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# è¯­æ³•åˆ†æžreview

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LL 1 语语语 The first “L” means we scan the input from left to right; the second “L” means we create a leftmost derivation; and the 1 means one input symbol of lookahead. A grammar G is LL(1) iff whenever A –> u | v are two distinct productions of G, the following conditions hold: - for no terminal a do both u and v derive strings beginning with a (i.e. first sets are disjoint) - at most one of u and v can derive the empty string - if v =>* ε then u does not derive any string beginning with a terminal in Follow(A)
The first set of a sequence of symbols u , written as First(u ) is the set of terminals which start all the sequences of symbols derivable from u . A bit more formally, consider all strings derivable from u by a leftmost derivation. If u =>* v , where v begins with some terminal, that terminal is in First(u). If u =>* ε , then ε is in First(u ). First

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Follow The follow set of a nonterminal A is the set of terminal symbols that can appear immediately to the right of A in a valid sentential form. A bit more formally, for every valid sentential form S =>* uAv , where v begins with some terminal, that terminal is in Follow(A).

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