Array indices must be positive integers or logical values.
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Writing function for Runge Kutta 4th order. ERROR --> Array indices must be positive integers or logical values.
command window call is
dydx = @(x,y) (1+4*x)*sqrt(y)
RungeKutta(dydx, 0, 1,0.25,1)
code below
function RungeKutta(f, L, U,h,y0)
%f = function
% L = lower bound
% U = upper bound
% y0 = inital condition y value
%h = step size
t = L:h:U;
n = length(t);
% need for loop
k1 = zeros (1,n);
k2 = zeros (1,n);
k3 = zeros (1,n);
k4 = zeros (1,n);
y = zeros(1, n);
for i = 0: n
k1(i) = f(t(i),y(i));
k2(i) = f(t(i)+0.5*h,y(i)+0.5*k1(i)*h);
k3(i) = f(t(i) + 0.5*h, y(i)+0.5*k2(i)*h);
k4(i) = f(t(i) + h, y(i) + k3(i)*h);
y(i+1) = y(i) + (1/6)*(k1(i)+2*k2(i) + 2*k3(i)+k4(i))*h;
end %end of fo loop
hold on
title('Runge-Kutta Method')
xlabel('X axis')
ylabel('Y axis')
plot(t, f(t),'b', x, y, 'r')
legend('Original Plot', 'True Plot')
end % end of function
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Answers (1)
James Tursa
on 26 Mar 2019
Edited: James Tursa
on 26 Mar 2019
At least two problems. First, you need to start your for loop indexing at 1 instead of 0:
for i = 1: n-1 % ending at n-1 so that y stays at n elements
Second, you don't set the initial state of y before entering the loop. You need something like this prior to the loop:
y(1) = y0;
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