Hi I'm new to matlab and I'm trying to run this code for Runge Kutta but I get an error at line 14.

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Nemo
Nemo 2016-5-4
回答： Walter Roberson 2016-5-4
function rk4_systems_trial()
alpha = [1 -1 2];
m = size(alpha,1);
if m == 1
alpha = alpha';
end
h = 0.02; %the step size
x(1) = 1; %initial conditions
w(:,1) = alpha; %col 1 of matrix w
for i = 1:4 %i=1,2,3,4
k1 = h*f(x(i), w(:,i));
k2 = h*f(x(i)+h/2, w(:,i)+0.5*k1);
k3 = h*f(x(i)+h/2, w(:,i)+0.5*k2);
k4 = h*f(x(i)+h, w(:,i)+k3);
w(:,i+1) = w(:,i) + (k1 + 2*k2 + 2*k3 + k4)/6;
x(i+1) = 1 + i*h;
end
[x' w']
function dy = f(x, y)
dy = [y(2);
y(3);
y(1)*(x+1)^2 +y(2)*(x+1) + y(3)*6 + cos(ln(x+1))];

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回答（1 个）

Walter Roberson
Walter Roberson 2016-5-4
MATLAB uses log rather than ln
In your lines
dy = [y(2);
y(3);
y(1)*(x+1)^2 +y(2)*(x+1) + y(3)*6 + cos(ln(x+1))];
the space between the ^2 and the +y(2) is being interpreted as if you have two adjacent array elements, sort of like if you had written
dy = [1;
2;
3 4]
This would not happen if you used a space after the "+",
dy = [y(2);
y(3);
y(1)*(x+1)^2 + y(2)*(x+1) + y(3)*6 + cos(log(x+1))];
Inside [], when there is a space followed by a "+" or a "-" that is not followed by a space, and the whole is not inside (), then the space is interpreted as indicating a new element, and the "+" or "-" are taken as "unary plus" or "unary minus". It is the difference between
[3 +4] or [3 -4]
and
[3 + 4] or [3 - 4] or [3+4] or [3-4]

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