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
The first few tasks do not need you to use a computer, nor should you. Do them by hand.
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 John, Nikiwe, Thabiso, Simon 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 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 "music" 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 attempt to describe a day's activities might be on the lines of
Radio = { TalkShow | NewsBulletin | "music" | "advert" } EOF .
NewsBulletin = "advert" NewsItem { NewsItem } [ Weather ] Filler .
NewsItem = "zuma" [ "shaik" ] | "strike" | [ "shaik" ] "zuma" | "mbeki" | "randFalls" | "accident" .
TalkShow = "host" { "listener" "host" } [ Filler ] .
Filler = "music" | "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's favourite line "Bob Bob". The following represent various ways of finding 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 3 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.
Develop a Cocol grammar BOOL.ATG for describing a list of Boolean expressions like those you should remember from your Boolean Algebra course in CSC 201. In this context, allow your Boolean expressions to be terminated with an = operator, and to include parentheses, single-letter variables, the constants FALSE and TRUE (sometimes written as 0 and 1), and the operators AND (written either as AND or "&" or as a "dot", or simply omitted), OR (written either as OR or as "|" or as a "plus") and NOT, written either as a prefix NOT or as a suffix apostrophe. So some examples of Boolean expressions might be (these are in the file BOOL.TXT)
a AND B OR (C OR NOT D) = a . b + (c + d') = a b + (c + d') =
NOT (a OR b) AND TRUE = (a + b)' . 1 = (a + b)' AND 1 =
b AND NOT C AND D = b . c' . d = b c' d =
Note that there is a precedence ordering among Boolean operators - parentheses take precedence over NOT, which takes precedence over AND, which takes precedence over OR.
Test your parser out on some examples like those just shown.
And here is something to think about. Draw out the parse tree that would correspond to the expression NOT (A OR B). Then draw out the parse tree that would correspond to NOT A AND NOT B. De Morgan will tell you that these expressions mean the same thing. Do you get the same tree in each case? Discuss the implications.
Develop a Cocol grammar that describes programs written in the ASM code that you struggled with in second year CS201 (the second-best course you have taken in Computer Science; no prizes for guessing which is the best course!). That should be pretty easy, but be careful to describe the language as tightly as possible. To refresh your memory here is a simple example program (BITS.ASM) (programs can get more complicated than this one, so be careful):
BEG ; Count the bits in a number
CLA ; CPU.A := 0
STA BITS ; BITS := 0
INI ; Read(CPU.A)
LOOP ; REPEAT
SHR ; CPU.A := CPU.A DIV 2
BCC EVEN ; IF CPU.A MOD 2 # 0 THEN
PSH ; save CPU.A on stack
LDA BITS
INC
STA BITS ; BITS := BITS + 1
POP ; restore CPU.A
EVEN BNZ LOOP ; UNTIL CPU.A = 0
LDA BITS ;
OTI ; Write(BITS)
HLT ; terminate execution
BITS DS 1 ; BYTE BITS
END
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 develop a grammar to describe the language, for which a long-winded summary can be found on the web at
http://www.scifac.ru.ac.za/lowlevel/app.htm
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 nnt 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?
Hand in sheet:
Task 1 - Listen to steam radio What form does your grammar take when you eliminate the meta-brackets? Which, if any, alternatives 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 2 - 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 3 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.