vdiode(i) - ls * R * (exp(Vdiode(i)/C) - 1);
% new Vdiode
Vdiode(i) = Vdiode(i) - f/fprime;
% store in vector
V_vec(i) = Vdiode(i);
% increment i
i = i + 1;
end
Vdiode
length(V_vec)

Cris LaPierre on 26 Oct 2020
Edited: Cris LaPierre on 26 Oct 2020
You create variable Vdiode so that it contains a single value, 1. Then in your while condition, you try to compare the second value with the first. There is no second value, resulting in the error you see. I've transcribed the relevant parts of your code below and run it so you can see.
clear
Vdiode(1) = 1
Vdiode = 1
Vdiode(2)
Index exceeds the number of array elements (1).
##### 2 CommentsShowHide 1 older comment
Cris LaPierre on 26 Oct 2020
This is going to be a cyclical issue until you fix the underlying logic. Step through your code one loop at a time. There is an error around either when you increment i, or which two values you compare (right now you are including the next value. Perhaps you want to look at the previous value instead? This means (i-1) and i).