[R] Discrete Event Simulation problem

[jism7690]

Hi All,

I am attempting to simulation an inventory model on R and I am having some
problems. I believe the problem is when I define my demand rate is stays
constant throughout so when I do need to reorder the model does not
recognise it as I have the initial supply arrival time set to infinity at
the start as no order has been played but I have an if statement saying, if
the level falls below a certain point then an order should be placed, I have
the expected time to order arrival set to be before the the sale but this is
not working.

Any Help would be appreciated. Thanks

maxStock = 100
minStock = 20
t.max=1100.0

LAST = t.max
START = 0

GetDemand<-function() START + runif(1,min=0,max=2)

main <- function(t.max,maxStock,minStock)
{
	index = 0
  t.current = START                               #### Starting Conditions
	t.demand = START
	t.supply = START
	inventory = 50
	order_costs = 0
	storage_costs = 0
	sales = 0
	sum = list(inventory = 50,order_costs = 0,storage_costs = 0,sales =0)

	while(index < LAST){
	index = index + 1
	t.demand = GetDemand()  ### expected time to next sale
	t.supply = Inf		### expected time to arrival of order, Infinity as order
has not been placed
	t.next =min(t.demand,t.supply)    ###next event either sale or supply is
the one with imminent arrival
	k = maxStock - inventory

  if(inventory > minStock)
  {
    	t.supply = Inf		### expected time to arrival of order, Infinity as
order has not been placed
  }
  if(inventory > 0)
      storage_costs = (t.next-t.current)*0.10*inventory
  t.current = t.next
  
	 if (inventory < minStock)
   {                       					###Need to Order
	 k = maxStock - inventory
	 order_costs = 50 + 0.02*k
	 sum$order_costs = sum$order_costs + order_costs
	 t.supply =  t.current + 1.0
    }

	if(t.current ==t.demand)
  {
  sum$inventory = sum$inventory - 1.0				                           
####Sale made
	sum$sales = sum$sales + 1.0
	t.demand = runif(1,min=0,max=2)
	}
	
	if(t.current == t.supply)
  {							                        ####Order Arrives
	sum$inventory = sum$inventory + k
	k = 0
	t.supply = Inf
	}
	
  if(inventory < maxStock)
  {
  k = maxStock - inventory
  sum$storage_costs = sum$storage_costs + storage_costs
  sum$order_costs = sum$order_costs + order_costs
  sum$inventory = sum$invneotry + k - sum$sales
  sum$sales = sum$sales + sales
  sum$sales = sum$sales + sales
  }

}
	sis = list(Time = index,StorageCosts =sum$storage_costs,OrderCosts=
sum$order_costs, FinalInventoryLevel=sum$inventory,sales=sum$sales ,
AverageCosts =((sum$order_costs + sum$storage_costs)/index))
	return(sis)
	}
main()

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