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FOLLOW Set in Syntax Analysis
  • Difficulty Level : Medium
  • Last Updated : 30 Sep, 2020

We have discussed following topics on Syntax Analysis. 

Introduction to Syntax Analysis 
Why FIRST and FOLLOW? 
FIRST Set in Syntax Analysis 

In this post, FOLLOW Set is discussed. 

Follow(X) to be the set of terminals that can appear immediately to the right of Non-Terminal X in some sentential form. 
Example: 

S ->Aa | Ac
A ->b  

      S                  S  
     /  \              /   \
    A    a            A     C  
    |                 |
    b                 b   

Here, FOLLOW (A) = {a, c}

Rules to compute FOLLOW set: 



1) FOLLOW(S) = { $ }   // where S is the starting Non-Terminal

2) If A -> pBq is a production, where p, B and q are any grammar symbols,
   then everything in FIRST(q)  except Є is in FOLLOW(B).

3) If A->pB is a production, then everything in FOLLOW(A) is in FOLLOW(B).

4) If A->pBq is a production and FIRST(q) contains Є, 
   then FOLLOW(B) contains { FIRST(q) – Є } U FOLLOW(A) 

Example 1: 
 

Production Rules:
E -> TE’
E’ -> +T E’|Є
T -> F T’
T’ -> *F T’ | Є
F -> (E) | id

FIRST set
FIRST(E) = FIRST(T) = { ( , id }
FIRST(E’) = { +, Є }
FIRST(T) = FIRST(F) = { ( , id }
FIRST(T’) = { *, Є }
FIRST(F) = { ( , id }

FOLLOW Set
FOLLOW(E)  = { $ , ) }  // Note  ')' is there because of 5th rule
FOLLOW(E’) = FOLLOW(E) = {  $, ) }  // See 1st production rule
FOLLOW(T)  = { FIRST(E’) – Є } U FOLLOW(E’) U FOLLOW(E) = { + , $ , ) }
FOLLOW(T’) = FOLLOW(T) =      { + , $ , ) }
FOLLOW(F)  = { FIRST(T’) –  Є } U FOLLOW(T’) U FOLLOW(T) = { *, +, $, ) }

Example 2: 

 

Production Rules:
S -> aBDh
B -> cC
C -> bC | 
D -> EF
E -> g | Є
F -> f | Є

FIRST set
FIRST(S) = { a }
FIRST(B) = { c }
FIRST(C) = { b , Є }
FIRST(D) = FIRST(E) U FIRST(F) = { g, f, Є }
FIRST(E) = { g , Є }
FIRST(F) = { f , Є }

FOLLOW Set
FOLLOW(S) = { $ } 
FOLLOW(B) = { FIRST(D) – Є } U FIRST(h) = { g , f , h }
FOLLOW(C) = FOLLOW(B) = { g , f , h }
FOLLOW(D) = FIRST(h) = { h }
FOLLOW(E) = { FIRST(F) – Є } U FOLLOW(D) = { f , h }
FOLLOW(F) = FOLLOW(D) = { h } 

Example 3: 
 

Production Rules:
S -> ACB|Cbb|Ba
A -> da|BC
B-> g|Є
C-> h| Є

FIRST set
FIRST(S) = FIRST(A) U FIRST(B) U FIRST(C) = { d, g, h, Є, b, a}
FIRST(A) = { d } U {FIRST(B)-Є} U FIRST(C) = { d, g, h, Є }
FIRST(B) = { g, Є }
FIRST(C) = { h, Є }

FOLLOW Set
FOLLOW(S) = { $ }
FOLLOW(A)  = { h, g, $ }
FOLLOW(B) = { a, $, h, g }
FOLLOW(C) = { b, g, $, h }

Note :

  1. Є as a FOLLOW doesn’t mean anything (Є is an empty string).
  2. $ is called end-marker, which represents the end of the input string, hence used while parsing to indicate that the input string has been completely processed.
  3. The grammar used above is Context-Free Grammar (CFG). The syntax of a programming language can be specified using CFG.
  4. CFG is of the form A -> B , where A is a single Non-Terminal, and B can be a set of grammar symbols ( i.e. Terminals as well as Non-Terminals)

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