什么是最左派生？

Question

请帮我理解Left Most Derivation第二个L的LL Parser是什么意思。

用一个最简单的例子来解释它。

我看到下面的图片解释最左推导，但我不明白：

Answer 1

语法规则被显示在左边与非终结符和终结符。非终结符号应该是大写字母，其他所有符号通常都是终端符号。在例子中，N和D是非终结符，0-9是终结符。最左派生总是让最左边的非终结符通过语法规则。试着格式化下面的例子。

N 
=> N D --Replaces the first/left most/only (which is "N") with the N => N D rule 
=> N D D --Replaces the first/left most nonterminal (which is "N") with the N => N D rule 
=> D D D --Replaces the first nonterminal (which is "N") with the N => D rule 
=> 1 D D --Replaces the first nonterminal ("D") with the D => 1 rule(our first terminal character!) 
=> 1 2 D --Replaces the first nonterminal ("D") with the D => 2 rule 
=> 1 2 3 --Replaces the first nonterminal ("D") with the D => 3 rule 
-- Only terminal characters remain, derivation/reduction is complete.