blog/content/post/2008/09/30/2008-09-30-00001041.md

---
title: 自分でmaxを定義する（汚い実装）
author: kazu634
date: 2008-09-30
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categories:
  - gauche
  - Lisp

---
<div class="section">
<p>
<a href="http://d.hatena.ne.jp/SaitoAtsushi/" onclick="__gaTracker('send', 'event', 'outbound-article', 'http://d.hatena.ne.jp/SaitoAtsushi/', 'id:SaitoAtsushi');">id:SaitoAtsushi</a>さんからせっかく教えてもらうも、applyをつかったうまい方法は思いつかず。。。とりあえずやっつけの汚い実装をする。問題は再帰で二回目以降が呼び出されるときに、引数が「<span class="footnote"><a href="/sirocco634/#f1" name="fn1" title="1 2 3 4 5">*1</a></span>」みたいになることが問題（それ以外の場合はあり得ない）のだから、
</p>

<ul>
<li>
      リストの要素が1
</li>
<li>
      (car リスト)がリストである
</li>
</ul>

<p>
    場合に、(car リスト)を返す手続きを作ってあげればとりあえずは解決する。つまり:
</p>

<p>
</p>

<pre class="syntax-highlight">
<span class="synSpecial">(</span>define <span class="synSpecial">(</span>select lis<span class="synSpecial">)</span>
<span class="synSpecial">(</span><span class="synStatement">if</span> <span class="synSpecial">(</span><span class="synStatement">and</span> <span class="synSpecial">(</span><span class="synStatement">=</span> <span class="synConstant">1</span> <span class="synSpecial">(</span>my_length lis<span class="synSpecial">))</span> <span class="synSpecial">(</span><span class="synStatement">list</span>? <span class="synSpecial">(</span><span class="synStatement">car</span> lis<span class="synSpecial">)))</span>
<span class="synSpecial">(</span><span class="synStatement">car</span> lis<span class="synSpecial">)</span>
lis<span class="synSpecial">))</span>
</pre>

<p>
    このような手続きだ。これをふまえた上で定義するとこうなった:
</p>

<pre class="syntax-highlight">
<span class="synSpecial">(</span>define <span class="synSpecial">(</span>my_max<span class="synStatement"> . </span>lis<span class="synSpecial">)</span>
<span class="synSpecial">(</span><span class="synStatement">cond</span> [<span class="synSpecial">(</span><span class="synStatement">null</span>? lis<span class="synSpecial">)</span> #f]
[<span class="synSpecial">(</span><span class="synStatement">=</span> <span class="synSpecial">(</span>my_length <span class="synSpecial">(</span>select lis<span class="synSpecial">))</span> <span class="synConstant">1</span><span class="synSpecial">)</span> <span class="synSpecial">(</span><span class="synStatement">car</span> <span class="synSpecial">(</span>select lis<span class="synSpecial">))</span>]
[<span class="synSpecial">(</span><span class="synStatement">=</span> <span class="synSpecial">(</span>my_length <span class="synSpecial">(</span>select lis<span class="synSpecial">))</span> <span class="synConstant">2</span><span class="synSpecial">)</span> <span class="synSpecial">(</span><span class="synStatement">cond</span> [<span class="synSpecial">(</span><span class="synStatement">&#60;=</span> <span class="synSpecial">(</span><span class="synStatement">car</span> <span class="synSpecial">(</span>select lis<span class="synSpecial">))</span> <span class="synSpecial">(</span><span class="synStatement">cadr</span> <span class="synSpecial">(</span>select lis<span class="synSpecial">)))</span> <span class="synSpecial">(</span><span class="synStatement">cadr</span> <span class="synSpecial">(</span>select lis<span class="synSpecial">))</span>]
[<span class="synSpecial">(</span><span class="synStatement">&#62;=</span> <span class="synSpecial">(</span><span class="synStatement">car</span> <span class="synSpecial">(</span>select lis<span class="synSpecial">))</span> <span class="synSpecial">(</span><span class="synStatement">cadr</span> <span class="synSpecial">(</span>select lis<span class="synSpecial">)))</span> <span class="synSpecial">(</span><span class="synStatement">car</span> <span class="synSpecial">(</span>select lis<span class="synSpecial">))</span>]<span class="synSpecial">)</span>]
[<span class="synSpecial">(</span><span class="synStatement">&#62;=</span> <span class="synSpecial">(</span>my_length <span class="synSpecial">(</span>select lis<span class="synSpecial">))</span> <span class="synConstant">3</span><span class="synSpecial">)</span> <span class="synSpecial">(</span><span class="synStatement">cond</span> [<span class="synSpecial">(</span><span class="synStatement">&#60;=</span> <span class="synSpecial">(</span><span class="synStatement">car</span> <span class="synSpecial">(</span>select lis<span class="synSpecial">))</span> <span class="synSpecial">(</span><span class="synStatement">cadr</span> <span class="synSpecial">(</span>select lis<span class="synSpecial">)))</span> <span class="synSpecial">(</span>my_max <span class="synSpecial">(</span><span class="synStatement">cons</span> <span class="synSpecial">(</span><span class="synStatement">cadr</span> <span class="synSpecial">(</span>select lis<span class="synSpecial">))</span> <span class="synSpecial">(</span><span class="synStatement">cddr</span> <span class="synSpecial">(</span>select lis<span class="synSpecial">))))</span>]
[<span class="synSpecial">(</span><span class="synStatement">&#62;=</span> <span class="synSpecial">(</span><span class="synStatement">car</span> <span class="synSpecial">(</span>select lis<span class="synSpecial">))</span> <span class="synSpecial">(</span><span class="synStatement">cadr</span> <span class="synSpecial">(</span>select lis<span class="synSpecial">)))</span> <span class="synSpecial">(</span>my_max <span class="synSpecial">(</span><span class="synStatement">cons</span> <span class="synSpecial">(</span><span class="synStatement">car</span> <span class="synSpecial">(</span>select lis<span class="synSpecial">))</span> <span class="synSpecial">(</span><span class="synStatement">cddr</span> <span class="synSpecial">(</span>select lis<span class="synSpecial">))))</span>]<span class="synSpecial">)</span>]<span class="synSpecial">))</span>
</pre>

<p>
    なんだかこれは面倒だ。絶対にもっと簡単な書き方があるような気がする。
</p>

<h4>
    追記:
</h4>

<p>
    ふと振り返ってみたら、<a href="http://d.hatena.ne.jp/asin/4873113482" onclick="__gaTracker('send', 'event', 'outbound-article', 'http://d.hatena.ne.jp/asin/4873113482', 'プログラミングGauche');">プログラミングGauche</a>でpick-greater, max-numberというのが書いてあった。fold手続きを使っている。可変長引数はリストになるからそれをそのまま渡せばいいのかな？
</p>

<pre class="syntax-highlight">
<span class="synSpecial">(</span>define <span class="synSpecial">(</span>max-number lis<span class="synSpecial">)</span>
<span class="synSpecial">(</span>fold
<span class="synSpecial">(</span><span class="synStatement">lambda</span> <span class="synSpecial">(</span>a b<span class="synSpecial">)</span>
<span class="synSpecial">(</span><span class="synStatement">if</span> <span class="synSpecial">(</span><span class="synStatement">&#60;</span> a b<span class="synSpecial">)</span>
b
a<span class="synSpecial">))</span>
<span class="synSpecial">(</span><span class="synStatement">car</span> lis<span class="synSpecial">)</span> <span class="synSpecial">(</span><span class="synStatement">cdr</span> lis<span class="synSpecial">)))</span>
<span class="synSpecial">(</span>define <span class="synSpecial">(</span>my_max<span class="synStatement"> . </span>args<span class="synSpecial">)</span>
<span class="synSpecial">(</span>max-number args<span class="synSpecial">))</span>
</pre>

<p>
    もしかしたら、可変長引数というのは結局リストになるのだから、「リストの中で一番大きい数を探す手続き」が書ければ、「可変長引数の中で一番大きな数を探す手続き」を書いたことになるのかな？
</p>

<p>
</p>

<h4>
    追記の追記:
</h4>

<p>
    apply使ってないや。。。
</p>
</div>

<div class="footnote">
<p class="footnote">
<a href="/sirocco634/#fn1" name="f1">*1</a>：1 2 3 4 5
</p>
</div>