context-free-grammar

问题 Given an arbitrary context-free grammar, how can I check whether it describes a regular language? I'm not looking for exam "tricks". I'm looking for a foolproof mechanical test that I can code. If it helps, here's an example of a CFG that I might receive as an input. Specifically, notice that the answer must be much more complicated than just looking for left- or right-recursion, since the presence of another type of recursion does not automatically imply the grammar is irregular. S: A B C D

问题 I have a set of grammar and I need to find the First and Follow from it. So far I've managed to make the First, but now I'm confused as to how to make the Follow. The set of grammar that I tried to solve: E -> -E | (E) | VT T -> -E | ε V -> id L L -> (E) | ε The First that I've come up with. If something's wrong, please inform me: First (E) = -, (, id First (T) = -, ε First (V) = id First (L) = (, ε Here's the Follow that I've managed to gather up so far: Follow (E) = $, ) Follow (T) = $, )

问题 So I copied this code from an SO answer: grammar Mu; parse : block EOF ; block : stat* ; stat : assignment | if_stat | while_stat | log | OTHER {System.err.println("unknown char: " + $OTHER.text);} ; assignment : ID ASSIGN expr SCOL ; if_stat : IF condition_block (ELSE IF condition_block)* (ELSE stat_block)? ; condition_block : expr stat_block ; stat_block : OBRACE block CBRACE | stat ; while_stat : WHILE expr stat_block ; log : LOG expr SCOL ; expr : expr POW<assoc=right> expr #powExpr |

问题 So I copied this code from an SO answer: grammar Mu; parse : block EOF ; block : stat* ; stat : assignment | if_stat | while_stat | log | OTHER {System.err.println("unknown char: " + $OTHER.text);} ; assignment : ID ASSIGN expr SCOL ; if_stat : IF condition_block (ELSE IF condition_block)* (ELSE stat_block)? ; condition_block : expr stat_block ; stat_block : OBRACE block CBRACE | stat ; while_stat : WHILE expr stat_block ; log : LOG expr SCOL ; expr : expr POW<assoc=right> expr #powExpr |

问题 Took a look at the declarationEmitter and for variable declarations, it has the function #emitVariableDeclaration which eventually calls #writeTypeOfDeclaration . This code does what is says---it takes a variable declaration and prints the variable and its type---this is exactly what I want to do. The problem is that when I replicate this code, the VariableDeclaration node has no symbol property...and thus, the type is always "any". Is there a missing step to initialize "symbols"? //sample

问题 I am trying to make a flex+bison scanner and parser for Newick file format trees in order to do operations on them. The implemented grammar an explanation is based on a simplification of (labels and lengths are always of the same type, returned by flex) this example. This is esentially a parser for a file format which represents a tree with a series of (recursive) subtrees and/or leaves. The main tree will always end on ; and said tree and all subtrees within will contain a series of nodes

问题 I have some problems to figure out a grammar for this special language, hope you could help: The language is: Σ={x,y,z} A = { w | w ∈ Σ^∗ ∧ |w|_x mod 2 >= |w|_y mod 2 } Because this one is so hard i tried first to put all properties together in one grammar, so |w|_x mod 2 >= |w|_y mod 2 and w ∈ Σ^∗ , but without getting all combination like cacbcacb etc What I get is something like: ccccc...caa...abcbbbcc and than i use ac -> ca etc to change the combination and to get every word I want. But

问题 NLTK (Natural Language Toolkit) lets you parse a FCFG grammar using nltk.FCFG.fromstring([grammar string here]). Where is the FCFG grammar format specification*? I googled it to death, but all I could find was this. *i.e. grammar language specification 回答1: From the demo: >>> from nltk import CFG >>> grammar = CFG.fromstring(""" ... S -> NP VP ... PP -> P NP ... NP -> Det N | NP PP ... VP -> V NP | VP PP ... Det -> 'a' | 'the' ... N -> 'dog' | 'cat' ... V -> 'chased' | 'sat' ... P -> 'on' |

问题 I am currently building a parser by hand. It is a LL(1) parser. At the moment, it is a great recognizer: its function parse(List tokens) decides whether or not tokens is a member of the language or not. Now, I want to build the corresponding AST for that input. However, I know how to implement it in a recursive descent way (already did it). That is, for the challenge, I implement my stack using a stack with the classical algorithm: next <- first token of the input stack <- START_SYMBOL do {

问题 The following grammar generates the language of regular expression S → A | B | C A → 0A1 | 1 | A1 B → 0B1 | 0 | 0B C → D10D D → empty | 0D | 1D Give leftmost and rightmost derivations of the following strings: 010100 000011 来源： https://stackoverflow.com/questions/62273171/i-cant-find-the-leftmost-and-rightmost-derivations-of-strings-because-there-is