Trying to plot R_K 4th order, but keep getting straight line?

The coding is below that i have used

clc;                                               % Clears the screen
clear all;

h=0.1;                                             % step size
x = 0:h:100;                                         % Calculates upto y(1)
s = zeros(1,length(x)); 

s(1) = 1;                                          % initial condition
i(1) = 0;
r(1) = 0;                                                    % initial condition
a = 0.180;
b = 0.037;
F_sir = @(s,i,r) -a*s*i;                                  % change the function as you desire
G_sir = @(s,i,r)  a*s*i- b*i;
H_sir = @(s,i,r) b*i;

for k=1:(length(x)-1)                              % calculation loop
    K_1 = F_sir(s(k),i(k),r(k));
    L_1 = G_sir(s(k),i(k),r(k));
    M_1 = H_sir(s(k),i(k),r(k));
    K_2 = F_sir(s(k)+0.5*h,i(k)+0.5*K_1,r(k)+0.5*h*L_1);
    L_2 = G_sir(s(k)+0.5*h,i(k)+0.5*K_1,r(k)+0.5*h*L_1);
    M_2 = H_sir(s(k)+0.5*h,i(k)+0.5*K_1,r(k)+0.5*h*L_1);
    K_3 = F_sir((s(k)+0.5*h),(i(k)+0.5*K_2),(r(k)+0.5*h*L_2));
    L_3 = G_sir((s(k)+0.5*h),(i(k)+0.5*K_2),(r(k)+0.5*h*L_2));
    M_3 = H_sir(s(k)+0.5*h,i(k)+0.5*K_2,r(k)+0.5*h*L_2);
    K_4 = F_sir((s(k)+h),(i(k)+K_3),(r(k)+L_3)); % Corrected        
    L_4 = G_sir((s(k)+h),(i(k)+K_3),(r(k)+L_3));
    M_4 = H_sir((s(k)+h), (i(k)+K_3), (r(k)+L_3)); 
    
    s(k+1) = s(k)+ (1/6)*(K_1+2*K_2+2*K_3+K_4)*h;  % main equation
    i(k+1) = (1/6)*(L_1+2*L_2+2*L_3+L_4)*h;
    r(k+1) = (1/6)*(M_1+2*M_2+2*M_3+M_4)*h;  % main equation
    
end
display(s(k+1));

Answer 1

[Alan Stevens]

Check these equations

F_sir = @(s,i,r) -a*s*i;                                  % change the function as you desire
G_sir = @(s,i,r)  a*s*i- b*i;
H_sir = @(s,i,r) b*i;

If you start with i = 0 these will all stay at zero.

Also, although you have r as an argument, it doesn't appear anywhere in the definition of these functions.

[Wasim Raza]

Thanks now it's working