compiler design

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Learn First and Follow

Learn First and Follow , Constructing LL(1) parsing tables and Example on First and follow
22 February, 2016     

Construction of LL(1) Parsing table:

  • Using First and follow we can construct parsing table.
  • Parsing table help us to parse the tree and till now we are directly using the parsing table now we will learn how to construct Parsing table.
  • First gives set of all terminals that may derived from alpha.
  • If First is a terminal like a,b,(,+,*,id etc..then it is first including empty set element .
Eg2: S-> AaAb | BBBa
         A-> a
         B-> B 
    Compute First S, A,B ???

Ans: First(S)= AaAb 
                               First of A that is a and first of BbBa that is b 
                          so first (S)= a,b
         First(A)= a
         First(B)= b
  • Follow always find on the right side of the production rule.
  • Like we have to find follow(E) so we have to look E->TE.  This E is our element which we need to find.
  • Follow(A)=  Set of all terminals that may follow immediately to the right of A.
  • like A->Aaaa These are terminals , they are follow of A.
  • Follow (A) is $ if A is start symbol.
  • If follow(A) is null or empty set then we will move to left side non-terminal set .
  • S->BA.. Find follow(A). So in this case we will find the value of Follow(A) is Follow(S).
  • if b is empty set because follow never contain empty set.
Que: Find First and follow
Solve the question nai toh move to next chapters so till den have fun bye tc..

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