MATLAB: Index exceeds the number of array elements (1264).

elementsindexMATLAB

I have been trying to work on a function however it continues to get stuck on this section. I have made sure that the matrix/vector isn't starting on zero, but I dont understand why I continue to get this error.

i=1;
for f=1:length(x)
    s(i)=x(i)+x(i+1);
    i=i+1;
end

Best Answer

Karlie - f iterates from 1 to the length of your x array. This is fine except for

s(i)=x(i)+x(i+1);

because when i (and you could use f here) is the length of your array, then i+1 is one larger than the length and so x(i+1) is invalid giving you the above error message. Instead, try something like

for k=1:length(x)-1
    s(k)=x(k)+x(k+1);
end

where we limit k to the interval [1, N-1] where N is the length your x array.

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What is the signature for ode1? I can't seem to find it in the doc. Where is your derivative function implemented? I don't see it in your code. If you look at the following link for RK4, the k's represent function derivatives at various predicted points, not integrated values as you seem to be doing.

http://en.wikipedia.org/wiki/Runge_kutta

E.g., I would have expected to see a derivative function something like this in your code somewhere:

function yp = y_deriv(x,y)
  yp= [y(2);-7*y(1)-0.5*y(2)];
end

And this derivative function would be used to calculate the k's. But I don't see anything like this in your code. E.g.,

    k1(:,ii)=y_deriv(x(ii),y(:,ii));
    k2(:,ii)=y_deriv(x(ii)+.5*h5,y(:,ii)+k1(:,ii)*.5*h5);
    k3(:,ii)=y_deriv(x(ii)+.5*h5,y(:,ii)+k2(:,ii)*.5*h5);
    k4(:,ii)=y_deriv(x(ii)+h5,y(:,ii)+k3(:,ii)*h5);

Also looks like you are missing a *h5 on the last line. (you could combine the last two lines into one vector equation)

Best Answer

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