managing the symbol table - hampden-sydney...

Managing the Symbol TableLecture 21

Sections 2.7, 5.1

Robb T. Koether

Hampden-Sydney College

Fri, Mar 6, 2015

Robb T. Koether (Hampden-Sydney College) Managing the Symbol Table Fri, Mar 6, 2015 1 / 26

1 Symbol Table FunctionsInitialization Functionsenter_block() and leave_block()id_lookup()install()

2 The Lexer and the Symbol Table

3 An Example

4 Assignment


Outline



3 An Example

4 Assignment


Symbol Table Members

Symbol Table MembersLinkedList id_tableint level

id_table – The linked list of hashtables of symbols.level – The current level in the linked list.


Symbol Table Functions

Symbol Table Functionsvoid init()

void init_keywords()

void install_keyword()

void enter_block()

void leave_block()

IdEntry id_lookup(String name, int blk_lev)

IdEntry install(String name, int blk_lev)


Outline



3 An Example

4 Assignment


The Function init()

The Function init()id_table = new LinkedList<Hashtable<String, IdEntry>>();id_table.addLast(null);

Create a linked list.Create a null hash table at level 0 (node 0).


The Function init_keywords()

The Function init_keywords()install_res_word(sym.INT, "int");install_res_word(sym.DOUBLE, "double");

...

Initialize the symbol table with all the keywords.


The Function install_keyword(kw)

The Function install_keyword(kw)IdEntry p = install(kw, 1);p.type = t;

Install the keyword s at level 1 of the symbol table.The install() function returns a reference to the IdEntry inthe table.Set the type attribute to t.


Outline



3 An Example

4 Assignment


The Function enter_block()

The Function enter_block()level++;id_table.addLast(new Hashtable<String, IdEntry>());

This function creates a new hash table at the next higher level.


The Function leave_block()

The Function leave_block()id_table.removeLast();level-;

This function removes the hash table from the highest level.


Outline



3 An Example

4 Assignment


The Function id_lookup(name, blk_lev)

The Function id_lookup()Identry id = null;Hashtable<String, Identry> ht = id_table.get(blk_lev);id = ht.get(name);

If blk_lev is positive, then id_lookup() searches only thehash table at that level.


The Function id_lookup(name, blk_lev)

The Function id_lookup()Identry id = null;for (int i = id_table.size() - 1; i > 0; i-){

Hashtable<String, Identry> ht = id_table.get(i);id = ht.get(name);if (id != null) break;

}

If blk_lev is 0, then id_lookup() searches all hash tables,beginning at the highest level.


Outline



3 An Example

4 Assignment


The Function install(name, blk_lev)

The Function install()Hashtable<String, Identry> ht = id_table.get(blk_lev);IdEntry id = new IdEntry();id.name = s;id.block_level = blk_lev;ht.put(s, id);

If blk_lev is 0, then install() installs the identifier at thecurrent level, level.If blk_lev is positive, then install() installs the identifier atthat level.


Outline



3 An Example

4 Assignment


The Lexer and the Symbol Table

The lexer must distinguish between identifiers and keywords.We could write a separate regular expression for each keyword:"int" {return new Symbol(sym.INT)}

However, this would make the lexer much more complicated.


The Lexer and the Symbol Table

When a token consists of a string of letters, the lexer will checklevel 1 of the symbol table.If the string is found there, then the lexer will return theappropriate keyword token.If the string is not found there, then the lexer will assume that it isan identifier and return an identifier token.


The Function lookup(name)

The Function lookup(name)IdEntry entry = SymbolTable.id_lookup(name, 1);if (entry != null)

return new Symbol(entry.type);else

return new Symbol(sym.ID, s);


The Function lookup(name)

The lookup() function is placed in the directive section of theflex file, within %{ and %}.


Outline



3 An Example

4 Assignment


Managing the Symbol Table

Lexer ParserSemantic

Action

SymbolTable

Processing the statement int count;




Action

SymbolTable

"int"

The Lexer sees “int”




Action

SymbolTable

"int"

lookup("int", 1)

Search the Symbol Table for keyword “int”




Action

SymbolTable

"int"

not null

Keyword “int” is found




Action

SymbolTable

"int" sym.INT

Send the INT token to the Parser




Action

SymbolTable

"int" sym.INT type ::= INT

The Parser matches type→ INT




Action

SymbolTable

"count"

The Lexer sees “count”




Action

SymbolTable

"count"

lookup("count", 1)

Search the Symbol Table for keyword “count”




Action

SymbolTable

"count"

null

Keyword “count” is not found




Action

SymbolTable

"count"sym.ID, "count"

Send the ID token to the Parser




Action

SymbolTable

"count" lval ::= ID

The Parser matches lval → ID




Action

SymbolTable

"count" lval ::= IDid("count")

Invoke the Semantic Action id("count")




Action

SymbolTable

"count"

id_lookup("count", 0)

lval ::= ID

Look up “count” in the Symbol Table




Action

SymbolTable

"count" lval ::= ID

null

“count” is not found




Action

SymbolTable

"count" lval ::= ID

install("count", 0)

IdEntryname = "count"

Install “count” in the Symbol Table




Action

SymbolTable

";"


The Lexer sees “;”




Action

SymbolTable

";"sym.SEMI


Send the SEMI token to the Parser




Action

SymbolTable

";"sym.SEMI

dcl ::= type ID ;


The Parser matches dcl → type ID ;




Action

SymbolTable

";" dcl ::= type ID ;dcl("count", 0)


Invoke the Semantic Action dcl("count", 0)




Action

SymbolTable

";"

install("count", 0)

dcl ::= type ID ;


Invoke install("count", 0)




Action

SymbolTable

";" dcl ::= type ID ;


IdEntry

Retrieve the IdEntry for count




Action

SymbolTable

";" dcl ::= type ID ;

IdEntryname = "count"type = INT

Update the type of count to INT


Outline



3 An Example

4 Assignment


Assignment

HomeworkRead Sections 2.7 and 5.1.


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