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Assignment #2: Object-Oriented Top-down Parsing

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CSE 305
Assignment #2: Object-Oriented Top-down Parsing

Note: This assignment may be done by a pair of students.
This assignment is about top-down parsing and type-checking for a simple programming language, called TinyPL,
whose grammar is given below in Chomsky notation with EBNF extensions:
program  ‘begin’ decls stmts ‘end’
decls  [ ‘int’ idlist ‘;’ ] [ ‘real’ idlist ‘;’ ] [ ‘bool’ idlist ‘;’ ]
idlist  id {‘,’ id}
stmts  { stmt }
stmt  assign | cond | loop | cmpd
assign  var ‘=’ expr ‘;’
loop  while ‘(‘ expr ‘)’ stmt
cond  if ‘(‘ expr ‘)’ stmt [ ‘else’ stmt ]
cmpd  ‘{‘ stmts ‘}’
expr  term [ (‘+’ | ‘-‘ | ‘||’) expr ]
term  factor [ (‘*’ | ‘/’ | ‘&&’) term ]
factor  literal | ‘!’ factor | ‘(‘ expr ‘)’
| ‘(‘ expr (‘<=’ | ‘=’ | ‘==’ | ‘!=’ | ‘<’ | ‘’) expr ‘)’
literal  ‘int’ | ‘real’ | ‘true’ | ‘false’ | ‘id’
Part I: Due Weds, Feb 25, 11:59 pm
Write an object-oriented top-down recognizer in Java to recognize well-formed TinyPL programs according to the
above grammar. This part does not require any type-checking; that is to be done in part II. Note the following:
1. All terminal symbols in the above grammar are given in quotes, and these are handled by the lexical
analyzer. For simplicity, the lexical analyzer assumes that an identifier is a single letter; an int is an
unsigned number; and a real is of the form digits.digits.
2. For each nonterminal of the TinyPL grammar, write one Java class, and put in its constructor the code for
the production rule associated with the nonterminal.
3. Develop your code starting from the files in: PiazzaResourcesHomeworksTinyPL.zip. There are four
Java files: TinyPL.java, Lexer.java, Buffer.java and Token.java. You have to complete TinyPL.java with the
code for the recognizer. The other three files pertain to lexical analysis – do not change these files.
4. The grammar exhibits the “dangling else” ambiguity. Resolve the ambiguity in your recognizer by matching
each ‘else’ with the closest previous unmatched ‘if’. Note: There is no need to rewrite the grammar.
5. Define four additional classes, called Int_Lit, Real_Lit, Bool_Lit, and Id_Lit corresponding to the literals for
int, real, bool, and id respectively. Make them subclasses of class Literal. Objects of these classes will hold
specific instances for the four kinds of literals: integers, reals, booleans, and identifiers respectively.
6. The input test cases will all be well-formed programs; no error-checking of the input is necessary.
7. Run your recognizer through JIVE and save the object diagram for each test case. The object diagram for
the i-th test case should be named obj_i.png. Use the “Stacked” option for the diagrams.
8. Submit TinyPL.java and your object diagrams as separate files using the ‘submit_cse305’ command.
JIVE Object Diagrams
1. In drawing an object diagram (parse tree), JIVE will consider references (pointers) that emanate from fields
of objects; it will not consider references from local variables of methods/constructors.
2. Once a parse-tree has been bound to a field of an object, one should not over-write it with another parsetree. Doing so will cause the original parse-tree to become disconnected from the diagram.
Part II: Due Sun, Mar 1, 11:59 pm
Augment your recognizer of Part I with code for performing type-checking, as described below.
1. The input test cases will be well-formed TinyPL programs, but they may have type-related errors:
(i) re-declaration of an identifier;
(ii) use of an undeclared variable in a statement; and
(iii) mismatch in the types of an operator and one of its operands or the presence of a nonboolean expression in the condition part of an ‘if’ or ‘while’ statement.
Sample error messages are given on the next page.
2. Corresponding to the above three type-related errors, declare three exception classes called, respectively,
Redeclaration, TypeUnknown, and TypeMismatch. Throw an exception when any of these errors is
detected and pass a string describing the error. Handle the exception in ‘main’ and terminate parsing.
3. Declare in class TinyPL a static field ST (for symbol table) initialized to a Java ‘HashMap’. Insert into ST each
identifier found in the declarations – but first check if it is already in the table before inserting. Throw the
‘Redeclaration’ exception if any identifier is declared more than once, passing the name of the re-declared
identifier as an argument to the exception.
4. Run your type-checker through JIVE and save the object diagram for each test case. The object diagram for
the i-th test case should be named part2_obj_i.png. Use the “Stacked” option for the diagrams.
5. Submit TinyPL.java and your object diagrams as separate files using the ‘submit_cse305’ command.
Test Cases and Online Submission
1. Test cases for the two parts will be posted on Piazza.
2. Both students working on the assignment should make a submission and should write their names at the
top of the file TinyPL.java. It is fine to work solo; write your name at the top of the file.
End of Assignment 2
Sample Error Messages for Part II
Statement Error Message
int x, y, z, x; Redeclaration: x
int y; real x, y; Redeclaration: y
if (x * y) … TypeMismatch: if condition must have bool type
while(a + b) … TypeMismatch: while condition must have bool type
x = x * 3.5 TypeMismatch: int * real (if x was declared int)
x = 2 * x / 3.5 TypeMismatch: int / real (if x was declared int)
x = x && true TypeMismatch: real && bool (if x was declared real)
int w = 10; TypeUnknown: w on LHS of assignment (if w was not declared)
int x = w; TypeUnknown: w in expression (if w was not declared
but x was declared)
while (x w) … TypeUnknown: w in expression (if w was not declared
but x was declared)
Sample Well-Formed Tiny PL Program without Type Errors:
begin
int s, i, n;
bool p;
n = 37;
s = 1;
while (s*s < n) s = s + 1;
i = 2;
p = true;
if (n 2)
while (p && (i < s+1)) {
if ((n/i)*i == n)
p = false;
i = i + 1;
}
end
The JIVE object diagram for the above program is shown on the next page.

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