ast generation prepared by manuel e. bermúdez, ph.d. associate professor university of florida...
TRANSCRIPT
AST Generation
Prepared by
Manuel E. Bermúdez, Ph.D.Associate ProfessorUniversity of Florida
Programming Language ConceptsLecture 9
Replacing Recursion with Iteration
• Not all the nonterminals are needed.
• The recursion in SL, X, Y and Z can be replaced with iteration.
Replacing Recursion with Iteration (cont’d)
proc S; {S → begin SL end→ id := E;
case Next_Token ofT_begin : Read(T_begin);
repeat S;until Next_Token {T_begin,T_id};Read(T_end);
T_id : Read(T_id);Read (T_:=);E;Read (T_;);
otherwise Error;end
end;Replaces recursion on Z.
Replaces call
to SL.
SLSL → S Z Z → S Z → }
Replacing Recursion with Iteration (cont’d)
proc E; {E → TYY → +TY → }
T;while Next_Token = T_+ do Read (T_+); T;
odend;
Replaces recursion on Y.
Replacing Recursion with Iteration (cont’d)
proc T; {T → PXX → *T → }
P;if Next_Token = T_*
then Read (T_*);T;
end;
Replaces call to X.
Replacing Recursion with Iteration (cont’d)
proc P;{P → (E)→ id }
case Next_Token ofT_(: Read (T_();
E; Read (T_));
T_id: Read (T_id);otherwise Error;end
end;
Construction of Derivation Tree for the Original Grammar (Bottom Up)
proc S; { (1)S → begin SL end (2)S → begin SL end → id := E; → id := E;
SL → SZ SL → SL S Z → SZ → S
→ }case Next_Token of
T_begin : Read(T_begin); S; Write (SL → S); while Next_Token in {T_begin,T_id} do
S;Write (SL → SL S);
od Read(T_end); Write (S → begin SL
end);
Construction of Derivation Tree for the Original Grammar (Bottom Up) (cont’d)
T_id : Read(T_id);Read (T_:=);E;Read (T_;);Write (S → id :=E ;);
otherwise Error;end
end;
Construction of Derivation Tree for the Original Grammar (Bottom Up) (cont’d)
proc E; {(1)E → TY (2) E → E+T Y → +TY → T → }
T;Write (E → T);while Next_Token = T_+ do
Read (T_+);T;Write (E → E+T);
odend
Construction of Derivation Tree for the Original Grammar (Bottom Up) (cont’d)proc T; {(1)T → PX (2) T → P*T
X → *T → P →
}P; if Next_Token = T_*
then Read (T_*);T;Write (T → P*T)
else Write (T → P);end;
Construction of Derivation Tree for the Original Grammar (Bottom Up) (cont’d)
proc P;{(1)P → (E) (2)P → (E) → id → id
}
// SAME AS BEFOREend;
Example
• Input String: begin id := (id + id) * id; end• Output:
P → idT → PE → TP → idT → PE → E+TP → (E)P → idT → P
T → P*TE → TS → id:=E;SL→ SS → begin SL end
Generating the Abstract Syntax Tree, Bottom Up, for the Original Grammar proc S; { S → begin S+ end 'block'
→ id := E; 'assign'var N:integer;
case Next_Token ofT_begin : Read(T_begin);
S;N:=1;while Next_Token in {T_begin,T_id} do
S;N:=N+1;
odRead(T_end);Build Tree ('block',N);
T_id : Read(T_id);Read (T_:=);E;Read (T_;);Build Tree ('assign',2);
otherwise Errorend
end;
Assume this builds a node.
Build Tree (‘x’,n) pops n trees from the stack, builds an ‘x’ node as their parent, and pushes the resulting tree.
Generating the Abstract Syntax Tree, Bottom Up, for the Original Grammar (cont’d)proc E; {E → E+T '+'
→ T }T;while Next_Token = T_+ do
Read (T_+)T;Build Tree ('+',2);
odend;
Left branching in tree!
Generating the Abstract Syntax Tree, Bottom Up, for the Original Grammar (cont’d)
proc T; {T → P*T '*' → P }
P;if Next_Token = T_*
then Read (T_*)T;Build Tree ('*',2);
end;
Right branching in tree!
Generating the Abstract Syntax Tree, Bottom Up, for the Original Grammar (cont’d)
proc P;{P → (E)→ id }
// SAME AS BEFORE, // i.e.,no trees builtend;
Example
• Input String: begin id1 := (id2 + id3) * id4; end
• Sequence of events:
id1
id2
id3
id4
BT('+',2)
BT('*',2)
BT('assign',2)
BT('block',1)
Summary
• Bottom-up or top-down tree construction.
• Original or modified grammar.• Derivation Tree or Abstract Syntax
Tree.
• Technique of choice (Hint: project)• Top-down, recursive descent parser.• Bottom-up tree construction for the
original grammar.
AST Generation
Prepared by
Manuel E. Bermúdez, Ph.D.Associate ProfessorUniversity of Florida
Programming Language ConceptsLecture 9