how can I improve the logic of this code

Im trying to build a relation between supply demand and price using 3 equations. the code I've generated is given below

clear all;
clc;
T=100;
P=zeros(1,T);
D=zeros(1,T);
R=zeros(1,T);
P(1)=10; %Initial Price 
D(1)=10; %Initial Demend
R(1)=5; % Initial Resource available
K=100; %Maximum demand 
M=10; %maximum resource
N=50; %Max Price K/N=2; N/K=0.5; N/M=5; M/K=0.1
for t=2:T
    D(t)=D(t-1)+((1-(P(t-1)/N))*(K/N));
    P(t)=P(t-1)-((1-(D(t-1)/K))*(N/K))+((1-(R(t-1)/M))*(N/M));
    R(t)=R(t-1)+((1-(D(t-1)/K)*(M/K)));
end
Xvals=1:T;
plot(Xvals,D,'b',Xvals,P,'r',Xvals,R,'g')
legend('Demand','Price','Resource')

when I run the code, the values get below zero. I want that whatever the results are but the values remain positive since neither of price nor demand nor resources can be negative. can anybody help me in improving these equations?

Answer 1

[Walter Roberson]

You could do things like

D(t) = max(0, D(t-1)+((1-(P(t-1)/N))*(K/N)) );

This will substitute 0 if the value would have been negative.

Answer 2

[Image Analyst]

To clip values to 0, use this code after the loop but before you call plot.

% If arrays are below zero, then clip arrays to 0.
D = max(D, 0);
P = max(P, 0);
R = max(R, 0);