LR(1) - 物品，向前看

Question

我很难理解 LR(1) 中的先行原理 - 项目。如何计算前瞻集？

举个例子，我有以下语法：

S -> AB
A -> aAb | b
B -> d

那么第一个状态会是这样的：

S -> .AB , {look ahead}
A -> .aAb, {look ahead}
A -> .b,   {look ahead}

我知道前瞻是什么，但我不知道如何计算它们。 我已经用谷歌搜索了答案，但找不到一个简单的解释这个问题的网页。

提前致谢

Answer 1

我会为你的例子写下前两种状态：

S -> AB
A -> aAb | b
B -> d

状态 0：

(1) S -> .AB, {$}   # Once we have done this rule it's EOF ($) 
(2) A -> .aAb, {d}  # from (1), after A there has to be a B whose first symbol has to be d
(3) A -> .b, {d}    # as above

状态 1：

(4) A -> a.Ab, {d}   # from (2)
(5) A -> .aAb, {b}   # from (4), the symbol after the A has to be b
(6) A -> .b, {b}     # from (4), as above
(7) A -> b., {d}     # from (3)
(8) S -> A.B, {$}    # from (1) and (7)
(9) B -> .B, {$}     # from (8)

以此类推，继续遵循与 LR(0) 解析器相同的移位/减少/关闭，但要跟踪 (lookahead for) 下一个符号...
（状态 2+ 较长，我不建议手动计算它们！）

我建议查看Udacity's Programming Languages 课程以了解有关词法分析和解析的更多信息。 还有an example on wikipedia 和related SO question。