The document discusses symbol tables, which are data structures used by compilers to store information about identifiers and other constructs from source code. A symbol table contains entries for each identifier with attributes like name, type, scope, and other properties. It allows efficient access to this information during analysis and synthesis phases of compilation. Symbol tables can be implemented using arrays, binary search trees, or hash tables, with hash tables being commonly used due to fast search times.

1. Akhil Kaushik
Asstt. Prof., CE Deptt.,
TIT Bhiwani
Symbol Table

2. Introduction

3. Analysis Part
• Breaks up the source program into constituent
pieces and imposes a grammatical structure on
them.
• It then uses this structure to create an intermediate
representation of the source program.
• If the analysis part detects that the source program
is either syntactically ill formed, then it must provide
informative messages, so the user can take
corrective action.

4. Analysis Part
• The analysis part also
collects information about
the source program and
stores it in a data structure
called a symbol table.
• It is passed along with the
intermediate representation
to the synthesis part.

5. Synthesis Part
• The synthesis part constructs the desired target
program from the intermediate representation and
the information in the symbol table.

6. Role of Symbol Table
Vital functions of a compiler –
• Record the variable names used in the source
program.
• Collect information about various attributes of each
name.

7. Role of Symbol Table
• These attributes may provide information about:-
– the storage allocated for a name,
– its type and its scope,
– procedure names,
– number and types of its arguments,
– the method of passing each argument and
– the type returned

8. Symbol Table Definition
• Symbol tables are data structures that are used by
compilers to hold information about source-program
constructs.
• A symbol table is a necessary component due to:-
– Declaration of identifiers appears once in a program.
– Use of identifiers may appear in many places of the
program text.

9. Symbol Table - Names
Symbol Table provides following info:-
• Which Identifier or name are used?
• What information is to be associated with a name?
• How do we access this information?

10. Symbol Table - Names

11. Symbol Table - Attributes
• Each piece of information associated with a name is
called an attribute.
• Attributes are language dependent.

12. Symbol Table - Attributes
• Identifiers and attributes are entered by the analysis
phases when processing a declaration of an
identifier.
• In block-structured languages with scopes and
explicit declarations:-
– The parser and/or semantic analyzer enter identifiers
and corresponding attributes.
– For every symbol, scanner can enter an identifier into a
symbol table if it is not already there.

13. Symbol Table - Entries
• Name and each entry in the symbol table is
associated with attributes that support the compiler
in different phases:
– The name (as a string).
– Size and Dimension
– The data type.
– Its scope (global, local, or parameter).
– Its offset from the base pointer (for local variables and
parameters only).

14. Symbol Table Capabilities
The capabilities of Symbol Table are:-
• Checking (determine whether the given information
is in the table).
• Adding or delete new Information.
• Access the information of Name.
• Generate intermediate or target code.
• Type checking - for verifying expressions &
assignments are semantically correct.

15. Symbol Table Capabilities
• Symbol tables typically need to support multiple
declarations of the same identifier within a program.

16. Symbol Table Implementation
A symbol table can be implemented in one of the
following ways:-
• Linear (sorted or unsorted) list
• Binary Search Tree
• Hash table

17. Symbol Table Implementation
• Required to make n-entries and m-inquiries.
Advantages:-
• It is easy to implement.
• Addition & retrieve information is easy
• Minimum space is required
Disadvantages:- Higher Access time

18. Symbol Table Implementation
• Use linear Array of records ,one record per name.
• Entries of S.T are not uniform.
• So to make it uniform, some information is kept
outside the table and pointer to this information
stored in S.T.
• Record (consist known no. of consecutive words of
memory, so names stored in record)

19. Symbol Table Implementation

20. Symbol Table Implementation
• If there is modest upper bound on the length of the
name, then the name can be stored in the symbol
table record itself.
• But If there is no such limit or the limit is already
reached then an indirect scheme of storing name is
used.
• A separate array of characters called a ‘string
table’ is used to store the name and a pointer to the
name is kept in the symbol table record

21. Symbol Table Implementation

22. Symbol Table Implementation
• Binary Search Tree: Efficient approach to organize
S.T with two fields: left & right.
• Algorithm for searching name in B.S.T
– P= initially a pointer to root
– 1) If Name = Name (P) then Return /* success */
– 2) Else if Name < Name (P) then
– P:= left(P) /* visit left child */
– 3) Else Name (P) < Name then
– P:= Right (P) /* visit right child */
• Addition: Firstly search, then add if not present.

23. Symbol Table Implementation
Hash Table:- Consists K words [0….k-1]
• Pointers into the storage table (linked list)
Searching Name in S.T:-
• Apply hash function to name
• h(Name) -> {0…..k-1 (integer) }
Addition new Name:-
• Create a record at available space in storage table
and link that record to h(Name)th list.

24. Symbol Table Implementation
Why use hashing:-
• Commonly used data
structure.
• Must be organized for
quicker search.
• keyword or identifier is
'hashed' to produce an
array subscript.

25. Akhil Kaushik
akhilkaushik05@gmail.com
9416910303
CONTACT ME AT:
Akhil Kaushik
akhilkaushik05@gmail.com
9416910303
THANK YOU !!!