CS 330   Fall, 2002
Program #1: Prolog
H. Hamburger & D. Richards
Due: October 16

Preparing:  Study chapter 7 of the text.  Section 7.2 has some practical details.  First try just starting and stopping (see table below).  Then try some of the built-in capabilities of Prolog like the list conventions and the “=” operator for matching; e.g., type this:

[X|Y] = [a,b,c].  Also try out the is operator to force evaluation of arithmetic expressions, like this: X is 3+2. The arithmetic operators work too, so 5>4. succeeds.

Files: Before entering a Prolog session, create a file (using any editor) that contains the facts and rules you wish to test. Start with simple predicates. 

Starting:  At the Unix prompt, type the Unix command prolog to start interacting with the Quintus Prolog interpreter on the GMU server.  To load your facts and/or rules from a file called file1 into the session, type [file1].  ending with a period.  If you entered Prolog while not in the directory containing the file, give the full path, within single quotes: [’pro/file1’]. You can load two files at once, like this: [factfile, rulefile]. If you unintentionally hit the <return> key without the period, just type a period and another <return>.  After loading your file(s), type in queries and you’ll get the answers.

Stopping:  Before you start anything, know how to stop it! To halt a Prolog session, use halt. which is a Prolog predicate with no arguments, with the side-effect of exiting Prolog.  To stop an unresponsive Prolog process that may be in an infinite loop, use <control-c>  a .

Tracing:  For debugging, you can have the Prolog processor trace the steps by which it reached its conclusions.  This can get pretty complex, though, so you may want to turn it off.  To start the process type trace. and to turn it off, use notrace.

Scripts: script is a Unix command that starts a script session. Everything you then type, plus the computer output, is copied to the file typescript (or to the file given as an argument, e.g., myoutfile) when you exit the script session by typing <control-d>. 

   

Starting ...   

and Stopping   

Prolog session:   

prolog   

halt.   

Prolog process:

pred(arg1, arg2).

<control-c> a

Tracing:   

trace.   

notrace.   

Script session:   

script myoutfile   

<control-d>   



Online help: To get Prolog help, type the special predicate help. which tells you about the special predicate manual(…).  You can also search Google for Quintus Prolog.

Assignment: You will be creating a database of airline flight schedules, along with rules for extracting useful relationships from that data. (Real information is easy to get, if you wish, by Google searches for “time zones” and “flight schedules.”)  The basic predicate, hop, is for a nonstop flight (that takes off, goes somewhere and lands).  Its 5 arguments are: a flight’s name (airline and number), the airport it comes from, the one it goes to, its local departure time and its local arrival time.  For example, if United Airlines flight 234 flies from LAX in Los Angeles to DTW near Detroit, the Prolog fact might be written

hop(flight(ua,234),lax,dtw,time(9,30),time(18,10)).

Prolog constants begin with a lowercase letter, so we use ua for United Airlines and lax for LAX.  A flight name is a structure containing an airline and a number, like flight(ua,234); a direct flight has consecutive hops with the same flight number.  Each time of day is also a structure, like time(18,10), which means 6:10 p.m., on a  24-hour clock, from time(0,0) to time(23,59). The duration of the above flight is 5 hours and 40 minutes because of a 3-hour time zone difference. 

In addition to hop, the only other predicate for facts is city, used for associating airports to their cities and time zones.  The fact that LAX is in Los Angeles, in time zone 16 (16 hours later than London, which is defined to be 0) is represented as follows.

city(lax,los_angeles,16).

Write Prolog facts for hop and city, as well as rules for the following predicates.

airport_hop(A1,A2)

There is a hop from airport A1 to airport A2.

city_hop(C1,C2)

There is a hop from an airport for city C1 to an airport for city C2.

two_hop(A1,A2)

It is possible to get from airport A1 to airport A2 in two hops.

time_diff(T1,T2,T)

T states how much later T2 is than T1.  Each variable is a time structure.

duration(A1,A2,T,F)

You can go from airport A1 to airport A2 in time T on flight F.

no_layover(A1,A2)

Like two_hop, but with between 0 and 2 hours between hops.

direct(A1,A2,F)

You can go from airport A1 to A2, staying on flight F.

path(A1,A2,L)

One can go from airport A1 to another one, A2, via list L of distinct airports.

(You’ll need to include other rules; e.g., member and different.

Use path on the right side of the rule, but not as the first predicate there.

Use specific airports as inputs.  Test first with non-looping data.)

Help: You may get help from anyone with Prolog, but do the assignment yourself. This pertains to choosing the facts, implementing the rules, and devising the tests.  See the CS Department project policy at http://cs.gmu.edu/honor-code.html .

Hand in: a discussion of your facts, rules, and queries, including a flight graph diagram showing your facts clearly. Provide a printout of a script of a session in which you…