CS 164, Spring 1993
Midterm #1
Professor Rowe


Problem #1 (10 points; 1 point each)
Circle T or F to indicate the true or false statements.

T F The following grammar is ambiguous.
S-->SaS
S-->SbS
S-->c

T F It is possible to build a deterministic finite state automation for the
language { a* b* } with only two states.

T F We use a scanner to convert characters to tokens rather than having the
parser do the conversion because it simplifies the parser and leads to
smaller, more time efficient compilers.

T F A sentential form is a string of terminals and non-terminals that can be
derived from the distinguished start symbol.

T F In OO93, two routines can have the same name or identifier.

T F In constructing a compiler, it is a good idea to use a program, like flex
or lex, because it simplifies the coding of a scanner.

T F A shared library contains code that can be shared by several processes
running different programs.

T F Dynamically allocated space is placed in the heap.

T F All objects in OO93 contain a pointer to an object that represents the
class of the object.

T F A grammar is a representation for a possibly infinite set of sentences.


Problem #2 (15 points)
(a) (5 points) Define a right-most derivation.

(b) (5 points) A possible definition for a floating point BINARY number input
format is the following: an optional + or - sign followed by a string of zero
or more 0's or 1's followed by a decimal point followed by a string of one or
more 0's or 1's.

Some valid numbers in this format are: +.1 10.01 -101.0.0

Some invalid numbers are: 1. -110

Show a finite state automation that will recognize BINARY floating point
numbers.

(c) (5 poinst) Explain a reduce/reduce confilict and give an example grammar with one.


Problem #3 (25 points)
Answer the questions below base on the following program fragment:
uses "stdobj.ooh"
List: class object is
         item: object;
         next: List;
         end;
Student: class object is
         name: string;
         login: string;
end;
WorkStudy: class Student is
         hours: int;
end;
BandMember: class Student is
         instrument: string;
end;
Employee: class object is
         name: string;
         jobtitle: string;
         hours: int;
end;
(a) (5 points) What is the type of "new(BandMember)"?

(b) What is the type of "new(Employee.classof)"?

(c) (10 points) Suppose you wanted to write a generic procedure named "works" that
returned true if the object passed was a work study student or an employee and
otherwise it returned false. Write the methods required to implement this procedure.
Note that the solution should be modular so that if we add a new category of student
who works, we don't have to modify your existing definitions.

(d) What does the following procedure do?
foo: procedure (x: List, y: object) : int is
begin
if (x<> nil) then
if (x.item.classof = y) then
return (1 + foo(x.next, y));
else
return (foo(x.next, y));
fi;
fi;
return (0);
end

(Hint: an example call is "foo(a_var, BandMember)" where a_var is a variable of
type List.)


Problem #4 (30 points)
Given the following LR parser tables and grammer rules, answer the following
question.

id ) ( $ S L
0 s2 s3 . . 1 .
1 . . . acc . .
2 r1 r1 r1 r1 . .
3 s2 s3 . . 5 4
4 s2 s3 s6 . 7 .
5 r3 r3 r3 r3 . .
6 r2 r2 r2 r2 . .
7 r4 r4 r4 r4 . .

The grammar is
1:S--> id
2:S--> ')' L '('
3:L--> S
4:L--> L S

(a) (10 points) Show the parse tree for the sentence ") ) id ) id ( ( (".


(b) (20 points) Show the parser configuration as it parses that input in the following
table. You must use state numbers on the syntax stack. (Hint: 25 configurations are
shown in the table -- the parse may take less than, more than, or equal to that number
steps.)

step stack input action
1   0 ) ) id ) id ( ( ( $ shift 3
2   . . .
3   . . .
4   . . .
5   . . .
6   . . .
7   . . .
8   . . .
9   . . .
10   . . .
11   . . .
12   . . .
13   . . .
14   . . .
15   . . .
16   . . .
17   . . .
18   . . .
19   . . .
20   . . .
21   . . .
22   . . .
23   . . .
24   . . .
25   . . .


Problem #5 (20 points)
Given the following grammar, construct the finite state automation that represents
the sets of collections of LR(0) items and the transitions between the sets.

S' --> S$
S --> A 'a'
S --> A 'a' S
A --> 'b' 'c'
A --> 'b' A 'c'

Notice that the rule for S' has already been added to the grammar.