The University of Melbourne
Department of Computing and Information Systems
Declarative Programming COMP30020/COMP90048
Semester 2, 2017
Project Specification
Project due 2 October 2017 at 5pm Worth 5% (for students of COMP90048)
The objective of this project is to practice your Prolog programming skills. You will write a few fairly simple Prolog predicates.
Note well: This project is only assigned for students enrolled in COMP90048. Other students are encouraged to do the exercise for practice, and may submit their work if they like, but it is not part of their assessment, so they will not receive credit for it.
The Assignment
You will implement the following Prolog predicates.
1. correspond(E1, L1, E2, L2)
this holds when in one place where list L1 has the value E1, L2 has E2. This must work in any mode in which L1 and L2 are proper lists (that is, either [] or a list whose tail is a proper list). For example:
correspond(e,[h,e,l,l,o],X,[1,2,3,4,5]) should have only the solution X = 2.
correspond(l,[h,e,l,l,o],X,[1,2,3,4,5]) should have solutions X = 3 and X = 4.
correspond(X,[h,e,l,l,o],4,[1,2,3,4,5]) should have only the solutions X = l.
correspond(X,[h,e,l,l,o],Y,[1,2,3]) should have solutions: X = h, Y = 1 and X = e, Y = 2 and X = l, Y = 3
correspond(o,[g,o,o,d,b,y,e],l,[h,e,l,l,o] should succeed. correspond(y,[g,o,o,d,b,y,e],X,[h,e,l,l,o] should fail.
2. interleave(Ls, L)
this holds when Ls is a list of lists, and L is a list of all the elements of all the lists in Ls, interleaved. That is, the first element of L is the first element of the first list in Ls, the second element of L is the first element of the second list in Ls, and so on for all the lists in Ls. After this, the next element of L is the second element of the first list in Ls, and so on, until two elements have been taken from all the lists in Ls. After this, come the third elements of all the lists in Ls, and so on, until all the elements of all the lists in Ls are included in L. All the lists in Ls must be the same length. For example:
interleave([[a,b],[c,d]],X) should have only the solution X=[a,c,b,d].
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interleave([X,Y,Z],[a,b,c,d,e,f]) shouldhaveonlythesolutionX=[a,d], Y=[b,e], Z=[c,f].
interleave([L],[1,2,3]) should have only the solution L=[1,2,3]. interleave([[3,1,2],[1,5],[4,9,6]],X) should fail.
3. partial eval(Expr0, Var, Val, Expr)
this holds when Expr is the arithmetic expression Expr0 with atom Var replaced by number Val, and any wholly numeric subexpressions fully evaluated to numbers. In this context an “arithmetic expression” is either:
• a number; or
• an atom (standing for an arithmetic variable); or
• a term of the form x+y, where x and y are arithmetic expressions; or • a term of the form x-y, where x and y are arithmetic expressions; or • a term of the form x*y, where x and y are arithmetic expressions; or • a term of the form x/y, where x and y are arithmetic expressions; or • a term of the form x//y, where x and y are arithmetic expressions.
Note that we are using Prolog atoms, rather than Prolog variables, for arithmetic vari- ables, thus you will need to traverse Expr0 replacing occurrences of Var with Val, and then evaluate any subexpressions containing no atoms. You need not, and should not, take advantage of algebraic equivalences like arithx*0 = 0 and arithx-x = 0; if x is not substituted with a value, both of these cases should be left as is in the output. Sim- ilarly, you should not exploit algebraic properties such as associativity, commutativity or distributivity, so expressions such as 6*x*7 should also be left as is. For example:
partial eval(6*7, x, 2, E) should have only the solution E=42.
partial eval(6*(3+x*x), x, 2, E) should have only the solution E=42.
partial eval(x*(3+y*y), y, 2, E) should have only the solution E=x*7.
partial eval((x*0+6)*(x-x+3+y*y), y, 2, E) should have only the solution E=(x*0+6)*(x-x+3+4).
This predicate need only work when Expr0, Var, and Val are ground, and it should produce only one solution for Expr.
Hint: Prolog typically does not use a special constructor to indicate a number or an atom, as one would with an algebraic type system. In Prolog, you can use the built-in predicates number(X) and atom(X) to test if the term X is a number or atom, respectively.
Note that the order in which solutions are found does not matter, but all listed answers must be found, and no extra solutions are permitted.
You must call your source file lab2.pl.
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Assessment
Your project will be assessed 100% on correctness. For this assignment, code quality will not be considered. However, for your own sanity, I do recommend commenting your code and programming it carefully, paying due attention to programming technique.
Note that timeouts will be imposed on all tests. Test cases will be rather small, so the timeouts should only affect you if you create an infinite recursion (infinite loop) or infinite backtracking loop.
Submission
You must submit your project from either of the unix servers dimefox.eng.unimelb.edu.au or nutmeg.eng.unimelb.edu.au. Make sure the version of your program source file you wish to submit is on this host, then cd to the directory holding your source code and issue the command:
submit COMP90048 lab2 lab2.pl
Important: you must wait a minute or two (or more if the servers are busy) after submitting, and then issue the command
verify COMP90048 lab2 | less
This will show you the test results from your submission, as well as the file(s) you submitted. If the test results show any problems, correct them and submit again. You may submit as often as you like; only your final submission will be assessed.
If you wish to (re-)submit after the project deadline, you may do so by adding “.late” to the end of the project name (i.e., lab2.late) in the submit and verify commands. But note that a penalty, described below, will apply to late submissions, so you should weigh the points you will lose for a late submission against the points you expect to gain by revising your program and submitting again.
It is your responsibility to verify your submission.
Your submission will be tested on one of the servers you are required to submit from. These servers run SWI Prolog version 6.6.6, which is probably older than the version you will develop on. You are advised to test your program on one of these servers before submitting; in the unlikely case that your program uses some Prolog features or libraries not supported by SWI 6.6.6, it will be much easier to discover this.
Note that these hosts are only available through the university’s network. If you wish to use these machines from off campus, you will need to use the university’s Virtual Private Network. The LMS Resources list gives instructions.
Windows users should see the LMS Resources list for instructions for downloading the (free) MobaXterm or Putty and Winscp programs to allow you to use and copy files to the department servers from windows computers. Mac OS X and Linux users can use the ssh, scp, and sftp programs that come with your operating system.
Late Penalties
Late submissions will incur a penalty of 0.5% of the possible value of that submission per hour late, including evening and weekend hours. This means that a perfect project that is
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much more than 4 days late will receive less than half the marks for the project. If you have a medical or similar compelling reason for being late, you should contact the lecturer as early as possible to ask for an extension (preferably before the due date).
Note Well:
This project is part of your final assessment, so cheating is not acceptable. Any form of material exchange between teams, whether written, electronic or any other medium, is considered cheating, and so is the soliciting of help from elec- tronic newsgroups. Providing undue assistance is considered as serious as receiv- ing it, and in the case of similarities that indicate exchange of more than basic ideas, formal disciplinary action will be taken for all involved parties. If you have questions regarding these rules, please ask the lecturer.
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