Hand in your solutions to this practical before lunch time on your next practical day, correctly packaged in a transparent folder with your cover sheets. Please do NOT come to a practical and spend the first hour printing or completing solutions from the previous week's exercises. Since the practical will have been done on a group basis, please hand in one copy of the cover sheet for each member of the group. These will be returned to you in due course, signed by the marker. Please make it clear whose folder you have used for the electronic submission, for example g03A1234. Lastly, please resist the temptation to carve up the practical, with each group member only doing one task. The group experience is best when you discuss each task together.
In this practical you are to
You will need this prac sheet and your text book. As usual, copies of the prac sheet are also available at http://www.cs.ru.ac.za/CSc301/Translators/trans.htm.
When you have completed this practical you should understand
This week you are required to hand in, besides the cover sheet:
I do NOT require listings of any Java code produced by Coco/R.
Keep the prac 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 and not given to demonstrators.
A rule not stated there, but which should be obvious, is that you are not allowed to hand in another group's or student'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. You are expected to be familiar with the University Policy on Plagiarism, which you can consult at:
http://www.scifac.ru.ac.za/plag.htm
There are several files that you need, zipped up this week in the file PRAC21.ZIP (Java version) or PRAC21C.ZIP (C# version)
j:
md prac21
cd prac21
copy i:\csc301\trans\prac21.zip
unzip prac21.zip
You will find the executable version of Coco/R and batch files for running it, frame files, and various sample programs and grammars, contained in files with extensions like
*.ATG, *.PAV, *.TXT *.BAD
In the kit you will find Calc.atg. This is essentially the calculator grammar on page 106 of the text, with a slight (cosmetic) change of name.
Use Coco/R to generate a parser for data for this calculator. You do this most simply by giving the command
cmake Calc
The primary name of the file (Calc) is case sensitive. Note that the .ATG extension is needed, but not given in the command. Used like this, Coco/R will simply generate three important components of a calculator program - the parser, scanner, and main driver program. Cocol specifications can be programmed to generate a complete calculator too (ie one that will evaluate the expressions, rather than simply check them for syntactic correctness), but that will have to wait for the early hours of another day.
(Wow! Have you ever written a program so fast in your life before?)
Of course, having Coco/R write you a program is one thing. But it might also be fun and interesting to run the program and see that it works.
A command like
crun Calc calc.txt
will run the program Calc and try to parse the file calc.txt, sending error messages to the screen. Giving the command in the form
crun Calc calc.bad -L
will send an error listing to the file listing.txt, which might be more convenient. Try this out.
Well, you did all that. Well done. What next?
For some light relief and interest you might like to look at the code the system generated for you (three .java files are created in a subdirectory Calc) - you don't have to comment this week, simply gaze in awe. Don't take too long over this, because now you have the chance to be more creative.
That's right - we have not finished yet. Modify the grammar so that you can use parentheses and leading signs in your expressions, and also give the calculator absolute and square root functions as in
abs(4 + (3 - sqrt(4)) * 7) .
Of course, the application does not have any real "calculator" capability - it cannot calculate anything (yet). It only has the ability to recognise or reject expressions at this stage. Try it out with some expressions that use the new features, and some that use them incorrectly.
Warning. Language design and grammar design is easy to get wrong. Think hard about these problems before you begin, and while you are doing them.
The file EBNF.ATG contains a Cocol grammar that describes EBNF using EBNF conventions, which might be familiar from lectures. Try this out "as is" to begin with, for example:
cmake EBNF
crun EBNF ebnf.txt
crun EBNF ebnf.bad -L
Next, use the grammar as a guide to develop a new system that will recognise or reject a set of productions written one to a line in "traditional" BNF notation, like those below:
<name> ::= <title> <first part> <surname>
<title> ::= Mr | Miss | Ms | Dr | Prof | eps
<first part> ::= <name> | <first part> <name> | eps
<surname> ::= <name>
<name> ::= <letter> | <letter> <name>
<letter> ::= <first half letter> | <second half letter>
<first half letter > ::= a | b | c | d | e | f | g | h | i | j | k | l | m
<second half letter> ::= n | o | p | q | r | s | t | u | v | w | x | y | z
Examples like this can be found in the files BNF.TXT and BNF.BAD. Note that we have used eps to denote the Greek letter e (epsilon) which cannot easily be represented in the character set you have available.
Parva really is a horrid little language, isn't it? But its simplicity means that it is easy to devise Terry Torture on the lines of "extend it".
In the prac kit you will find the grammar for the first level of Parva, taken from page 164. Generate a program from this that will recognise or reject simple Parva programs, and verify that the program behaves correctly with two of the sample programs in the kit, namely VOTER.PAV and VOTER.BAD.
cmake Parva
crun Parva voter.pav
crun Parva voter.bad
Now modify the grammar to add various features. Specifically, add (and check that the additions work):
Here are some examples of code that should give you some ideas.
void main () {
// Demonstrate various statements, including else
int age;
bool beenKissed;
read("How old are you, and have you been kissed? ", age, beenKissed);
if (age == 16) {
write("sweet sixteen");
if (! beenKissed) write(" and never been kissed?");
}
else
if (age == 21) {
write("party time!");
int beers = 20, headache = 0, strain = 0;
while (beers > 0) {
beers--; strain++; ++headache;
if (beers % 8 == 0) strain = 0;
}
}
else write("do your practical");
}
void main () {
// Demonstrate syntax for various loops and conditionals
// (not supposed to do anything useful!)
int i = 1, k = 0;
while (i < 10) {
do {
k = k + i;
if (k % 2 == 0) continue;
write(k);
} while (k < i);
}
for i = k to k + 25
write(2 * i, " is an even number");
}
void main () {
// Demonstrate various statements, including if - else
int age;
const tooMuch = 5;
read(age);
switch (age) {
case 16:
write("sweet sixteen"); break;
case 21: {
write("party time!");
int beers, headache = 0, strain = 0;
for beers = 20 to 0 by -1 {
beers--; strain++; ++headache;
if (headache > tooMuch) break;
if (beers % 8 == 0) strain = 0;
}
break;
}
case 30:
write("over the hill, bru"); break;
case 40:
case 50:
case 60:
write("take a new lover"); break;
default:
write("boring"); break;
}
}
These little programs are in the kit (test*.pav), and you can easily write some more of your own.
Note: Read that phrase again: "that should give you some ideas". And again. And again. Don't just rush in and write a grammar that will recognise only some restricted forms of statement. Think hard about what sorts of things you can see there, and think hard about how you could make your grammar fairly general.
Hint: All we require at this stage is the ability to describe these features. You do not have to try to give them any semantic meaning or write code to allow you to use them in any way. In later pracs we might try to do that, but please stick to what is asked for this time, and don't go being over ambitious.
In the prac kit you will find an extract from the index to my forthcoming bestseller "Hacking out a Degree", in the file INDEX.TXT. It looks something like this:
abstract class 12, 45
abstraction, data 165
advantages of Java and Modula-2 1-99, 100-500, Appendix 4
aegrotat examinations -- see unethical doctors
aggregate pass, chances of 0
class attendance, intolerable 15, 745
class members 31
deadlines, compiler course -- see sunrise
horrible design (C and C++) 34, 45, 56-80
lectures, missed 1, 3, 5-9, 12, 14-17, 21-25, 28
loss of DP certificate 2007
no chance of supp 2008
prac tests 10, 25, 27, 30
probable exclusion from Rhodes 2007
recursion -- see recursion
senility, onset of 21-24, 105
subminimum 40
supplementary exams (first courses only) 45 - 49
wasted years 2005 - 2007
Develop a grammar Index.atg that describes this and similar indexes (indices?) and create a program that will analyse an index and accept or reject it (syntactically).
For ease of use with the Java file and directory naming conventions, please
Make sure that the grammar includes the "pragma" $CN. The COMPILER line of your grammar description should thus always read something like
COMPILER Gram $CN
The laboratory installation of UltraEdit has been configured to link to Coco/R by using an option in the "advanced" pull down menu. To apply Coco/R to the file in the "current window", invoke UltraEdit from your working directory, for example
UEDIT32 Gram.atg
It is easy enough to configure a copy of UltraEdit you might have installed on your own computer to incorporate the option to invoke Coco/R. Use the Advanced ->; Tool Configuration pull down, and set up an option to read as in the example below
If the Coco/R generation process succeeds, the Java compiler is invoked automatically to try to compile the application. If this does not succeed, a Java compiler error listing is redirected to the file ERRORS, where it can be viewed easily by opening the file in UltraEdit.
You can run the Java version of Coco/R in free standing mode with a command like:
cmake Gram
Like that, error messages are a little cryptic. In the form
cmake Gram -options m
the system will produce you a listing of the grammar file and the associated error messages, if any, in the file LISTING.TXT. If the Coco/R generation process succeeds the Java compiler is invoked automatically to try to compile the application. If this does not succeed, a Java compiler error listing is redirected to the file ERRORS, where it can be viewed easily by opening the file in UltraEdit.
Error checking by Coco/R takes place in various stages. The first of these relates to simple syntactic errors - like leaving off a period at the end of a production. These are usually easily fixed. The second stage consists of ensuring that all non-terminals have been defined with right hand sides, that all non-terminals are "reachable", that there are no cyclic productions, no useless productions, and in particular that the productions satisfy what are known as LL(1) constraints. We shall discuss LL(1) constraints in class in the next week, and so for this practical we shall simply hope that they do not become tiresome. The most common way of violating the LL(1) constraints is to have alternatives for a nonterminal that start with the same piece of string. This means that a so- called LL(1) parser (which is what Coco/R generates for you) cannot easily decide which alternative to take - and in fact will run the risk of going badly astray. Here is an example of a rule that violates the LL(1) constraints:
assignment = variableName ":=" expression
| variableName index ":=" expression.
index = "[" subscript "]" .
Both alternatives for assignment start with a variableName. However, we can easily write production rules that does not have this snag:
assignment = variableName [ index ] ":=" expression .
index = "[" subscript "]" .
A moment's thought will show that the various expression grammars that we have discussed in class - the left recursive rules like
expression = term | expression "-" term .
also violate the LL(1) constraints, and so have to be recast as
expression = term { "-" term } .
to get around the problem.
For the moment, if you encounter LL(1) problems, please speak to the long suffering demonstrators, who will hopefully be able to help you resolve all (or most) of them.