How can I get N to be a different value for a different t?

Im working on an SIR model for my physics lab. And to get a sort of general idea behid quarantine measures, I would want N in the following code to change for different intervals, ie be N=100000 for t>=0 & t<100 and N=30000 for t>100 but am unsure how to implement this. Keep in mind coding is very very new to me and would like some simple models.

%To solve the equations dS/dt = -(k/N)*S* I, dI/dt=(k/N)*S*I-g*I, dR/dt=g*I
%I defined values for k, N, and g in accordance with Alexander Ware's work to see if I could reach a similar graph
k=3/14;  
N=100000;
g=1/14;
t=linspace(0,365,365);
f = @(t,x) [-(k/N)*x(1)*x(2);(k/N)*x(1)*x(2)-g*x(2);g*x(2)];
%The @(t,x) term is just the function handle. Here I have created a function with three pieces, each representing the right sides of the equations above. 
%In place, I have x(1)=S, x(2)=I, and x(3)=R
[t,xa]=ode45(f,[0 365], [N 500 0]);
%ode45 takes each term in the function f and integrates the function from 0 to 365. [N 500 0] represents the inital conditions for S, I, and R respectively
a1=plot(t,xa(:,1)); M1='S';
hold on
%Hold allows for the addition of more plots without deleting the first
a2=plot(t,xa(:,2),'k'); M2='I';
a3=plot(t,xa(:,3),'r'); M3='R';
hold off
%Blue curve represents S, red curve represents R, and black represents I.
legend([a1, a2 a3], M1, M2, M3)

Answer 1

[darova]

Try this modification