blog/content/post/2008/09/06/2008-09-06-00001022.md

---
title: ソフト開発の過去問で出てきた再帰をGaucheで書く
author: kazu634
date: 2008-09-06
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categories:
  - つれづれ

---
<div class="section">
<p>
    ソフトウェア開発者試験ででてきた再帰を使った関数をGaucheで書いてみた。元の関数をCで書くとこんな感じになる:
</p>

<pre class="syntax-highlight">
<span class="synComment">/* ======================= */</span>
<span class="synComment">/* === Include library === */</span>
<span class="synComment">/* ======================= */</span>
<span class="synPreProc">#include </span><span class="synConstant">&#60;stdio.h&#62;</span>
<span class="synComment">/* ==================== */</span>
<span class="synComment">/* === Main Routine === */</span>
<span class="synComment">/* ==================== */</span>
<span class="synType">void</span> prnt_digit(<span class="synType">int</span> n)
{
<span class="synStatement">if</span> (<span class="synConstant"></span> == n){
<span class="synStatement">return</span>;
}
printf(<span class="synConstant">&#34;</span><span class="synSpecial">%d</span><span class="synConstant">&#34;</span>, n);
prnt_digit(n - <span class="synConstant">1</span>);
printf(<span class="synConstant">&#34;</span><span class="synSpecial">%d</span><span class="synConstant">&#34;</span>, n);
}
<span class="synType">int</span> main(<span class="synType">int</span> argc, <span class="synType">char</span> *argv[])
{
prnt_digit(<span class="synConstant">5</span>);
printf(<span class="synConstant">&#34;</span><span class="synSpecial">\n</span><span class="synConstant">&#34;</span>);
<span class="synStatement">return</span> <span class="synConstant"></span>;
}
</pre>

<p>
    これをgaucheで書いてみる:
</p>

<pre class="syntax-highlight">
<span class="synComment">;; lambdaを使わないバージョン</span>
<span class="synSpecial">(</span>define <span class="synSpecial">(</span>proc n<span class="synSpecial">)</span>
<span class="synSpecial">(</span>define <span class="synSpecial">(</span>tmp_proc temp<span class="synSpecial">)</span>
<span class="synSpecial">(</span><span class="synStatement">print</span> n<span class="synSpecial">)</span>
<span class="synSpecial">(</span>proc <span class="synSpecial">(</span><span class="synStatement">-</span> n <span class="synConstant">1</span><span class="synSpecial">))</span>
<span class="synSpecial">(</span><span class="synStatement">print</span> n<span class="synSpecial">))</span>
<span class="synSpecial">(</span><span class="synStatement">if</span> <span class="synSpecial">(</span><span class="synStatement">=</span> n <span class="synConstant"></span><span class="synSpecial">)</span>
n
<span class="synSpecial">(</span>tmp_proc <span class="synSpecial">(</span><span class="synStatement">-</span> n <span class="synConstant">1</span><span class="synSpecial">))))</span>
<span class="synComment">;; がんばってlambda使ってみたバージョン</span>
<span class="synSpecial">(</span>define <span class="synSpecial">(</span>proc_1 n<span class="synSpecial">)</span>
<span class="synSpecial">(</span><span class="synStatement">if</span> <span class="synSpecial">(</span><span class="synStatement">=</span> n <span class="synConstant"></span><span class="synSpecial">)</span>
n
<span class="synSpecial">((</span><span class="synStatement">lambda</span> <span class="synSpecial">(</span>temp<span class="synSpecial">)</span>
<span class="synSpecial">(</span><span class="synStatement">print</span> n<span class="synSpecial">)</span>
<span class="synSpecial">(</span>proc_1 <span class="synSpecial">(</span><span class="synStatement">-</span> n <span class="synConstant">1</span><span class="synSpecial">))</span>
<span class="synSpecial">(</span><span class="synStatement">print</span> n<span class="synSpecial">))</span>
<span class="synSpecial">(</span><span class="synStatement">-</span> n <span class="synConstant">1</span><span class="synSpecial">))))</span>
</pre>

<p>
    lambdaは手続きを返すんだから「(」を二重に重ねないと引数をlambdaの無名関数に渡すことができないわけですね。なるほど。
</p>

<p>
    追記:
</p>

<p>
    condは暗黙でbeginが省略されていると解釈される。ifはそうじゃない。だから今回の場合は
</p>

<pre class="syntax-highlight">
<span class="synSpecial">(</span>define <span class="synSpecial">(</span>proc_2 n<span class="synSpecial">)</span>
<span class="synSpecial">(</span><span class="synStatement">if</span> <span class="synSpecial">(</span><span class="synStatement">=</span> n <span class="synConstant"></span><span class="synSpecial">)</span>
n
<span class="synSpecial">(</span>begin
<span class="synSpecial">(</span><span class="synStatement">print</span> n<span class="synSpecial">)</span>
<span class="synSpecial">(</span>proc_1 <span class="synSpecial">(</span><span class="synStatement">-</span> n <span class="synConstant">1</span><span class="synSpecial">))</span>
<span class="synSpecial">(</span><span class="synStatement">print</span> n<span class="synSpecial">))))</span>
</pre>

<p>
    こうでもいいわけか。
</p>
</div>