Why do I receive Index exceeds the number of array elements (1)?

close all
clear all
clc

tend = 10;

k = 0.1;
alpha = 0.09;
beta = 0.4;
gamma = 0.08;

t(1) = 0;
L(1) = 10;
R(1) = 10;
C(1) = 0;
i = 1;

while t(end) < tend
  
    props = zeros(1,4);
    props(1) = k * L(i) * R(i);
    props(2) = alpha * L(i);
    props(3) = beta * R(i);
    props(4) = gamma * C(i);
    
    prop_sum = sum(props);
    
    r1 = rand(1);
    tau = (1/prop_sum) * log(1/r1);
    
    t(i+1) = t(i) + tau;
    
    r2 = rand(1);
    
    if r2 * prop_sum <= props(1)
        L(i+1) = L(i) - 1;
        R(i+1) = R(i) - 1;
        C(i+1) = C(i) - 1;
        
        else if r2 * prop_sum > props(1) && r2 * prop_sum <= props(1)+props(2)
            L(i+1) = L(i) - 1;
        
            else if r2 * prop_sum > props(1)+props(2) && r2 * prop_sum <= props(1)+props(2)+props(3)
                R(i+1) = R(i) - 1;  
        
                 else if r2 * prop_sum > props(1)+props(2)+props(3) && r2 * prop_sum <= props(1)+props(2)+props(3)+props(4)
                    C(i+1) = C(i) - 1;   
                 end
            end
        end
    end
 
     i = i + 1;
    
end

plot(t,L,Y,C)

After I run the program, the command window shows

"Index exceeds the number of array elements (1).

Error in untitled4 (line 20)

props(1) = k * L(i) * R(i);"

What is the problem and how should I fix this error?

Answer 1

[John D'Errico]

You would be best served to learn to use the debugger. But I can see what happens.

At the first iteration, i = 1. And all of your variables are initially scalars, thus L, T, R, and C.

But inside the while loop, you add new elements to those variables, that were once scalars. But you ONLY append a new element under a specific condition. Thus, we see:

if r2 * prop_sum <= props(1)

What happens if that test fails?

If it fails, then L and C and P and R are NOT expanded.

But then you increment i. And you ALWAYS increment i on EVERY iteration. So what happens after the first iteration? i is now 2. But if that test failed, then those variables are still scalars. And on the second iteration, you try to access L(2), and C(2), and R(2).

Worse, you will continue to have this problem in the future for later iterations, every time that test fails. And since the test is dependent on a random value in r2, it may fail often enough.

So I have no idea what is your goal in writing this code. But it has a bug built into it. You need to rethink what you are doing.

Answer 2

[Awais Saeed]

Your L = 10(a scaler). After second iteration, i = 2 and L(2) does not exist. That is why you are seeing that error. Secondly, only a single if statement will execute per iteration and only one element will be appended to either L, R, or C. Which means if an element is added to R(3), then L and C remain the same and you cannot access L(3) and C(3) because they does not exist.

[John D'Errico]

Um, no. That is not the answer. Well, let me say that your answer misses the problem.

[Awais Saeed]

I completely meant what you said. I just made it short assuming that the OP wrote the code and he will pick up the point by hinting to the problem.