CHOOSE Command / START

and

SELECT OK

Your plot should look like this.

Figure 16—2. Sine Wave Plot.

As you can see, the familiar sine wave unfolds for both variables. When the simulation completes, examine the Standard Report. Compare the x_ variable with the Savevalue x_exact, which is calculated from the analytic solution. Do the values agree?

When you have the solution to the differential equations, consider yourself lucky and use it everywhere you can. Most real world systems of equations are much too unwieldy for that. Usually, you will be forced to play out the integration numerically, using one or more INTEGRATE Commands, which takes a lot more computer time.

Now, close all open windows.

We.., that's all for now. In the next lesson, we will learn about GPSS World's built-in Programming language called PLUS.

Lesson 17 - PLUS

This lesson considers PLUS, the Programming Language Under Simulation. See the "PLUS Primer" at the end of this lesson for practical examples of the use of PLUS.

As we have learned, Model Statements are the building blocks of Models, and they can be sent interactively to an existing simulation.

However, the only PLUS Statements that are Model Statements are the

CHOOSE File / Save As

Then

TYPE Fox

for the name of the model and

SELECT Save

CHOOSE Command / Create Simulation

We have included the Procedure SetPop as part of the Model and then created the Simulation. Any invocations of SetPop interactively or within the simulation will now use this definition.

CHOOSE Command / SHOW

and in the dialog box

TYPE SetPop(100)

and

SELECT OK

Notice that the return value from the SetPop Procedure was 0. Let’s look at the population level.

CHOOSE Command / SHOW

and in the dialog box

TYPE Foxes

and

SELECT OK

See? The User Variable Foxes was updated when we did the first SHOW. Now lets redefine the SetPop Procedure interactively.

CHOOSE Command / Custom

TYPE PROCEDURE SetPop(level)

Foxes = level # 10 ;

and

SELECT OK

Now, when the SetPop Procedure is invoked, the population will be set to ten times the argument. Again

CHOOSE Command / SHOW

and in the dialog box

TYPE SetPop(100)

and

SELECT OK

Notice that the return value from the SetPop Procedure was 0 again. We could have inserted a RETURN Statement into SetPop. In fact, SetPop could be

nothing more than a single RETURN Statement.

Let’s look at the population level.

CHOOSE Command / SHOW

and in the dialog box

TYPE Foxes

and

SELECT OK

As you can see, although the simulation had already been sent to the Simulation Object, we were able to redefine the Procedure interactively. This is very useful when you need to debug your own PLUS Procedures, as we will discover next.

Debugging PLUS Procedures

Let’s define a Procedure with an error in it.

CHOOSE File / New and since Model is already highlighted

SELECT OK

Enter the following PLUS Procedure.

TYPE PROCEDURE RabbitRate() BEGIN

TEMPORARY BirthRate, DeathRate, TotRate; BirthRate = 100;

DeathRate = 80 ;

RETURN TotRate; END;

Don’t forget the final semicolon. Now, save the model.

CHOOSE File / Save As

Then for the name of the file

TYPE Rates

SELECT Save

Next create the simulation

CHOOSE Command / Create Simulation

So far, so good. If you inadvertently made typing errors, use Search / Next Error in the Model Window to find and correct them. Now invoke the Procedure.

CHOOSE Command / SHOW

and in the dialog box

TYPE RabbitRate()

and

SELECT OK

We got an error message. Take a close look at each line in the message. It tells us the line number where the error occurred. Let’s put the cursor at that point. Make sure that the Model Window has the focus.

CHOOSE Search / Go To Line

TYPE 5

and

SELECT OK

Apparently, this line is trying to use a value from a Temporary User Variable that was never initialized. As we can see this is true for the TotRate Variable. We forgot to initialize it. Just above the line in error,

TYPE TotRate = BirthRate - DeathRate ;

This time we are NOT going to Retranslate the Model. We are going to redefine the Procedure and still maintain the existing simulation environment. We do this by using an Interactive INCLUDE Command. This may seem cumbersome, but it is meant to show you the power of the INCLUDE Command.

First, copy the text in the Model Window to the Window Clipboard. Next, we will create a text file.

CHOOSE File / New

SELECT Text File

SELECT OK

and then paste the Clipboard contents into the new Text File and save the file.

CHOOSE File / Save As

TYPE RateDef

for the file name and

SELECT Save

This creates a TEXT file that can be used as the Include File. All Include Files must be text files. Now, let’s redefine SetPop( ) in the existing simulation. That means using INCLUDE, not Retranslate.

CHOOSE Command / Custom

and in the dialog box

TYPE INCLUDE "RateDef.txt"

and

SELECT OK

So far, so good. Notice that we did not Retranslate. Now invoke the Procedure.

CHOOSE Command / SHOW

and in the dialog box

TYPE RabbitRate()

There. That fixed the problem. It was easy to redefine the Procedure interactively. Although it was not so important here, in a big simulation you may not want to disturb the simulation. That’s when interactive redefinition really pays off. In such cases, you may want to load an INCLUDE Command into a function key, and use it to read in a small file containing nothing but the test definition of the Procedure. The main thing to remember is that you must save changes before you use INCLUDE. A DO Command is also useful for debugging Plus Procedures. See Chapter 8 in the Reference Manual.

You may find it convenient to make minor modifications to the PLUS Procedure when you are testing it. First, you may want to "comment out" TEMPORARY Statements, as long as none of the Temporary User Variable or Matrix Names clash with global entities. If there is a conflict, global variables could be altered. It is best to keep the Temporary names unique. Simply place /* at the beginning of each TEMPORARY or TEMPORARY MATRIX Statement, and */ after it. This will cause your intermediate results to be available after the Procedure ends. Second, you can add additional Assignment Statements and RETURN Statements to stop the Procedure wherever you like.

Then to test the results, all you have to do is redefine the Procedure and invoke it. Each action could be done with a single stroke of a Function Key. The highly interactive GPSS World environment makes Procedure redefinition an effective debugging technique.

A few other items of interest include the fact that PLUS Procedures can call other PLUS Procedures and can be accessed from any block operand in your model as well as from the PLUS block.

Hopefully, reviewing the example models in this lesson and the examples in the Plus Primer that follows will help you to understand some of the potential of PLUS. Also, the sample model PREDATOR.GPS in Chapter 2 of this manual uses a PLUS Procedure. There’s a World of power waiting for you.

Lesson 18 - Debugging Your

Models

GPSS World’s interactivity makes debugging and the testing of design alternatives much easier than in older versions of GPSS. In the last lesson we saw how much easier it is to debug PLUS Procedures in an interactive environment. In this lesson, we’ll try a few simple debugging techniques that can be very useful when you apply them to complex models.

The model is shown below.

;GPSS World Sample File - BARBER.GPS.

***********************************************************************

* *

* Barber Shop Simulation *

* *

***********************************************************************

Waittime QTABLE Barber,0,2,15 ;Histogram of Waiting times GENERATE 3.34,1.7 ;Create next customer

TEST LE Q$Barber,1,Finis ;Wait if line 1 or less

;else leave shop

SAVEVALUE Custnum+,1 ;Total customers who stay

ASSIGN Custnum,X$Custnum ;Assign number to customer

QUEUE Barber ;Begin queue time

SEIZE Barber ;Own or wait for barber

DEPART Barber ;End queue time

ADVANCE 6.66,1.7 ;Haircut takes a few minutes

RELEASE Barber ;Haircut done. Give up the barber

Finis TERMINATE 1 ;Customer leaves

CHOOSE File / Open

SELECT BARBER

and

SELECT Open

Before we go any further, let’s put some SHOW commands into Function Keys. Open the Model Settings Notebook

CHOOSE Edit / Settings and select the Function Keys page.

Next to s and t type the following two SHOW Commands. Place the mouse pointer in each box and click once before typing.

TYPE SHOW P$Custnum

and

TYPE SHOW X$Custnum

SELECT OK

You have now loaded SHOW commands into two function keys. While debugging a simulation you can interactively examine many values at a touch of a single key. We’ll use them later.

Now create the simulation.

CHOOSE Command / Create Simulation

Let’s also open the Blocks Window. In the Main Menu

CHOOSE Window / Simulation Window / Blocks Window

Now, let’s put a Stop Condition on Transaction 5.

CHOOSE Command / Custom

and in the dialog box

TYPE Stop 5

SELECT OK

You should also arrange the windows so you can see both the Blocks Window and the Journal Window.

Your screen should look something like this.

Figure 18—1. The Blocks, and Journal Windows in BARBER.GPS.

In the Main Menu

CHOOSE Command / START

and in the dialog box replace the 1

TYPE 100

and

SELECT OK

Now you see a message indicating that Transaction 5 has stopped on its first attempted Block entry in the simulation. In this case it happens to be the first GENERATE Block.

Figure 18—2. Message after Stop is executed in the Journal Window.

Now, we’ll step through the simulation using the function key that is loaded in the Model Settings Notebook with STEP 1. If you want to check the list of preassigned function key settings, go to the Function Keys Page of the Model Settings Notebook.

Now that we have control of the active Transaction, let’s first take off the Stop Condition.

CHOOSE Window / Simulation Snapshot / User Stops

CLICK ON The "5"

CLICK ON The Remove Button

SELECT OK

PRESS p

You could also execute this command by pressing b+a + S or choosing Command / STEP in the Main Menu.

Transaction 5 will enter the GENERATE Block. You’ll be able to see this in the trace messages in the Journal Window. Now STEP one more time to cause the Transaction to enter the TEST Block.

PRESS p

When this Transaction enters the TEST Block, it will find that the queue for the barber is greater than 1. This customer will then choose not to wait and will leave the shop. You will see it has been scheduled for the TERMINATE Block.

Now, before going on, let’s examine some other values related to the simulation.

CHOOSE Command / SHOW

and in the dialog box

TYPE Q$Barber

and

SELECT OK

and see that the size of the queue (2) is returned to the Status Line in the Main

Window, but can also be seen in the Journal Window. Now, let’s use the SHOW Commands that we loaded into the function keys.

PRESS t

The value 4 will be returned in the Status Line. This tells us the number of customers who have stayed at the shop. Remember, you can also use the Expressions Window to watch a whole series of values as you step through a simulation. The Expressions Window was discussed in detail in the Lesson 11. Now let’s continue to Step through the simulation.

PRESS p

to move Transaction 5 through the TERMINATE Block. Now let’s look at the number of customers that have stayed in the shop

PRESS t

The value returned will be 4. Transaction 5 did not wait and was not added to our counter.

We’ll use the preloaded function key for CONTINUE to do this

PRESS m

If the Blocks Window is open, the simulation will run more slowly than with all the windows closed. Close the Blocks Window now, but minimize the Journal Window for later use.

CLICK ON The X-Upper Right of the Blocks Window

then

CLICK ON The leftmost of the three buttons-Upper Right of the Journal Window

The simulation will quickly run to completion and a report will be written. Let’s also take a quick look at one of the other windows, the Table Window.

CHOOSE Window / Simulation Window / Table Window

and in the drop-down box, you’ll see Waittime, since this is the only Table in the model.

SELECT OK

In this particular histogram you can see the wait times experienced in the queue for the barber. Mean waiting time was 10.709 minutes with a standard deviation of 2.702 minutes. The Table Window is dynamic, and is updated as the state of the simulation changes.

Figure 18—3. The Waittime Table

Before we go on, let’s restore the Journal Window. On the Desktop CLICK ON The Minimized Icon in the lower left of the screen

Now we can watch Journal messages as we work a little more with the use of Stops.

CHOOSE Window / Simulation Window / Blocks Window and open the Blocks Window to a comfortable viewing size.

Now, in the Blocks Window select the RELEASE Block (Block number 9) in the model. You can do this by moving the mouse pointer over the Block labeled REL and clicking mouse button 1.

CLICK ON The Release Block in the Blocks Window then

CLICK ON The Place Icon in the Debug Toolbar

A Stop Condition has now been put on Block 9. You can do this through the Command menu in the Main Window, as well using a Custom Command, but using the Debug Toolbar is easier. Now start the simulation.

CHOOSE Command / START

and in the dialog box replace the 1

TYPE 100

and

SELECT OK

The simulation will now stop when the first Transaction is ready to enter the RELEASE Block.

As you can imagine, these techniques will become even more valuable as your model becomes more complex.

The change will be implemented immediately. First Now let’s alter the structure

of the model without leaving the Session.

PRESS s

Remember, we loaded a SHOW Command for the customer number Parameter

into s early in the lesson. The value returned in the Status line of the Main Window is 54 for the Customer Number.

Let’s remove the STOP on the RELEASE Block.

CHOOSE Window / Simulation Snapshot / User Stops

CLICK ON The "9"

CLICK ON The Remove Button

SELECT OK

Now let’s assume you have decided you no longer want each Transaction to carry the customer number in a Parameter. We’ll eliminate the ASSIGN Block. In the Model Window, position the mouse at the beginning of the ASSIGN Block line.

CLICK and DRAG The Cursor to the End of the Line

and

PRESS c

Now in a flash you can retranslate the model and start it running again.

CHOOSE Command / Retranslate

and then

CHOOSE Command / START

and in the dialog box replace the 1

TYPE 11111

and

SELECT OK

Let the simulation run for a second or two and

PRESS o

to halt the simulation and then

PRESS s

Chances are you got an error message saying that the Parameter doesn’t exist since it is no longer being assigned to new Transactions. Let’s take a look at a way we can make interactive changes in a simulation without having to retranslate the model. We’ll read in a new model to examine this option.

The model below deserves a little study before you see how GPSS World allows interactive changes to your simulation while you are in the development and test phases of a project. Take a minute to look at the model shown below.

; GPSS World Sample File - SAMPLE7.GPS

**********************************************************************

**

*Automobile Arrival Simulation *

**

*For simplicity, this model only deals with one-way traffic *

*in North-South and East-West directions. *

**********************************************************************

GENERATE 20,10 ;Create next automobile. QUEUE Eastwest

TEST E X$EWlight,F$Intersection ;Block until green, and

*the intersection is * * free

SEIZE Intersection

DEPART Eastwest ;End queue time. ADVANCE 10 ;Cross the intersection. RELEASE Intersection

TERMINATE 1 ;Auto leaves intersection.

*

GENERATE 30,10 ;Create next automobile. QUEUE Northsouth

TEST E X$NSlight,F$Intersection ;Block until green and

*the intersection is free SEIZE Intersection

DEPART Northsouth ;End queue time. ADVANCE 10 ;Cross the intersection. RELEASE Intersection

TERMINATE 1 ;Auto leaves intersection.

**********************************************************************

**

*Traffic Light Simulation *

**

**********************************************************************

GENERATE ,,,1

Begin1 SAVEVALUE NSlight,Red ;North-South light turns red SAVEVALUE EWlight,Green ;East-West light turns green ADVANCE Greentime ;Light is green

SAVEVALUE NSlight,Green ;North-South light turns green SAVEVALUE EWlight,Red ;East-West light turns red ADVANCE Redtime ;Light is red

TRANSFER ,Begin1 Greentime EQU 200

*When the light is Green (value 0) and the intersection is not busy

*(the State Variable(SNA) F$Intersection evaluates as 0), a car may

*pass into the intersection. These conditions are tested at the TEST

*Block. When the light is red (value 100) or the intersection is busy

*(SNA F$Intersection returns the value 1), the condition at the TEST

*Block will not be met and the car will not proceed.

Green EQU 0

Red EQU 100 Redtime EQU 300

*

*Do START 4000 EW Congestion builds. Try Greentime EQU 1000. Fine

*but NS congestion builds. Greentime EQU 400 works for both.

This model is of an intersection of two one-way streets. By changing the length of the green light, you can experiment with the effects on the traffic flow. We will open the Plot Window with two variables, one for North-South traffic and one for East-West traffic.

First, Open the model

CHOOSE File / Open

and

SELECT SAMPLE7

and

SELECT Open

Then create the simulation and open the Plot Window.

CHOOSE Command / Create Simulation

and

CHOOSE Window / Simulation Window / Plot Window

When the Edit Plot Window appears, enter the values you see below. Remember to position the mouse pointer at the start of each line and click once

before you start to type or use the v key to go from box to box in a forward direction. Don’t try to use e to go from box to box.

Figure 18—4. The Edit Plot Window.

The label field contains the label used in the legend at the bottom of the plot Next, the Expression box indicates the variable to be plotted. Finally, in the Title field we’ll choose a name that will describe both items that we want to plot, namely the Queues of cars in both directions. The Y-axis values have defaults of 0 and 100. We’ll change only the Max value to 150.

Finally, the time range for each view of the plot must be added. Our time range is 8000. You will want to make this time duration long enough so that it does not flash by as the simulation runs, but not so long that your plot is jammed together. You may have to experiment once or twice to get the correct value. The Plot Window will page horizontally when it reaches the end of the window. You should also be aware that by default only 10,000 plot points are saved for each expression. You can change this setting in the Model Settings Notebook. If you refresh or resize your window or do anything that causes the window to refresh automatically, such as dragging another window over the top of the Plot Window, only the remembered points can be redrawn. Of course, the more points you save, the more memory you will use for this purpose. Since GPSS World makes huge amounts of memory available to you, this is not usually a problem.

To add these values to the plot

CLICK ON The Plot button

and ifyou want to save the information with this simulation

PRESS o

Open up a Custom Command dialog box

CHOOSE Command / Custom

and

TYPE Greentime EQU 1000

SELECT OK

then

PRESS m

This will cause the CONTINUE command to be executed. Now notice that North-South congestion builds. Interrupt the simulation again

PRESS o

Open up a Custom Command dialog box

CHOOSE Command / Custom

and

TYPE Greentime EQU 400

SELECT OK

then

PRESS m

Now the traffic is flowing reasonably in both directions. Let the simulation run to completion or you can halt it when you are done watching the Plot.

PRESS o

if you prefer.

Close all Windows related to the Traffic Model.

CLICK ON The X-Upper Right of each Window

GPSS World can also call in Command Files consisting of INITIAL statements or other Commands. You can do this either by interactively typing an INCLUDE Command in a Custom Command dialog box or by placing this statement in a Model File.

Read in the Barbershop model.

CHOOSE File / Open

and in the dialog box

SELECT BARBER

then

SELECT Open

Create the simulation

CHOOSE Command / Create Simulation

The following is an example of the command to call in an ASCII file containing Initial Statements as described above. The first command shows how it would be done interactively, the second as part of your program.

CHOOSE Command / Custom

and in the dialog box

TYPE INCLUDE "INIT.TXT"

SELECT OK

Remember, if you have altered the location of the sample files, then, you must assert the file path in this INCLUDE Command.

The contents of this file are: INITIAL X$One,45

INITIAL X$Two,765

Now show the values of these Savevalues. The SAVEVALUE entities are created as they are given their initial values.

CHOOSE Command / SHOW

and in the dialog box

TYPE X$One

SELECT OK

You should see the value 45 in the Status Line of the Main Window and in the Journal Window. Repeat this process to see the second SAVEVALUE.

CHOOSE Command / SHOW

and in the dialog box

TYPE X$Two

SELECT OK

and check the value in the Status Line.

Add the following line as the last line of the model that you currently see in the Model Window

INCLUDE "INIT.TXT"

Now let’s retranslate this model.

CHOOSE Command / Retranslate

When the model is retranslated, it will automatically read the external file and create and initial the Savevalues. Show the Savevalues as we did above, if you wish.

INCLUDE Files can also be used as we did in the ANOVA Lesson. They make it easy to manage multiple runs of a simulation. Also, unless suppressed, a report will be produced for each completed simulation run. See the GPSS World Reference Manual and Lesson 14 of this manual for more information on report options.

In this lesson, you have used some of the interactive capabilities of GPSS