Predictive parsers

. A non recursive top down parsing method

. Parser "predicts" which production to use

. It removes backtracking by fixing one production for every non-terminal and input token(s)

. Predictive parsers accept LL(k) languages

- First L stands for left to right scan of input

- Second L stands for leftmost derivation

- k stands for number of lookahead token

. In practice LL(1) is used

In general, the selection of a production for a non-terminal may involve trial-and-error; that is, we may have to try a production and backtrack to try another production if the first is found to be unsuitable. A production is unsuitable if, after using the production, we cannot complete the tree to match the input string. Predictive parsing is a special form of recursive-descent parsing, in which the current input token unambiguously determines the production to be applied at each step. After eliminating left recursion and left factoring, we can obtain a grammar that can be parsed by a recursive-descent parser that needs no backtracking . Basically, it removes the need of backtracking by fixing one production for every non-terminal and input tokens. Predictive parsers accept LL(k) languages where:

. First L : The input is scanned from left to right.

. Second L : Leftmost derivations are derived for the strings.

. k : The number of lookahead tokens is k.

However, in practice, LL(1) grammars are used i.e., one lookahead token is used.