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Unrecognized function or variable 'ode4'.

31 vues (au cours des 30 derniers jours)
Khang Nguyen
Khang Nguyen le 11 Oct 2022
Commenté : Torsten le 11 Oct 2022
I am trying to use ode4 to validate with the Runge-Kutta method, but keep getiing this error when I call ode4 "Unrecognized function or variable 'ode4'."
h=0.1; % step size
x = 0:h:60;
%y(1) = [-0.5;0.3;0.2];
y(1) = -0.5;
F_xy = @(t) -3*cos(t/2) + 4*sin(t/2) + 1;
for i=1:(length(x)-1) % calculation loop
k_1 = F_xy(x(i));
k_2 = F_xy(x(i)+ 0.5*h*k_1);
k_3 = F_xy((x(i)+0.5*h*k_2));
k_4 = F_xy((x(i)+k_3*h));
y(i+1) = y(i) + (1/6)*(k_1+2*k_2+2*k_3+k_4)*h; % main equation
end
% validate using a decent ODE integrator
y0 = -0.5;
yx = ode4(F_xy, 0,h,60, y0)
plot(x,y,'o-', x, yx, '--')

Réponse acceptée

Walter Roberson
Walter Roberson le 11 Oct 2022
Modifié(e) : Walter Roberson le 11 Oct 2022
  2 commentaires
Khang Nguyen
Khang Nguyen le 11 Oct 2022
Modifié(e) : Khang Nguyen le 11 Oct 2022
I still get the same error.
h=0.1; % step size
x = 0:h:60;
F_xy = @(t) -3*cos(t/2) + 4*sin(t/2) + 1;
y0 = -0.5;
L = length(x)
tspan = linspace(0,60,L)
yx = ode4(F_xy, tspan, y0)
Walter Roberson
Walter Roberson le 11 Oct 2022
Did you download the .zip file from that Question, and unzip it and place the directory on your MATLAB path ?

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Plus de réponses (1)

Torsten
Torsten le 11 Oct 2022
Modifié(e) : Torsten le 11 Oct 2022
Look below at how k_1,...,k_4 must be computed in the case that your ODE function only depends on t, not y.
The classical Runge-Kutta boils down to the usual Simpson's rule.
h=0.1; % step size
x = 0:h:60;
%y(1) = [-0.5;0.3;0.2];
y(1) = -0.5;
F_xy = @(t) -3*cos(t/2) + 4*sin(t/2) + 1;
for i=1:(length(x)-1) % calculation loop
k_1 = F_xy(x(i));
k_2 = F_xy(x(i)+0.5*h);
k_3 = F_xy(x(i)+0.5*h);
k_4 = F_xy(x(i)+h);
y(i+1) = y(i) + (1/6)*(k_1+2*k_2+2*k_3+k_4)*h; % main equation
end
% validate using a decent ODE integrator
y0 = -0.5;
yx = ode4(@(t,y)F_xy(t), 0,h,60, y0);
plot(x,y,'o-', x, yx, '--')
function yout = ode4(F,t0,h,tfinal,y0)
% ODE4 Classical Runge-Kutta ODE solver.
% yout = ODE4(F,t0,h,tfinal,y0) uses the classical
% Runge-Kutta method with fixed step size h on the interval
% t0 <= t <= tfinal
% to solve
% dy/dt = F(t,y)
% with y(t0) = y0.
% Copyright 2014 - 2015 The MathWorks, Inc.
y = y0;
yout = y;
for t = t0 : h : tfinal-h
s1 = F(t,y);
s2 = F(t+h/2, y+h*s1/2);
s3 = F(t+h/2, y+h*s2/2);
s4 = F(t+h, y+h*s3);
y = y + h*(s1 + 2*s2 + 2*s3 + s4)/6;
yout = [yout; y]; %#ok<AGROW>
end
end
  3 commentaires
Walter Roberson
Walter Roberson le 11 Oct 2022
The error between what two values? You cannot calculate the error unless you have an analytic solution.
h=0.1; % step size
x = 0:h:60;
y(1) = -0.5;
F_xy = @(t) -3*cos(t/2) + 4*sin(t/2) + 1;
syms t C
intF = int(F_xy(t), t)
intF = 
eqn = subs(intF, t, 0) + C == y(1)
eqn = 
sol = solve(eqn)
sol = 
intF = subs(intF + C, C, sol)
intF = 
... that should be the analytic solution.
Torsten
Torsten le 11 Oct 2022
If you compare your code and ode4, you'll see that they are identical. So there should be no difference in the results.

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