blog/content/post/2010/11/24/2010-11-24-00001430.md

---
title: Ninety-nine Lisp Problems – P10の解答
author: kazu634
date: 2010-11-24
url: /2010/11/24/_1632/
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categories:
  - gauche
  - Lisp

---
<div class="section">
<p>
    P9の解答で作成した関数を元にして、リスト空リストへの変換を行う形で解答を作成してみました。つまり
</p>
  
<ol>
<li>
      packでリストを作成する
</li>
<li>
      packで作られたリストの中のリストを変換する手続きencode-listを作成する
</li>
<li>
      packに対してencode-listをmapする
</li>
</ol>
  
<p>
    感じでやってみました。
</p>
  
<h4>
    ソース
</h4>
  
<pre class="syntax-highlight">
<span class="synComment">;; P10 (*) Run-length encoding of a list.</span>
<span class="synComment">;;     Use the result of problem P09 to implement the so-called run-length encoding data compression method. Consecutive duplicates of elements are encoded as lists (N E) where N is the number of duplicates of the element E.</span>
<span class="synComment">;;     Example:</span>
<span class="synComment">;;     * (encode '(a a a a b c c a a d e e e e))</span>
<span class="synComment">;;     ((4 A) (1 B) (2 C) (2 A) (1 D)(4 E))</span>
<span class="synSpecial">(</span>define <span class="synSpecial">(</span>pack-seed front sublist result l<span class="synSpecial">)</span>
<span class="synSpecial">(</span><span class="synStatement">cond</span>
[<span class="synSpecial">(</span><span class="synStatement">null</span>? l<span class="synSpecial">)</span> <span class="synSpecial">(</span><span class="synStatement">append</span> result <span class="synSpecial">(</span><span class="synStatement">list</span> sublist<span class="synSpecial">))</span>]
[<span class="synSpecial">(</span><span class="synStatement">eq</span>? front <span class="synSpecial">(</span><span class="synStatement">car</span> l<span class="synSpecial">))</span> <span class="synSpecial">(</span>pack-seed front
<span class="synSpecial">(</span><span class="synStatement">append</span> sublist
<span class="synSpecial">(</span><span class="synStatement">list</span> front<span class="synSpecial">))</span>
result
<span class="synSpecial">(</span><span class="synStatement">cdr</span> l<span class="synSpecial">))</span>]
[else <span class="synSpecial">(</span>pack-seed <span class="synSpecial">(</span><span class="synStatement">car</span> l<span class="synSpecial">)</span>
<span class="synSpecial">(</span><span class="synStatement">list</span> <span class="synSpecial">(</span><span class="synStatement">car</span> l<span class="synSpecial">))</span>
<span class="synSpecial">(</span><span class="synStatement">if</span> <span class="synSpecial">(</span><span class="synStatement">null</span>? result<span class="synSpecial">)</span>
<span class="synSpecial">(</span><span class="synStatement">if</span> <span class="synSpecial">(</span><span class="synStatement">null</span>? sublist<span class="synSpecial">)</span>
<span class="synSpecial">'()</span>
<span class="synSpecial">(</span><span class="synStatement">append</span> result <span class="synSpecial">(</span><span class="synStatement">list</span> sublist<span class="synSpecial">)))</span>
<span class="synSpecial">(</span><span class="synStatement">append</span> result <span class="synSpecial">(</span><span class="synStatement">list</span> sublist<span class="synSpecial">)))</span>
<span class="synSpecial">(</span><span class="synStatement">cdr</span> l<span class="synSpecial">))</span>]<span class="synSpecial">))</span>
<span class="synSpecial">(</span>define <span class="synSpecial">(</span>pack l<span class="synSpecial">)</span>
<span class="synSpecial">(</span>pack-seed #f <span class="synSpecial">'()</span> <span class="synSpecial">'()</span> l<span class="synSpecial">))</span>
<span class="synSpecial">(</span>define <span class="synSpecial">(</span>encode-list l<span class="synSpecial">)</span>
<span class="synSpecial">(</span><span class="synStatement">let</span> <span class="synSpecial">((</span>num <span class="synSpecial">(</span><span class="synStatement">length</span> l<span class="synSpecial">)))</span>
<span class="synSpecial">(</span><span class="synStatement">append</span> <span class="synSpecial">(</span><span class="synStatement">list</span> num<span class="synSpecial">)</span>
<span class="synSpecial">(</span><span class="synStatement">list</span> <span class="synSpecial">(</span><span class="synStatement">car</span> l<span class="synSpecial">)))))</span>
<span class="synSpecial">(</span>encode-list <span class="synSpecial">'(</span>a a a a a<span class="synSpecial">))</span>
<span class="synSpecial">(</span>define <span class="synSpecial">(</span>encode l<span class="synSpecial">)</span>
<span class="synSpecial">(</span><span class="synStatement">map</span> <span class="synSpecial">(</span><span class="synStatement">lambda</span> <span class="synSpecial">(</span>l<span class="synSpecial">)</span>
<span class="synSpecial">(</span>encode-list l<span class="synSpecial">))</span>
<span class="synSpecial">(</span>pack l<span class="synSpecial">)))</span>
<span class="synSpecial">(</span>encode <span class="synSpecial">'(</span>a a a a b c c a a d e e e e<span class="synSpecial">))</span>
</pre>
</div>