September 2014
Intermediate to advanced
336 pages
7h 53m
Japanese
Content preview from アンダースタンディング コンピュテーション ―単純な機械から不可能なプログラムまで
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132
4
章 能力を高める
この実行トレースを見ると、機械がシンボル規則とトークン規則を行ったり来たりしているのがわか
ります。シンボル規則はスタックのトップがトークンに置き換わるまで、シンボルを繰り返し展開して
いきます。そして、トークン規則はスタックのトップがシンボルに当たるまで、スタック（と入力）を消
費していきます。入力文字列が文法規則によって生成できる限り、行ったり来たりしながら、最終的に
空のスタックが得られます
†
。
実行の各ステップにおいて、PDAはどうやって選ぶべき規則を知るのでしょうか。これが非決定性
の能力です。NPDAシミュレーションは可能な規則をすべて試します。空のスタックに到達するパス
がある限り、それは必ず見つかります。
4.3.3
実用性
このパース手順は非決定性を頼りにしていますが、実際の応用では、非決定性を避けたほうがよい
でしょう。なぜなら、非決定性PDAよりも決定性PDAのほうが、ずっと高速でシミュレートしやすい
ためです。幸運なことに、ほとんどの場合、入力トークンそのものを使って各ステージでどのシンボル
規則を適用するかを判断することで（先読み（lookahead）と呼ばれるテクニック）、非決定性を取り除
くことができます。ただし、CFGからPDAへの変換はもっと複雑になります。
また実際のところ、有効なプログラムを認識できるだけでは十分ではありません。「2.6 パーサの実
装」で見たように、プログラムをパースすることの本質は、プログラムを構造化された表現に変換し、
そ
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O’Reilly covers everything we've got, with content to help us build a world-class technology community, upgrade the capabilities and competencies of our teams, and improve overall team performance as well as their engagement.Julian F.
I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
I’ve been on the O’Reilly platform for more than eight years. I use a couple of learning platforms, but I'm on O'Reilly more than anybody else. When you're there, you start learning. I'm never disappointed.Amir M.
I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.