At a high level, the difference between LR(0), LALR(1), and LR(1) is the following: An LALR(1) parser is an “upgraded” version of an LR(0) parser that keeps track of more precise information to disambiguate the grammar. An LR(1) parser is a significantly more powerful parser that keeps track of even more precise information than an … Read more
Examples from wikipedia LL(1) grammar S -> F S -> ( S + F ) F -> a input ( a + a ) parsing steps S -> “(” S “+” F “)” -> ( “F” + F ) -> ( “a” + F ) -> ( a + “a” ) LR(0) grammar (1) E … Read more
To check if a grammar is LL(1), one option is to construct the LL(1) parsing table and check for any conflicts. These conflicts can be FIRST/FIRST conflicts, where two different productions would have to be predicted for a nonterminal/terminal pair. FIRST/FOLLOW conflicts, where two different productions are predicted, one representing that some production should be … Read more
Both LR(0) and SLR(1) parsers are bottom-up, directional, predictive parsers. This means that The parsers attempt to apply productions in reverse to reduce the input sentence back to the start symbol (bottom-up) The parsers scan the input from left-to-right (directional) The parsers attempt to predict what reductions to apply without necessarily seeing all of the … Read more
SLR, LALR and LR parsers can all be implemented using exactly the same table-driven machinery. Fundamentally, the parsing algorithm collects the next input token T, and consults the current state S (and associated lookahead, GOTO, and reduction tables) to decide what to do: SHIFT: If the current table says to SHIFT on the token T, … Read more
At a high level, the difference between LL parsing and LR parsing is that LL parsers begin at the start symbol and try to apply productions to arrive at the target string, whereas LR parsers begin at the target string and try to arrive back at the start symbol. An LL parse is a left-to-right, … Read more