blog/content/post/2008/09/28/2008-09-28-00001040.md

---
title: 可変長引数を受け取る手続きを書くときに再帰で処理する際はどうすればいいのかしら？
author: kazu634
date: 2008-09-28
url: /2008/09/28/_1117/
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categories:
  - gauche
  - Lisp

---
<div class="section">
<p>
    自力でmaxを定義するの続き。どうしても定義できない。問題は
</p>

<pre class="syntax-highlight">
<span class="synSpecial">(</span>define <span class="synSpecial">(</span>my_length lis<span class="synSpecial">)</span>
<span class="synSpecial">(</span><span class="synStatement">if</span> <span class="synSpecial">(</span><span class="synStatement">null</span>? lis<span class="synSpecial">)</span>
<span class="synConstant"></span>
<span class="synSpecial">(</span><span class="synStatement">+</span> <span class="synConstant">1</span> <span class="synSpecial">(</span>my_length <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>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 lis<span class="synSpecial">)</span> <span class="synConstant">1</span><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">=</span> <span class="synSpecial">(</span>my_length 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> lis<span class="synSpecial">)</span> <span class="synSpecial">(</span><span class="synStatement">cadr</span> lis<span class="synSpecial">))</span> <span class="synSpecial">(</span><span class="synStatement">cadr</span> lis<span class="synSpecial">)</span>]
[<span class="synSpecial">(</span><span class="synStatement">&#62;=</span> <span class="synSpecial">(</span><span class="synStatement">car</span> lis<span class="synSpecial">)</span> <span class="synSpecial">(</span><span class="synStatement">cadr</span> lis<span class="synSpecial">))</span> <span class="synSpecial">(</span><span class="synStatement">car</span> lis<span class="synSpecial">)</span>]<span class="synSpecial">)</span>]
[<span class="synSpecial">(</span><span class="synStatement">&#62;=</span> <span class="synSpecial">(</span>my_length 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> lis<span class="synSpecial">)</span> <span class="synSpecial">(</span><span class="synStatement">cadr</span> 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> lis<span class="synSpecial">)</span> <span class="synSpecial">(</span><span class="synStatement">cddr</span> lis<span class="synSpecial">)))</span>]
[<span class="synSpecial">(</span><span class="synStatement">&#62;=</span> <span class="synSpecial">(</span><span class="synStatement">car</span> lis<span class="synSpecial">)</span> <span class="synSpecial">(</span><span class="synStatement">cadr</span> 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> lis<span class="synSpecial">)</span> <span class="synSpecial">(</span><span class="synStatement">cddr</span> lis<span class="synSpecial">)))</span>]<span class="synSpecial">)</span>]<span class="synSpecial">))</span>
</pre>

<p>
    としたときに(my_max (cons (cadr lis) (cddr lis)))とかの部分で引数として渡されると、結果として<span class="footnote"><a href="/sirocco634/#f1" name="fn1" title="1 2 3 4 5">*1</a></span>みたいなリストが渡されてmy_lengthの結果が1になってしまうこと。考えたのは
</p>

<ul>
<li>
      my_lengthを拡張して、'(1 2 (3 4) (5 6))とかでも認識できるようにする
</li>
</ul>

<p>
    でも、for-eachを使ってみればいいのかなーと思ったんだけど、うまく再帰の形で要素数をカウントできず。。。しょうがないから、
</p>

<ul>
<li>
      &#8216;(1 2 (3 4) (5 6))を'(1 2 3 4 5 6)にする
</li>
</ul>

<p>
    を考えているところ。
</p>

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