# COMP30020 Declarative Programming Assignment 2 solution

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## Description

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. replace(E1, L1, E2, L2)
this holds when list L1 is the same as L2, except that in one place where L1 has the
value E1, L2 has E2. Note that only one occurrence of E1 is replaced. This must work
in any mode in which at least one of L1 or L2 is a proper list (that is, either [] or a list
whose tail is a proper list). For example:
replace(2,[1,2,3,4],5,X) should have only the solution X = [1,5,3,4].
replace(2,[1,2,3,2,1],5,X) should backtrack over the solutions X = [1,5,3,2,1]
and X = [1,2,3,5,1].
replace(2,X,5,[1,5,3,5,1]) should backtrack over the solutions X = [1,2,3,5,1]
and X = [1,5,3,2,1].
replace(X,[a,b,c,d],Y,[a,e,c,d]) should have only the solution X = b, Y = e.
replace(X,[1,2,3,2,1],Y,[1,5,3,5,1]) should have no solutions (it should fail).
2. zip(As, Bs, ABs)
this holds when As, Bs, and ABs are lists of the same length, and each element of ABs
is a term of the form A-B where A is the corresponding element of As and B is the
corresponding element of Bs. This should work whenever at least one of the arguments
is a proper list. Note that – is an infix operator here, but aside from syntax, A-B is an
ordinary term whose functor is – and whose two arguments are A and B. For example:
zip([1,2,3,4],[a,b,c,d],L) should have only the solution L=[1-a,2-b,3-c,4-d]].
zip(X,Y,[1-a,2-b,3-c,4-d]) should have only the solution X=[1,2,3,4], Y=[a,b,c,d].
zip([1,2,3,4],Y,[1-a,2-b,3-c,4-d]) should have only the solution Y=[a,b,c,d].
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zip(X,[a,b,c,d],[1-P,2-Q,3-R,4-S]) should have only the solution X=[1,2,3,4],
P=a, Q=b, R=c, S=d.
zip([1,2,3],[a,b,c,d],L) should fail.
3. sublist(Xs, Ys)
this holds when Xs is a list containing some of the elements of Ys, in the same order
they appear in the list Ys. This should work whenever Ys is a proper list. For example:
sublist([a,c,e],[a,b,c,d,e]) should succeed.
sublist([a,e,c],[a,b,c,d,e]) should fail.
sublist([a,X,d],[a,b,c,d,e]) should have the two solutions X=b and X=c.
sublist(X,[a,b,c]) should have the eight solutions X=[]; X=[c]; X=[b]; X=[b,c];
X=[a]; X=[a,c]; X=[a,b]; and X=[a,b,c].
Note that for all of these predicates, 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 assignment2.pl.
Assessment
This assignment will not be assessed for credit; that is, it is worth 0% of your final mark. To
make it useful as practice for later, assessed work, your code will be tested for correctness.
Your submission will only be checked for 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.
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 the unix server nutmeg2.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 COMP30020 assignment2 assignment2.pl
Important: you must wait a minute or two (or more if the servers are busy) after
submitting, and then issue the command
verify COMP30020 assignment2 | 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., assignment2.late) in the submit and verify commands.
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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