Thread: Use of await statement in problem

Hi JPF. I'm having trouble with a question given to us by our lecturer. The question is as follows

In a toy train circuit (figure of eight) trains can enter via section 5 or section 14, do a figure of eight circuit and then leave via the same section that they entered.

A train joining the circuit must first make sure the section ahead and the section immediately to the right/left of them is free (e.g. A train entering via section 14 must ensure the section 14 and section 13, are not occupied by another train at that time. To avoid train crashes only one train at any time can be in any of the available sections. The crossover protocol at the junction (i.e. section 0) in the centre of the figure of eight is that the first train to get access to the junction is allowed to proceed across it.

The code that I have so far is below

public class traintrack {
 
	/*declare and initialise global variables */
	final int NUMBER_OF_TRAINS= 20;
	int numberOnTrack = 0;
 
	//slots on the roundabout
	String sectionsOnTrack [] = {"[.....]", "[.....]", "[.....]", "[.....]","[.....]","[.....]","[.....]","[.....]","[.....]",
									"[.....]","[.....]","[.....]","[.....]","[.....]","[.....]","[.....]","[.....]","[.....]"};
}
 
	for (int count = 1; NUMBER_OF_TRAINS < 18; count++);
	{
		int entry = 5 & 14;
		int exit = 5 & 14; 
 
		trainProcess (entry, exit);
 
	} //end for loop
} //end main process
 
Process trainProcess (int s, int t);
	{ 
		<await> (numberOnTrack <= 18, numberOnTrack++)>; //wait if track too crowded
 
	<await(sectionsOnTrack[2..s]=="[.....]"); AND (sectionsOnTrack[2..s+7])>
 
}	
 
}

the reason it cuts off around the await section is because I am rather a beginner in using java and despite being a final year student have managed to get along doing modules that are not heavily focused on coding; but also I couldn't find much on java await statements by googling.

Any advice/nudges in the right direction would be appreciated.
Thanks

see thats what i'm unsure of. Our lecturer provided some code that he said would work straight out of the box for java, it was as follows:

Global Invariant: (NumberOfTrainsInAnySection <= 1) AND (TotalTrainsOnTrack <= 8)
          process main {
	//declare and initialise global variables 
	Const NUMBER_OF_TRAINS=20;
	int numberOnTrack = 0;
          slots on the roundabout 
	string  section[0..11] = { &#8220;[.....]&#8221;, &#8220;[.....]&#8221;, &#8220;[.....]&#8221;, &#8220;[.....]&#8221;, &#8220;[.....]&#8221;, &#8220;[.....]&#8221;, &#8220;[.....]&#8221;, 
						&#8220;[.....]&#8221;,&#8220;[.....]&#8221;, &#8220;[.....]&#8221;, &#8220;[.....]&#8221;,&#8220;[.....]&#8221; };
           //create and set the trains moving 
	 for (count= 1 to NUMBER_OF_TRAINS){ 
                   	trainProcess (int n);
               } /* end  for */
           } // end main process
 
           process trainProcess (int n) { 
    <await (numberOnTrack<=8) numberOnTrack++;> /*    /* Wait if track is full
     <await (section[0) ==”[.....]”)  section[0]=”[“+n+”]”;> /* Wait to get into section[0]
    /* wait for clearance before moving on to junction */
    <await ( (section[10]==&#8221;[.....]&#8221;) AND (section[1]==&#8221;[.....]&#8221;) ) section[1]= &#8221;[&#8220;+n+&#8221;]&#8221;; section[0]= &#8221;[.....]&#8221;; >
   /* move off the junction
   <await ( section[2]==”[.....]”) section[2]= ”[“+n+”]”; section[1]= ”[.....]”; >
    /* move around the loop
    int currentPosition = 2;
    do   {
          <await (section[currentPosition+1]==”[.....]”)   section[currentPosition+1]=section[currentPosition]; 
                                                                                        section[currentPosition]=”[.....]”; >
          currentPosition = currentPosition+1;
    } while (currentPosition <=10)
  /* wait for clearance before moving on to junction */
    <await ( section[1]==&#8221;[.....]&#8221;)  section[1]= &#8221;[&#8220;+n+&#8221;]&#8221;; section[10]= &#8221;[.....]&#8221;; >
    /* wait to move forward off junction */
     <await (section[11] ==&#8221;[.....]&#8221;)  section[11]=&#8221;[&#8220;+n+&#8221;]&#8221;; section [1]= &#8221;[.....]&#8221; >
    < section[11]=&#8221;[.....]&#8221;; >    /* move off the track */
    < numberOnTrack--;> /* decrement the number of trains on the track
} /* end carProcess */

but it doesn't work at all when I paste it in, as he says it should.

My understanding is that the wait statement is just to ensure that there is enough room on the track for another train to enter.

thanks for having a look at my thread too

I agree with Norm, that doesn't look much like java.

Is the wait command absolutely necessary? This seems like a problem about determining when the trains are to enter through the station if they are all traveling at a constant "velocity" (Sections per iteration). A wait function seems to be for a time base instead of an iterative base. There are a few controlling variables for the solution: How many sections does each train take up? How long is the figure 8 track? Most importantly, what is the distance of each station from the junction within the figure 8?

This problem could be solved without programming it; I'd say watch/imagine a train going through the track and observe how many iterations after departing from the starting station which pass before the train enters the junction. This allows you to see the walls - moments where a collision is possible - created by any train leaving the station.

I really like this problem, could you post the picture that goes along with it?

heres the picture attached... ive since found that the code I posted was psuedocode. and the aim is to make psuedocode in that same format as mentioned above for this track.

The psuedocode I posted earlier is from a different question (just to serve as an example)

Here's the question for the above image

"A toy train circuit (figure of eight) is show below. Trains can enter via section 5 or section 14, do a figure of eight circuit and then leave via the same section that they entered. The route they take is indicated in figure 1.

Traintrack.jpg

A train joining the circuit must first make sure the section ahead and the section immediately to the right/left of them is free (e.g. A train entering via section 14 must ensure the section 14 and section 13, are not occupied by another train at that time. To avoid train crashes only one train at any time can be in any of the available sections. The crossover protocol at the junction (i.e. section 0) in the centre of the figure of eight is that the first train to get access to the junction is allowed to proceed across it.

Assuming that shared memory is available
(a) Design a solution using the await statement, that adhers to the above
behaviour protocol and hence allows trains to enter, travel around and leave safely. (25%)
(b) translate the design in part(a) into a semaphore based design. (25%)
"


At the minute Im working on changing the psuedocode so that it applies to the question above

Originally Posted by Norm

There is no await statement in java. Where have you heard about an await statement?

First time I've heard is from this question which is what's confusing. But this is just psuedocode now, it signifies waiting until a slot on the track is empty before another train can enter.

Oh I see, its one of those exercises. I would start with creating objects and passing variables between them which act within the constraints. Maybe you could clear this up, is semaphore design trying to determine the memory allocation for operating this track?

The first task of identifying the objects has already been done: TrackSegments, Trains, and Stations.
It seems there are two different reference points for deciding how to draft the "sight" of the program. Junctions, being the point of conflict, could be created to control the trains, but seeing this exercise I am pretty sure you are supposed to focus on passing variables from TrackSegments to the Trains.