Merge three initial values into one code by runge-kutta

Question

Mariam Gasra 2019-5-8

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/460895-merge-three-initial-values-into-one-code-by-runge-kutta

回答： KSSV 2019-5-8

h=0.5;                                             % step size
x = 0:h:100;                                         % Calculates upto y(3)
Y = zeros(1,length(x)); 
%y(1) = [0.2;0.3;0.2];
y(1) = 0.2
; % redo with other choices here. 
    % initial condition
F_xy = @(t,r) 3.*exp(-t)-0.4*r;                    % change the function as you desire
for i=1:(length(x)-1)                              % calculation loop
    k_1 = F_xy(x(i),y(i));
    k_2 = F_xy(x(i)+0.5*h,y(i)+0.5*h*k_1);
    k_3 = F_xy((x(i)+0.5*h),(y(i)+0.5*h*k_2));
    k_4 = F_xy((x(i)+h),(y(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  
tspan = [0,100]; y0 = -0.5;
[tx, yx] = ode45(F_xy, tspan, y0)
plot(x,y,'o-', tx, yx, '--')

how can i run this code for three initial value and plot it in same graph?

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Answer 1

KSSV 2019-5-8

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/460895-merge-three-initial-values-into-one-code-by-runge-kutta#answer_374073

在 MATLAB Online 中打开

h=0.5;                                             % step size
x = 0:h:100;                                         % Calculates upto y(3)
ibc = [0.2;0.3;0.2];
Y = zeros(length(ibc),length(x));
for j = 1:length(ibc)
    y = zeros(1,length(x)) ;
    y(1) = ibc(j) ;
    % redo with other choices here.
    % initial condition
    F_xy = @(t,r) 3.*exp(-t)-0.4*r;                    % change the function as you desire
    for i=1:(length(x)-1)                              % calculation loop
        k_1 = F_xy(x(i),y(i));
        k_2 = F_xy(x(i)+0.5*h,y(i)+0.5*h*k_1);
        k_3 = F_xy((x(i)+0.5*h),(y(i)+0.5*h*k_2));
        k_4 = F_xy((x(i)+h),(y(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
    Y(j,:) = y ;
    
    % validate using a decent ODE integrator
    tspan = [0,100]; y0 = -0.5;
    [tx, yx] = ode45(F_xy, tspan, y0) ; 
    plot(x,y,'o-', tx, yx, '--')
    hold on
end