Modelling reactions usting randsamples

I am quite new on matlab so sorry for bothering you. I am trying to model the following reactions (all are molecules)

A+A-----C rate constant k1=0.001 A decreases by 2 Also given that a reaction happens the probability is k1*A(A-1)*R

A+B-----D rate constant k2=0.01 A and B decreases by 1 probability is A*B*(k2)/(R

u-----A rate constant k3=1.2 A increases by 1 probability is (k3)/R

u----B rate constant k4=1) B increases by 1 probability is (k4)/R

And t can be calculated from the exprnd function

Now i have that the total rate is R= A(A-1)*k1+AB*k2+k3+k4

The thing i need to is to create an array of three columns time, A and B and an appropriate number of columns. Now using the rate constants above create a for loop, calculate total R, calculate t from exprnd function, and finally use a random sample to determine which reaction occurs at t+dt. Then i would need to update the values of A, B and time in my inital vector accordingly. PLEASE i am quite new with matlab may someone help me writing this code!

I have written the following, but its just producing the final value of A and B

iterations= 15

A= 2

B= 2

T=0

tC= zeros([iterations+1,3])

%tC(1,:)= [T,A,B]

k1=0.001;

k2=0.01;

k3=1.2;

k4= 1;

for n=1:iterations

    rate1= k1*A.*(A-1);

    rate2= A.*B*k2;

    rate3= k3;

    rate4= k4;

    rate=A.*(A-1)*k1+A.*B*k2+k3+k4; 

    taus=exprnd(1./(k*rate),[n+1,1]);

    p1=k1*A*(A-1)/rate;

    p2=A*B*k2/rate;

    p3=k3/rate;

    p4= k4/rate;

    y=randsample([1,2,3,4],1,true,[p1,p2,p3,p4]);

    if y==1

        A=A-2

    elseif y==2

        A=A-1

        B=B-1

    elseif y==3

        A=A+1

    elseif y==4

        B=B+1

    end

        A=(A)

    end

tC(:,1)=taus

tC(:,2)=A

tC(:,3)=B

PLEASE HELP ME