Page 33 - DCAP601_SIMULATION_AND_MODELING

Unit 1: Introduction to System Simulation

an order (to be delivered three days hence), by stating the amount ordered and the day it is due Notes
to be received. We repeat this procedure for 180 days’. Initially we set day number i = 1, stock
S = 115, number of units due UD = 0 (because there is no outstanding order), and the day they are
due DD=0.
The demand, DEM, for each day is not a fixed quantity but a random variable. It could assume
any integral value from 00 to 99 and each with equal probability. We will use a special subroutine,
which generates a 2-digit random integer, and will use the numbers thus produced as the daily
demands.
The following Fortran program (format free) implements the flowchart for evaluating the total
cost for a given replenishment policy (P, Q) for 180 days. Statement No. 110 in the program
makes use of the subprogram RNDY1 (DUM) which is a subprogram to generate a real
pseudorandom number between zero and one. The argument of this function can be any dummy
number or variable.
Notice how condition 6, that there is no more than one reorder outstanding, has been taken care
of. In Statement 130, we add the number of units due (if any) UD to the current stock S to get the
equivalent stock, ES. It is this number which is compared to P before an order is placed. Since
UD> P if we already have a replenishment order outstanding another order will not be placed.
This is an extremely simple model of an inventory-control system.
INTEGER P, Q, S, ES, UD, DD. DEM
READ, P, Q
C = 0.0
S = 115
I = 1
UD = O
DD = O
100 IF (DD .NE. I) GO TO 110
S = S + Q
UD = 0
110 DEM = RNDY1 (DUM)* 100.0
IF (DEM .LE. S) GO TO 120
C = C + (FLOAT (DEM) – FLOAT (S))*18.0
S = 0
GO TO 130
120 S = S – DEM
C = C + FLOAT (S)*.75
130 ES = S + UD
IF (ES .GT. P) GO TO 140
UD = Q
DD = I + 3
C = C + 75.0
140 I = 1 + 1
IF (I .LE. 180) GO TO 100
PRINT, P, Q, C
STOP
END

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