Not sure how to fix index out of bounds

Hey guys here is my code

%%Intitial Conditions
N = 1000;
a = 0; 
b = 2;
p = 0;
q = 0;
h = (b-a)/N;
x = linspace(a,b,N);
y = exp(-x.^2);
%%Simpsons Rule
S = (h/3).*y(1);
for i = 2:N/2
    p = p + (h/3).*y(2*i-1);
end

for i = 2:N/2-1
    q = q +(h/3).*y(2*i);
end
S = S + 4*p + 2*q + (h/3)*y(N);
disp(S)
%%Order of error
In(i) = S;
for i = 1:N   
r = (In(i)-In(2.*i))./(In(2.*i)-In(4.*i));
end
P = log(r)/log(2);
disp(P)

I get this error

??? Attempted to access In(500); index out of bounds because numel(In)=499.

Error in ==> Simp1 at 24 r = (In(i)-In(2.*i))./(In(2.*i)-In(4.*i));

I understand that In has a value at 499 and that it cant take on a value at 500 but Im clueless as to how to fix this.

Any assistance would be greatly appreciated.

Answer 1

[Guillaume]

A few problems with your code. First:

 In(i) = S;

Looks very suspicious. As i is the last value reached in the last for loop. If that's really what you wanted to do then

 In = zeros(1, 499);
 In(499) = S;

would be a lot clearer. But I suspect that's not what you wanted to do and you're missing somewhere a declaration of In.

Secondly:

 for i = 1:N   
   r = (In(i)-In(2.*i))./(In(2.*i)-In(4.*i));
 end

Three problems here.

- You haven't put anything in In so r = 0/0 = NaN

- Since i goes up to N = 1000, you need at least 4000 elements in In for In(4*i) to be valid

- You're overwriting r on each iteration of the loop.