1. SYMBOL TABLE Chuen-Liang Chen Department of Computer Science
and Information Engineering
National Taiwan University
Taipei, TAIWAN

2. Introduction (1/2) Introduction (1/2)
function
dictionary
interface (operations)
typedef char string[MAXSTRING];
typedef struct symtab { ... } *symbol_table; /* a pointer */
typedef struct id_entry { ... } id_entry;
/* Create a new (empty) symbol table . */
extern symbol_table create(void);
/* Remove all entries in table and destroy it. */
extern void destory(symbol_table table);
name
semantic
meaning
attribute

3. Introduction (2/2)
/* Enter name in table; return a reference to the entry corresponding to name
* and a flag to indicate whether the name was already present. */
extern void enter( symbol_table table, const string name,
id_entry *entry, boolean *present );
/* Search for name is table; return a reference to the entry corresponding to name
* (if there is one) and a flag to indicate whether the name was present. */
extern void find( const symbol_table table,
const string name,
/* Associate the attrs record with entry. */
extern void set_attributes( id_entry *entry,
const attributes *attrs );
/* Get the attributes record associated with entry. */
extern void get_attributes( const id_entry entry,
attributes *attrs );

4. Basic implementation techniques
considerations
insert time u search time
storage utilization (especially, “name” field)
QUIZ: how to handle “name” field efficiently?
and so on
unordered list
array u linked list
ordered list
array
binary search tree
with balancing technique
hash table
with chaining (linked list, binary tree)
QUIZ: comparison

5. Block-structured symbol tables
nested name scopes
declare
H, A, L : Integer;
begin
X, Y : Real;
... H, A(integer), L, X, Y are visible
end;
A, C, M : Character;
... H, L, A(character), C, M are visible
problem -- a variable may be visible in one place and invisible in another place
approaches 1. an integrated table 2. individual table per scope

6. Integrated table for all scopes
name + scope number
global hash table implementation of nested scopes
hash key : name only
QUIZ: how about binary tree implementation?
QUIZ: what to do, when a scope is closed? drawbacks?
A(3)
hash table
chained entries for names
(with scope numbers)
L(1)
A(1)
C(3)
H(1)
M(3) .

7. Individual table per scope
scope stack
scopes are opened and closed in LIFO manner
interface
extern void sts_push(const symbol_table table);
extern symbol_table sts_pop(void);
extern symbol_table sts_current_scope(void); /* for insertion */
/* Search stack of tables for name; return a reference to the entry corresponding to
* name (if there is one) and a flag to indicate whether the name was present. */
extern void sts_find( const string name,
id_entry *entry,
boolean *present );
QUIZ: drawbacks?
QUIZ: comparison of two approaches
H,A,L
A,C,M
individual symbol tables

8. Symbol table for fields/records
example record declaration l approach 2
A, R : record
A : Integer;
X : record
A : real;
C : boolean;
end;
example field references -- A.X.A R.X.C
approach 1
(A,0) 1
(R,0) 2
(A,1) -
(X,1) 3
(A,3) -
(C,3) -
(A,2) -
(X,2) 4
. . . A R C
real
bool X
int
(A,4) -
(C,4) -

9. Advanced features export rules import rules altered search rules
Pascal’s with
Ada’s use
implicit declaration
overloading
forwarding reference
QUIZ: how?