Elimination of left recursion
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This presentation discusses eliminating left recursion from context-free grammars. Left recursion occurs when the leftmost symbol on the right side of a production rule is the same as the non-terminal on the left side, which can cause infinite loops in recursive-descent parsers. The presentation provides an example of a left-recursive grammar for expressions and shows how to eliminate the left recursion by introducing new non-terminals and production rules to define expressions and terms without left recursion. It notes that both immediate and indirect left recursion must be eliminated.
Elimination of left recursion
- 1. ELEMINATION OF LEFT RECURSION Wellcome to our presentation. Slide 1
- 3. What is left recursion? Eliminating left Recursion... Example of Eliminating Left Recursion Conclusion Reference Code part Slide 3 OVERVIEW
- 4. In the formal language theory of computer science , left recursion is a special case of recursion where a string is recognized as part of a language by the fact that it decomposes into a string from that same language (on the left) and a suffix (on the right). For instance 1+2+3 can be recognized as a sum because it can be broken into 1+2, also a sum, and +3, a suitable suffix. Slide 4
- 5. Eliminate left Recursion Context Free Grammar Slide 5
- 6. LEFT RECURSION The production is left-recursive if the leftmost symbol on the right side is the same as the non terminal on the left side. For example, expr → expr + term. If one were to code this production in a recursive-descent parser, the parser would go in an infinite loop. Slide 6
- 7. We can eliminate the left-recursion by introducing new nonterminals and new productions rules. For example, the left-recursive grammar is: E → E + T | T E → T * F | F F → (E) | id. We can redefine E and T without left-recursion as E → TE` E`→ + TE` | E T → FT` T → * FT` | E F → (E) | id Getting rid of such immediate left recursion is not enough. One must get rid of indirect left recursion too, where two or more nonterminals are mutually left- recursive Slide 7