什麼是最左派生？

Question

請幫我理解Left Most Derivation第二個L的LL Parser是什麼意思。

用一個最簡單的例子來解釋它。

我看到下面的圖片解釋最左推導，但我不明白：

Answer 1

語法規則被顯示在左邊與非終結符和終結符。非終結符號應該是大寫字母，其他所有符號通常都是終端符號。在例子中，N和D是非終結符，0-9是終結符。最左派生總是讓最左邊的非終結符通過語法規則。試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.