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# Why do I keep getting the error message array indices must be positive integers or logical values?

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Thomas Clontz on 27 Sep 2019
Closed: MATLAB Answer Bot on 20 Aug 2021
Array indices must be positive integers or logical values.
Error in Drift (line 12)
X(h) = X(0) + mu*h + sigma*Z;
for the following code...
T = 10;
N = 100;
h = T/N;
mu = 0.1;
sigma = 0.2;
X = zeros(1,N+1);
for k = 1:100
for i = 1:N+1
Z = normrnd(0,h);
X(h) = X(0) + mu*h + sigma*Z;
end
plot(Time,X);
hold on
end
John D'Errico on 27 Sep 2019
MATLAB has an index origin of 1, not 0. So X(0) will fail, and give you exactly that error.

Matt J on 27 Sep 2019
Edited: Matt J on 27 Sep 2019
Looks like this line was accidentally using C-indexing syntax.
X(i) = X(1) + mu*h*(i-1) + sigma*Z;

John D'Errico on 27 Sep 2019
MATLAB has an index origin of 1, not 0. So X(0) will fail, and give you exactly that error. (Matt pointed that out.)
But also
T = 10;
N = 100;
h = T/N;
...
X = zeros(1,N+1);
X(h) = X(0) + mu*h + sigma*Z;
What is h? h=10/100 = 0.1.
X is a vector. Can you index a vector at the 0.1 location? Where is that? Sort of between location 0 (which does not even exist in MATLAB anyway) and memory location 1?
Sorry. Start with the getting started tutorials. You need to learn how MATLAB does indexing.

Catalytic on 27 Sep 2019
Edited: Catalytic on 27 Sep 2019
Why do I keep getting the error message array indices must be positive integers
Because the h in X(h) is non-integer and the 0 in X(0) is non-positive.