Why do i keep on receiving the same error? Please show me how to fix the code!

I tried to work on the code based on the predator-prey with chaos case study but I still get error after countless times of fixing it.

[Alan Stevens]

Post the actual code, not images.

[Nur Maisarah]

% Parameter values
a = 1.2;
b = 0.6;
c = 0.8;
d = 0.3;

% Initial conditions and time span
y0 = [2; 1];
tspan = [10, 40];
h = 0.0625;

% Solve the equations using Euler's method
[tp_euler, yp_euler] = eulersys(@predprey, tspan, y0, h, a, b, c, d);

% Solve the equations using RK4 method
[tp_rk4, yp_rk4] = rk4sys(@predprey, tspan, y0, h, a, b, c, d);

% Plot the results
figure;

subplot(2, 2, 1);
plot(tp_euler, yp_euler(:, 1), 'b', tp_euler, yp_euler(:, 2), 'r');
title('Euler time plot');
xlabel('Time');
ylabel('Population');
legend('Prey', 'Predator');

subplot(2, 2, 2);
plot(yp_euler(:, 1), yp_euler(:, 2));
title('Euler phase plane plot');
xlabel('Prey');
ylabel('Predator');

subplot(2, 2, 3);
plot(tp_rk4, yp_rk4(:, 1), 'b', tp_rk4, yp_rk4(:, 2), 'r');
title('RK4 time plot');
xlabel('Time');
ylabel('Population');
legend('Prey', 'Predator');

subplot(2, 2, 4);
plot(yp_rk4(:, 1), yp_rk4(:, 2));
title('RK4 phase plane plot');
xlabel('Prey');
ylabel('Predator');

% Predator-Prey Models and Chaos

% Function for Lotka-Volterra equations
function yp = predprey(t, y, a, b, c, d)
yp = [a * y(1) - b * y(1) * y(2); -c * y(2) + d * y(1) * y(2)];
end

% Function for Euler's method
function [tp, yp] = eulersys(dydt, tspan, y0, h, varargin)
t = tspan(1):h:tspan(2);
N = length(t);
y = zeros(N, length(y0));
y(1, :) = y0;

for i = 1:N-1
    y(i+1, :) = y(i, :) + h * feval(dydt, t(i), y(i, :), varargin{:});
end

tp = t';
yp = y;
end

% Function for fourth-order Runge-Kutta method
function [tp, yp] = rk4sys(dydt, tspan, y0, h, varargin)
ti = tspan(1);
tf = tspan(2);
t = ti:h:tf;
N = length(t);
y = zeros(N, length(y0));
y(1, :) = y0;

for i = 1:N-1
    tt = t(i);
    hh = t(i+1) - t(i);
    
    k1 = h * feval(dydt, tt, y(i, :), varargin{:})';
    ymid = y(i, :) + k1./2;
    
    k2 = h * feval(dydt, tt + hh/2, ymid, varargin{:})';
    ymid = y(i, :) + k2./2;
    
    k3 = h * feval(dydt, tt + hh/2, ymid, varargin{:})';
    yend = y(i, :) + k3;
    
    k4 = h * feval(dydt, tt + hh, yend, varargin{:})';
    
    phi = (k1 + 2*(k2 + k3) + k4) / 6;
    y(i+1, :) = y(i, :) + phi';
end

tp = t';
yp = y;
end

Answer 1

[Torsten]

Your function "predprey" must return a row vector, not a column vector:

Use

function yp = predprey(t, y, a, b, c, d)
yp = [a * y(1) - b * y(1) * y(2), -c * y(2) + d * y(1) * y(2)];
end

instead of

function yp = predprey(t, y, a, b, c, d)
yp = [a * y(1) - b * y(1) * y(2); -c * y(2) + d * y(1) * y(2)];
end

[Nur Maisarah]

I tried with that code but still error occur:

predator_prey_script

Unable to perform assignment because the size of the left side is 1-by-2 and the size of the right side

is 2-by-2.

Error in eulersys (line 7)

y(i+1, :) = y(i, :) + h * feval(dydt, t(i), y(i, :)', varargin{:});

Error in predator_prey_script (line 10)

[tp_euler, yp_euler] = eulersys(@predprey, tspan, y0, h, a, b, c, d);

[James Tursa]

I haven't looked at your code, but if you are getting a 2x2 on the RHS then that may be the result of adding a 1x2 row vector to a 2x1 column vector. Double check that all of your vectors are either row vectors or column vectors for consistency.

[Torsten]

You can't get this error message because I tested the code and it worked after the change.

Although Euler and Runge-Kutta gave different results - but that's another issue.

[Nur Maisarah]

iT WORKED NOW THANK YOUU FOR THE HELP!

[Image Analyst]

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