This, the first graded assignment of the semester, asks you to solve a simple problem in each of five very different programming languages: Ada 95, C#, Python, Prolog, and Scheme. Specifically, you are to write a program that takes two values, k and n, and outputs all combinations of k numbers chosen from {1, 2, ..., n}. So if, for example, you specify the values 2 and 4, you should see (in some order)
1 2
1 3
1 4
2 3
2 4
3 4
You may print the members of each combination in ascending or descending
order (your choice); but please don't otherwise scramble them.
Finding combinations is a naturally recursive problem (iterative solutions
are also possible). The easiest and most elegant
solutions employ a recursive set of iterators, which are
abstractions used to drive
a for loop. We will study iterators in Section 6.5.3; you
may want to read ahead. In the terminology of that section,
you’ll find that Python and C# have “true”
iterators.
Prolog's search mechanism can be used to create the equivalent of
iterators, and yields a very elegant solution (my version is ony 4 lines
long).
Ada and Scheme have no special iterator support; for these you'll have to
work with lists (or find some other solution—e.g., tasks in
Ada).
For what it's worth, Java and C++ (which you can try for extra credit)
have iterator objects, which are sort of half of what you
want.
If you already knew all five languages, you’d probably find your task easiest in Prolog and hardest in Ada, with the other three somewhere in the middle. (Of course you probably don’t know all five languages already, so the unfamiliar ones will be the hardest.) A hint: the CD that comes with the textbook contains working versions of all the nontrivial examples in the book. For Ada and C# you might find it helpful to start with one of these: it will already import appropriate libraries and contain examples of the control constructs, I/O calls, etc.
When run, your Ada, C#, and Python programs should take k and
n (in that order) as command-line arguments, and then write
combinations, one per line, to
standard output.
For Scheme, which runs in an interpreter, please arrange for
(combinations k n) to return a
list of combinations, each of which is a (nested) list.
For Prolog, please arrange for combinations(k, n, L)
to produce successive combinations (values for L) in response
to a semicolon prompt.
Your programs should all run in time proportional to the length of the output. Not all “obvious” programs will do so. Behind this link, for example, is a Python program that produces the correct output, but takes time exponential in n, regardless of k. In particular, it takes exponential time when k = n, even though the output is only one line long (try it for, say, k = n = 20). (For what it's worth, my version of the better Python solution is actually a shorter program.)
You may work alone on this project or in teams of two. If you split up
the languages, whoever takes Ada should probably do two; the other
person should do three.
However you divide the programming,
each team member must write his or her own
README file (no sharing of text on this allowed), and turn in
the project separately (with all five programs, which will be the
same as the partner's code).
This means, of course, that you'll need to really understand your
partner's code.
In addition to the usual requirements, your write-up must compare and contrast the programming experience in the different languages (all five of them). What was easy? What was hard? Are there noticeable differences in speed? What do you like/dislike? Did you find iterators to be helpful?
Be sure to follow all the rules on the Grading page.
To turn in your code, use the following procedure, which will be the
same for all assignments this semester:
Put your write-up in a README.txt or
README.pdf file in the same directory as your code, and
(while still in that directory) run
the script ~cs254/bin/TURN_IN on a csug machine.
The script will package the contents of the directory (and any
subdirectories) into a bundle and send it to the TA for grading (so
clean up any mess you might have in the directory first).
Be sure your README file describes any features of your
code that the TA might not immediately notice.
We will be using the following language implementations:
gnatmake (a wrapper for the
GNU Ada translator). It’s in /u/cs254/bin,
which you
should add to your PATH environment variable (ask a
friend or one of the TA if you
don’t know how). It produces native executables.
gmcs (the Mono project C#
compiler) and run with the mono JIT/run-time system.
Both are located in /u/cs254/bin.
pl interpreter.
It's in /usr/bin.
python interpreter. It’s
in /usr/bin.
mzscheme or, under X, with the drscheme GUI.
Both are located in /u/cs254/bin.
You are welcome to do development using other language implementations and/or platforms, but you must ensure that your final versions compile and run correctly using the implementations listed above. We will be testing using only these.
I won’t be devoting lecture time to how to use these languages. You’ll need to find on-line tutorials or other resources, and teach yourself. Here are some decent starting points:
Before the beginning of class on Tuesday, September 8, send e-mail
to cs254 containing answers to the following
questions:
"hello,
world" to standard output in C#? In Ada?
