Hand in your solutions to the second part of 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/courses/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/plagiarism_policy.pdf
The first few tasks do not need you to use a computer, nor should you. Do them by hand.
One of last week's tasks was to extend the four function calculator. A solution to that task is given below.
Modify the solution to give a syntactically equivalent grammar, but your solution may not use the { } or [ ] "metabrackets", although it can use the ( ) parentheses if you like. The modified grammar must still be LL(1).
COMPILER Calc1 $CN
/* Simple four function calculator (extended)
P.D. Terry, Rhodes University, 2011 */
CHARACTERS
digit = "0123456789" .
hexdigit = digit + "ABCDEF" .
TOKENS
decNumber = digit { digit } [ "." { digit } ]
| "." digit { digit } .
hexNumber = "$" hexdigit { hexdigit } .
IGNORE CHR(0) .. CHR(31)
PRODUCTIONS
Calc1 = { Expression "=" } EOF .
Expression = [ "+" | "-" ] Term { "+" Term | "-" Term } .
Term = Factor { "*" Factor | "/" Factor } .
Factor = Primary { "!" } .
Primary = decNumber | hexNumber | "(" Expression ")" | "abs" "(" Expression ")" .
END Calc1.
Hint: You may not yet have noticed that Coco allows you to write an empty option to a production simply by leaving it empty, for example
Title = "Mr" | "Mrs" | "Ms" | "Dr" | Professor" | . /* nothing beteween the last | and the . */
As an example of checking the LL(1) conditions for a grammar expressed in EBNF, let us consider how we might describe the daily activities of SAFM - "your news and information leader" (more realistically described as a non- stop party political broadcast). Hour by hour Gillian, Xolani, Nancy, Ashraf, Elvis, Krivani and their comrades present what are generously called shows. These include music and advertisements (since people pay no licence fees, the station has to raise revenue through advertising). What passes as "information" is provided by news bulletins, weather reports and talk shows. News bulletins consist of a homogeneous stream of stories, while talk shows consist of interchanges between the host and listeners who are keen enough to phone in - with the person hosting the exchange getting the first and last word on the subject, of course.
Items like "advert", "rain" and "song" are really in the category of the lexical terminals which a scanner (in the person of someone listening to the radio) would recognize as key symbols while parsing a broadcast. So one playful attempt to describe a day's activities might be on the lines of
Radio = { TalkShow | NewsBulletin | "song" | "advert" } EOF .
NewsBulletin = "advert" NewsItem { NewsItem } [ Weather ] Filler .
NewsItem = "ANC" [ "cosatu" ] | "strike" | [ "cosatu" ] "ANC" | "zuma"
| "corruption" | "malema" "song" | Accident .
TalkShow = "host" { "listener" "host" } [ Filler ] .
Accident = "collision" "claims" "another" number "lives" .
Filler = "song" | "advert" .
Weather = { "snow" | "rain" | "cloudy" | "windy" } .
Analyze this grammar in detail. One way is to begin by recasting it in a form that introduces further non- terminals that allow for the elimination of the [ ] and { } meta-brackets. If it proves out to be non-LL(1), try to find an essentially equivalent description that is LL(1), or argue why this should be impossible.
Palindromes are character strings that read the same from either end, like "Hannah" or my brother Peter's favourite line when he did the CTM advertisements: "Bob Bob". The following represent various attempts to find grammars that describe palindromes made only of the letters a and b:
(1) Palindrome = "a" Palindrome "a" | "b" Palindrome "b" .
(2) Palindrome = "a" Palindrome "a" | "b" Palindrome "b" | "a" | "b" .
(3) Palindrome = "a" [ Palindrome ] "a" | "b" [ Palindrome ] "b" .
(4) Palindrome = [ "a" Palindrome "a" | "b" Palindrome "b" | "a" | "b" ] .
Which grammars achieve their aim? If they do not, explain why not. Which of them are LL(1)? Can you find other (perhaps better) grammars that describe palindromes and which are LL(1)?
Which of the following statements are true? Justify your answer.
(a) An LL(1) grammar cannot be ambiguous.
(b) A non-LL(1) grammar must be ambiguous.
(c) An ambiguous language cannot be described by an LL(1) grammar.
(d) It is possible to find an LL(1) grammar to describe any non-ambiguous language.
Hand in your solutions to tasks 1 through 4 before continuing.
There are several files that you need, zipped up this week in the file PRAC22.ZIP (Java version) or PRAC22C.ZIP (C# version)
Copy the prac kit into a newly created directory/folder in your file space in the usual way:
j:
md prac22
cd prac22
copy i:\csc301\trans\prac22.zip
unzip prac22.zip
You will find the executable version of Coco/R and batch files for running it, frame files, and various sample programs and grammars, including ones for the grammars given in tasks 1, 2 and 3.
After unpacking this kit attempt to make the parsers, as you did last week. Ask the demonstrators to show you how to get Coco/R to show you the FIRST and FOLLOW sets for the non-terminals of the grammar, and verify that the objections (if any) that Coco/R raises to these grammars are the same as you have determined by hand.
The grammar for expressions in Parva employs very few levels of operator precedence, corresponding exactly to the levels found in Pascal and its derivatives. Modify the Parva grammar from last week's practical so that it recognizes expressions whose precedence rules are equivalent to those found in C# or Java. Do not try to include ++ and -- operators in your expressions, however.
Since I am a kindly old soul, this week's prac kit contains a correct version of Parva.atg that you can use if you did not get your own solution completed.
Something else to think about. Why do you suppose languages derived from C have so many levels of precedence and the rules they have for associativity? What do all these levels offer to a programmer that languages modelled on Pascal might appear to withhold? Do Pascal-like languages really withhold these features? Why do you suppose Wirth, in defining Pascal, did not use the C hierarchy?
Develop a Cocol grammar that describes programs written in the PVM code that you struggled with in Practical 20. That should be pretty easy, but be careful to describe the language as tightly as possible. Then, just to make the language more attractive, suppose you had been able to use optional alphanumeric labels in your code as the destinations of branching instructions - as exemplified here:
DSP 2
LOOP LDA 1
INPI
LDL 1
BZE EXIT
BRN LOOP
EXIT LDL 1
PRNI
BRN 2
HALT
Of course your parser-only system won't actually be able to assemble the code. In a few weeks time we might extend it further to do that - for the moment simply describe the language.
Wait! we are not finished yet. You might have noticed that the assembler you used last week also generated a .COD file which, for the example above, would have looked like this
ASSEM
BEGIN
{ 0 } DSP 2
{ 2 } LDA 1
{ 4 } INPI
{ 5 } LDL 1
{ 7 } BZE 11
{ 9 } BRN 2
{ 11 } LDL 1
{ 13 } PRNI
{ 14 } BRN 2
{ 16 } HALT
END.
What you might not have noticed was that this .COD file could also be assembled by the assembler - the addresses in { braces } were treated as comments. Now that you have been told this, ensure that the language you describe can handle this feature as well.
After such a musical introduction to this section of the course you will be intrigued to learn that Scottish pipe bands often compete at events called Highland Gatherings where three forms of competition are traditionally mounted. There is the so-called "Slow into Quick March" competition, in which each band plays a single Slow March followed by a single Quick March. There is the so-called "March, Strathspey and Reel" competition, where each band plays a single Quick March, followed by a single Strathspey, and then by a single Reel; this set may optionally be followed by a further Quick March. And there is also the "Medley", in which a band plays a selection of tunes in almost any order. Each tune falls into one of the categories of Quick March, Strathspey, Reel, Slow March, Jig and Hornpipe but, by tradition, a group of one or more Strathspeys within such a medley is always followed by a group of one or more Reels.
Develop a grammar to describe the activity at a Highland Gathering at which a number of competitions are held, and in each of which at least one band performs. Competitions are held in one category at a time, with a short break between competitors, and between events, and with a suitable announcement being made to introduce the category of competition and the name of each band to the audience. Regard concepts like "QuickMarch", "Reel", "AnnounceCompetition", "break" and so on as terminals - of course, in reality, there are many different possible tunes of each sort, of course, but you may have to be a piper to distinguish one tune from another.
Hand in sheet: - Available in the kit in the ASCII file HANDIN.TXT if you prefer
Task 1 - Eliminating Meta-brackets What form does your grammar take when you eliminate the meta-brackets? Task 2 - Listen to Steam Radio What form does your grammar take when you eliminate the meta-brackets? Which, if any, productions break the LL(1) rules, and why? Can you find an equivalent grammar that does obey the LL(1) constraints? If so, give it. If not, explain why you think it canot be done. Task 3 - Palindromes Does grammar 1 describe palindromes? If not, why not? Is it an LL(1) grammar? If not, why not? Does grammar 2 describe palindromes? If not, why not? Is it an LL(1) grammar? If not, why not? Does grammar 3 describe palindromes? If not, why not? Is it an LL(1) grammar? If not, why not? Does grammar 4 describe palindromes? If not, why not? Is it an LL(1) grammar? If not, why not? Can you find a better grammar to describe palindromes? If so, give it, if not, explain why not. Task 4 Which of the following statements are true? Justify your answers. (a) An LL(1) grammar cannot be ambiguous. (b) A non-LL(1) grammar must be ambiguous. (c) An ambiguous language cannot be described by an LL(1) grammar. (d) It is possible to find an LL(1) grammar to describe any non-ambiguous language.