This prac is due for submission by lunch time on your next practical day, correctly packaged in a transparent folder as usual (unpackaged and late practical submissions will not be accepted - you have been warned). Pracs should please be deposited in the hand-in box outside the lab. Only one set of listings is needed for each group, but please enclose as many copies of the cover sheet as are needed, one for each member of the group. These will be returned to you in due course.
In this practical you are to
The exercises for this week are not really difficult, although they may take longer than they deserve simply because you may be unfamiliar with the systems.
Copies of this handout, the cover sheet, the Parva language report, and descriptions of the library routines for input, output, string handling and set handling in Java and C# are available on the course web site at http://www.cs.ru.ac.za/courses/CSc301/Translators/trans.htm .
When you have completed this practical you should understand
This week your group is required to hand in, besides the individual cover sheets for each member:
Keep the cover sheet and your solutions until the end of the semester. Check carefully that your mark has been entered into the Departmental Records.
You are referred to the rules for practical submission which are clearly stated in our Departmental Handbook. However, for this course pracs must be posted in the "hand-in" box outside the laboratory before the next practical session and not given to demonstrators during the session.
A rule not stated there, but which should be obvious, is that you are not allowed to hand in another student's or group's work as your own. Attempts to do this will result in (at best) a mark of zero and (at worst) severe disciplinary action and the loss of your DP. You are allowed - even encouraged - to work and study with other students, but if you do this you are asked to acknowledge that you have done so on all cover sheets and with suitable comments typed into all listings. You are expected to be familiar with the University Policy on Plagiarism, which you can consult at:
http://www.scifac.ru.ac.za/plagiarism_policy.pdf
In this practical course you will be using a lot of simple utilities, and usually work at the "command line" level rather than in a GUI environment. Note in particular:
LPRINT Queens.pav Queens.java
The listings come out in a small font which enables long lines to be read easily and with narrow line spacing (so that you get more listing for your money). Please use this utility, which prints listings in a small courier font to produce all listings submitted on this course, as it makes my job of reading the submissions much easier. Program listings in "proportional font" are awkward to read.
For this prac it is recommended that you simply work in the Hamilton lab, rather than begging, borrowing or stealing copies of a whole host of software for home use. In future pracs you will mostly use Java or C# only, and the prac kits will, hopefully, contain all the extras you need.
We shall make use of zipped prac kits throughout the course; you will typically find sources for each week's prac in a file pracNN.zip on the server. Copy prac19.zip and xtacy.zip needed for this week, either directly from the server on I:\CSC301\TRANS (or by using the WWW link on the course page), and extract the sources when you need them, into your own directory/folder, perhaps by using UNZIP.
copy i:\csc301\trans\prac19.zip
unzip prac19.zip
Use UNZIP or WINZIP, as the file contains files with long file names which PKUNZIP cannot handle.
In the past there has been a problem with running applications generated by the C# compiler if these are stored on the network drives. If you have difficulties in this regard, for those parts of the practical that involve the use of C#, work from the local D: drive instead. After opening a command window, log onto the D: drive, create a working directory and unpack a copy of the prac kit there:
d:
md d:\G01T1111
cd d:\G01T1111
unzip I:\csc301\trans\prac19.zip
In the prac kit you will find various versions of a famous program for finding a list of prime numbers using the method known as the Sieve of Eratosthenes. You will also find various versions of a program for solving the N Queens problem, some "empty" programs, some other bits and pieces, including a few batch files to make some of the following tasks easier and a long list of prime numbers (primes.txt) for checking your own!
You may not be a Pascal expert, but in the kit you will find some Pascal programs, including SIEVE.PAS that determines prime numbers using a Boolean array to form a "sieve". Study and compile these programs - you can do this from the command line quite easily by issuing commands like
FPC SIEVE.PAS
FPC QUEENS.PAS
FPC EMPTY.PAS
to use the 32-bit Windows version of the Free Pascal compiler. Make a note of the size of the executable (use the command DIR SIEVE.EXE and DIR QUEENS.EXE and DIR EMPTY.EXE).
You may be able to produce a slightly faster version of the executable program for the Sieve example by suppressing the index range checks that Pascal compilers normally include for code that accesses arrays:
FPO SIEVE.PAS
How do the sizes of the executables compare? Why do you suppose the "empty" program produces the amount of code that it does?
Here is something more demanding: By experimenting with the CONST declaration, find out how large a sieve the program can handle. What is the significance of this limit? Hint: you should find that funny things happen when the sieve gets too large, though it may not immediately be apparent. Think hard about this one!
You may not be a Modula-2 expert either, but examine, and then compile and run the equivalent Modula-2 code supplied in the files SIEVE.MOD, EMPTY.MOD and QUEENS.MOD. You can do this quickly using commands like
M2C QUEENS (note that the .MOD extension is not quoted here) or
M2O SIEVE (for the version that suppresses subscript checks)
Make a note of the size of the executables produced. How do they compare with the Pascal executables? Approximately how big a sieve can the compiler handle? Why do you suppose there is a difference, when the source programs are all so similar?
The kit also includes C and C++ versions of these programs. Compile these and experiment with them in the same way:
BCC SIEVE.C (using the Borland compiler in C mode)
BCC SIEVE.CPP (using the Borland compiler in C++ mode)
CL SIEVE.C (using the WatCom compiler in C mode)
CL SIEVE.CPP (using the WatCom compiler in C++ mode)
Once again, make a note of the size of the executables, and in particular, compare them with the earlier versions. Can you think of any reason why the differences are as you find them?
There are two Java compilers available for your use. The JDK one is called javac and there is also the (much faster) one called jikes (Jikes will only handle Java 1.4 level source, but that covers most things). Both of these are conveniently invoked from within UltraEdit. You can also compile a Java program directly from the command line with commands like
javac Sieve.java (using the (slow) JDK compiler)
jikes Sieve.java (using the (fast) Jikes compiler)
You can compile the C# versions of these programs from the command line, for example:
csharp Sieve.cs
(You may have to do this on the local D: drive) Make a note of the size of the ".NET assemblies" produced (SIEVE.EXE, EMPTY.EXE and QUEENS.EXE). How do these compare with the other executables?
On the course web page you will find a description of Parva, a toy language very similar to C, and a language for variations on which we shall develop a compiler and interpreter later in the course. The main difference between Parva and C/Java/C# is that Parva is stripped down to bare essentials.
Learn the Parva system by studying the language description where necessary, and trying the system out on the supplied code (SIEVE.PAV and QUEENS.PAV). There are various ways to compile Parva programs. The easiest is to use a command line command:
parva Sieve.pav simple error messages
parva -o Sieve.pav slightly optimized code
parva -l Sieve.pav error messages merged into listing.txt
You will have to do this on the local D: drive.
More conveniently, we have set up UltraEdit to allow for an option to compile Parva programs. If you want to add this to your home systems, use the Advanced->Tool Configuration pull down, then set the following fields
Command Line Parva %n%e
Working Directory %p
Menu Item Name Parva
Save all files first Selected
Output to List Box Selected
Capture Output Selected
and then click Insert. After this you can choose the Parva option on the Advanced menu to compile (and, when successful, run) the program in the "current window". The demonstration programs Sieve.pav and Queens.pav in the kit have a few fairly obvious errors. Learn the syntax and semantics of Parva by correcting the errors until the programs run correctly. Once again, experiment to see how large a sieve you can set up.
In the kit you will also find various equivalent programs that solve the famous N Queens problem. These use a back-tracking approach to determine how to place N Queens on an N * N chess board in such a way that no Queen threatens or is threatened by any other Queen - noting that a Queen threatens another Queen if the two pieces lie on a common vertical, horizontal or diagonal line drawn on the board. Here is a solution showing how 4 Queens can be placed safely on a 4 * 4 board:
.---------------.
| | | Q | |
|---+---+---+---|
| Q | | | |
|---+---+---+---|
| | | | Q |
|---+---+---+---|
| | Q | | |
`---=---=---=---'
Compile one or more of these programs and try them out. For example
FPC QUEENS.PAS
QUEENS
There are two versions written in each of Pascal, Modula-2, Java and C#. One version uses parameters to pass information between the routines, the other version uses global variables. At some stage you could usefully spend a little time studying Tutorial 19 on the web site, which explains the technique behind the solution.
Complete the table on the hand-in sheet to determine the number of solutions as a function of N. Do you see a pattern in this?
It may help amplify the material we are discussing in lectures if you put some simple Modula-2 programs through a high-level translator we have available, and then look at, and compile, the C code to see the sort of thing that happens when one performs automatic translation of a program from one high-level language to another.
We have a demonstration copy of a system (Russian in origin), that translates Modula-2 or Oberon-2 source code into C. The system is called Extacy (a poor pun on "X to C", it seems). Whether or not the C code one obtains is usable depends, obviously, on having C translations of all of one's Modula-2 libraries as well. In principle all one has to do is convert these libraries using the same system. Some very simple libraries came with the demonstration kit, and we have produced one or two more, but we would have to pay many Roubles and do an awful lot of work to get the system fully operational.
XC =m SOURCE.MOD
will produce all the .H and .C files needed for a "make" of the parent program SOURCE.MOD
Convert the sample programs in this kit (SIEVE.MOD and QUEENS.MOD) and the various support modules to C, and then use a C++ compiler to compile and run the resulting code. Most simply, run the C compiler directly from the command line:
BCC SIEVE.C EASYIO.C X2C.C
BCC QUEENS.C EASYIO.C X2C.C
or
CL SIEVE.C EASYIO.C X2C.C
CL QUEENS.C EASYIO.C X2C.C
Take note of, and comment on, such things as the kind of C code that is generated (is it readable; is it anything like you might have written yourself?), and of the relative ease or difficulty of using such a system. You might also like to comment on the size of the object code produced.
Different compilers - even for very similar programs - can produce code of very different quality. In particular "interpretive" systems (of which the Parva implementation is one example) produce programs that run far more slowly than do "machine" or "native" code systems. Carry out some tests to see these effects for yourselves, and how severe they are, by comparing the execution times of some of the programs.
Summarize your findings on page 2 of the cover sheet, explaining briefly how you come to the figures that you quote. Do the N Queens programs using parameters perform better/worse than those using global variables? Is Java better/worse than C# (the source code in each case is almost identical)? Do 16-bit compilers fare better or worse than 32-bit compilers?
Hint: the machines in the Hamilton Labs are very fast, so you should try something like this: modify the programs to comment out nearly all the output statements (since you are not interested in seeing the solutions to the N Queens problem a zillion times, or a zillion lists of prime numbers, or measuring the speed of I/O operations), and then run the programs and time them with a stop watch. Choose sizes for the sieve or chessboard (and a suitable number of iterations) that will produce measurable times of the order of a few seconds.
Although Java is often touted as being an interpreted language, in fact the latest versions of the Java "interpreter" - the program executed when you give the java command - actually indulge in "just in time" compiling (see textbook page 32) and "JIT" the code to native machine code as and when it is convenient - which results in spectacularly improved performance. It is possible to frustrate this by issuing the java command with a directive -Xint:
javac Sieve.java
java -Xint Sieve
to run the program in interpretive mode. Try this out as part of your experiment.
In lectures you were told of the existence of decompilers - programs that can take low-level code and attempt to reconstruct higher level code. There are a few of these available for experiment.
jad a decompiler that tries to construct Java source from Java class files
javap a decompiler that creates pseudo assembler source from a Java class file
gnoloo a decompiler that creates JVM assembler source from a class file
oolong an assembler that creates Java class files from JVM assembler source
ildasm a decompiler that creates CIL assembler source from a .NET assembly
ilasm an assembler that creates a .NET assembly from CIL assembler source
peverify a tool for verifying .NET assemblies
Try out the following experiments or others like them:
(a) After compiling Sieve.java to create Sieve.class, decompile this:
jad Sieve.class
and examine the output, which will appear in Sieve.jad
(b) Disassemble Sieve.class
javap -c Sieve >Sieve.jvm
and examine the output, which will appear in Sieve.jvm
(c) Disassemble Sieve.class
gnoloo Sieve.class
and examine the output, which will appear in Sieve.j
(d) Reassemble Sieve.j
oolong Sieve.j
and try to execute the resulting class file
java Sieve
(e) Be malicious! Corrupt Sieve.j - simply delete a few lines in the section that corresponds to the Main function (use lines with opcodes on them). Try to reassemble the file (as above) and to re-run it. What happens?
(f) Compile Sieve.cs and then disassemble it
csharp Sieve.cs
Disassemble Sieve (calls ildasm from a batch file, produces Sieve.cil)
and examine the output, which will appear in Sieve.cil
(g) Reassemble Sieve.cil
Reassemble Sieve (calls ilasm from a batch file, produces new Sieve.exe))
and try to execute the resulting class file
Sieve
(h) Be malicious! Corrupt Sieve.cil - simply delete a few lines in the section that corresponds to the Main function (use lines with opcodes on them). Try to reassemble the file (as above) and to re-run it. What happens?
(i) Experiment with the .NET verifier after step (g) and again after step (h)
NetVerify Sieve (calls peverify from a batch file)
- and don't use a search engine!
Nothing you have done so far should have extended your programming talents very much. To get the old brain cells working a little harder, turn your minds to the following tasks.
Pat Terry's problems are sometimes reputed to be hard. They only get very hard if you don't think very carefully about what you are trying to do, and they get much easier if you think hard and spend time discussing the solutions with the tutors or even the Tyrant himself. His experience of watching the current generation of students suggests that some of you get beguiled by glitzy environments and think that programs just "happen" if you can guess what to click on next. Don't just go in and hack. It really does not save you any time, it just wastes it. Each of the refinements can be solved elegantly in a small number of lines of code if you think them through carefully before you start to use the editor, and I shall be looking for elegant solutions.
Remember a crucial theme of this course - "Keep it as simple as you can, but no simpler". I am looking for imaginative, clear, simple solutions to the problems.
An easy one to start you off.
Consider the following program, which is a Parva version of an algorithm you have surely seen before (Fibo.pav):
// Print a table of Fibonacci numbers using (slow) recursive definition
// P.D. Terry, Rhodes University, 2011
int fib(int m) {
// Compute m-th term in Fibonacci series 0,1,1,2 ...
if (m == 0) return 0;
if (m == 1) return 1;
return fib(m-1) + fib(m-2);
} // fib
void main() {
int limit;
read("Supply upper limit ", limit);
int i = 0;
while (i <= limit) {
write(i, "\t", fib(i), "\n");
i = i + 1;
} // while
} // main
If you compile and run this for a fairly large Limit you will easily see that it takes longer and longer as i increases. In fact, the algorithm is easily shown to be O(1.6N).
Of course, if all you want is a simple table of Fibonacci numbers and not a text-book demonstration of a recursive function it is much simpler and faster to proceed as follows (fib1.pav):
// Print a table of Fibonacci numbers using (fast) iterative method
// P.D. Terry, Rhodes University, 2011
void main() {
int
term = 0,
first = 0,
second = 1,
limit;
read("Supply upper limit ", limit);
write(term, "\t", first, "\n");
while (term < limit) {
term = term + 1;
write(term, "\t", second, "\n");
int next = first + second;
first = second;
second = next;
} // while
} // main
or, perhaps, with the while loop expressed using only two variables, as follows (fib2.pav):
while (term < limit) {
term = term + 1;
write(term, "\t", second, "\n");
second = first + second;
first = second - first;
} // while
Well, suppose we do want a recursive function (perhaps you don't, but this is not a democracy!). The reason that the program runs ever slower is, of course, that evaluating the function for a large argument effectively sets up a tree of calls to functions whose values have already been computed previously many, many times. Try drawing the tree, if you need convincing.
A way of improving on this is as follows. Each time a value is computed for a hitherto unused value of the argument, store the result in a (global) array indexed by the value of the argument as well as "returning" it. Then, if another call is made for this value of the argument, obtain the value of the function from the array rather than making the two interior recursive calls. So, for example, if one had evaluated fib(2) ... fib(5) the array would contain:
.-----------------------------------------------------------------------------.
| 0 | 1 | 1 | 2 | 3 | 5 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | ......
`-----------------------------------------------------------------------------'
0 1 2 3 4 5 6 7 8 9 10 11 12 ......
and evaluating fib(6) would amount to a single addition only, while evaluating fib(10) at this point still be much faster than in the simple version in Fibo.pav (why?).
Wait! This is a very well known exercise. You are bound to be able to find many solutions on the web. Resist the temptation. Work out your own solution, please.
(The same goes for some other exercises in this course. You will get far more out of them if you try them by yourselves, or within your group, rather than hunting for previous solutions.)
This sort of optimization can be very useful in cases where a recursive function might be called many thousands of times in a system for values of an argument within a known range (so that the array size is easily fixed). The technique has a special name. After you have developed your solution, see if you can find out what this name is.
You will need to become acquainted with various library classes to solve the next three tasks, which are to be done in Java or C# as you prefer, and are designed to emphasize some useful techniques.
Descriptions of the relevant I/O library routines can be found on the course website, and a simple sample program using some of the library routines can be found in the kit as the program SampleIO.java (listed below) or SampleIO.cs (essentially the same). Note the precautionary error checking.
It is important that you learn to use the IO libraries InFile, OutFile and IO. These will be used repeatedly in this course. Please do not use other methods for doing I/O, or spend time writing lots of exception handling code or using other ways to extract tokens from complete lines.
A local bus service (like those in big cities like Grahamstown) links a large number of stops. Suppose we are given the travel time from any one stop to the next (assume that this time would be the same for return journeys) and that this information is captured in a long list of times given in minutes, with the stop names (abbreviated if necessary to 8 letters) between them. For example, if we had 8 stops we might have a list like
College 8 Hamilton 5 Oakdene 12 Gino's 25 Mews 9 Union 12 Steers 17 Athies
The railway company want to put up a poster at each stop from which one can easily determine the total travel time between any one stop and any other one. For the data given this might look like this (see file TRAINS):
College Hamilton Oakdene Gino's Mews Union Steers Athies
College 0 8 13 25 50 59 71 88
Hamilton 8 0 5 17 42 51 63 80
Oakdene 13 5 0 12 37 46 58 75
Gino's 25 17 12 0 25 34 46 63
Mews 50 42 37 25 0 9 21 38
Union 59 51 46 34 9 0 12 29
Steers 71 63 58 46 21 12 0 17
Athies 88 80 75 63 38 29 17 0
Write a program to create such a table. Use the ArrayList class to store the original data - you will need a small auxiliary class to record the successive pairs of names and travel times - and then set up a two-dimensional matrix to contain the computed values (note that this matrix will be symmetric).
Goldbach's conjecture is that every even number greater than 2 can be expressed as the sum of two prime numbers. Write a program that examines every even integer N from 4 to Limit, attempting to find a pair of prime numbers (A , B) such that N = A + B. If successful the program should write N, A and B; otherwise it should write a message indicating that the conjecture has been disproved. This might be done in various ways. Since the hidden agenda is to familiarize you with the use of a class for manipulating "sets", you must use a variation on the sieve method suggested by the code you have already seen: create a "set" of prime numbers first in an object of the IntSet class, and then use this set intelligently to check the conjecture.
A photographer is about to photograph a class of students (yes, we know, two weeks ago duncan took ages to get it right) and wants them neatly arranged by height. The plan is to have the taller people in the back rows, and for each row to slope down from the centre towards each edge. Given a class of students and their heights, Duncan needs a plan of where each should stand. The program should choose the number of rows intelligently, depending on the size of the class.
In the prac kit you can find an executable version of a program that does this (PHOTO.EXE), and a data file from which you could select the first N students (STUDENTS) to try out your ideas.
This code is in the file SampleIO.java in the prac kit. There is an equivalent C# one in the file SampleIO.cs.
import library.*;
class SampleIO {
public static void main(String[] args) {
// check that arguments have been supplied
if (args.length != 2) {
IO.writeLine("missing args");
System.exit(1);
}
// attempt to open data file
InFile data = new InFile(args[0]);
if (data.openError()) {
IO.writeLine("cannot open " + args[0]);
System.exit(1);
}
// attempt to open results file
OutFile results = new OutFile(args[1]);
if (results.openError()) {
IO.writeLine("cannot open " + args[1]);
System.exit(1);
}
// various initializations
int total = 0;
IntSet mySet = new IntSet();
IntSet smallSet = new IntSet(1, 2, 3, 4, 5);
String smallSetStr = smallSet.toString();
// read and process data file
int item = data.readInt();
while (!data.noMoreData()) {
total = total + item;
if (item > 0) mySet.incl(item);
item = data.readInt();
}
// write various results to output file
results.write("total = ");
results.writeLine(total, 5);
results.writeLine("unique positive numbers " + mySet.toString());
results.writeLine("union with " + smallSetStr
+ " = " + mySet.union(smallSet).toString());
results.writeLine("intersection with " + smallSetStr
+ " = " + mySet.intersection(smallSet).toString());
} // main
} // SampleIO