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<!DOCTYPE html>
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<title>SICP Exercise 1.6</title>
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SICP Exercise 1.6
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<span class="label">Date</span>
<abbr class="published" title="2013-02-24T17:09:00+09:00">
<i class="icon-calendar"></i>일 24 2월 2013
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<span class="label">By</span>
<a href="/author/wonseok-choi.html"><i class="icon-user"></i>Wonseok Choi</a>
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<a href="/category/sicp.html"><i class="icon-folder-open"></i>SICP</a>.
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<a href="/tag/sicp.html"><i class="icon-tag"></i>sicp</a>
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<p>SICP (Structure and Interpretation of Computer Programs)를 공부하는 사람이 꽤
많은 듯 하다. 관련 블로그도 많고, 다 볼 수 있을지 모르겠지만,
How To Design Programs 살 때 덤으로 산건데 이걸 먼저 보고 있다..</p>
<p>아무튼 Exercise 1.6 은 <code>cond</code> 를 이용해 <code>if</code> 를 대체하는 procedure를 만들어
사용하면 어떻게 되느냐 이다. 책의 앞에서 다루었던
applicative-order evaluation과 관련이 있는데,
내가 사용하는 mit-scheme에서는 다음과 같은 메시지가 출력된다.</p>
<div class="highlight"><pre><span></span>;Aborting!: maximum recursion depth exceeded
</pre></div>
<p><em>procedure의 인자들이 평가되는데, <code>sqrt-iter</code>의 <code>else-clause</code>에서
<code>sqrt-iter</code>를 호출하기 때문에 무한 루프에 빠지게 된다.</em></p>
<p>SICP Exercise 1.6</p>
<div class="highlight"><pre><span></span><span class="p">(</span><span class="k">define </span><span class="p">(</span><span class="nf">new-if</span> <span class="nv">predicate</span> <span class="nv">then-clause</span> <span class="nv">else-clause</span><span class="p">)</span>
<span class="p">(</span><span class="k">cond </span><span class="p">(</span><span class="nf">predicate</span> <span class="nv">then-clause</span><span class="p">)</span>
<span class="p">(</span><span class="k">else </span><span class="nv">else-clause</span><span class="p">)))</span>
<span class="p">(</span><span class="k">define </span><span class="p">(</span><span class="nf">sqrt-iter</span> <span class="nv">guess</span> <span class="nv">x</span><span class="p">)</span>
<span class="p">(</span><span class="nf">new-if</span> <span class="p">(</span><span class="nf">good-enough?</span> <span class="nv">guess</span> <span class="nv">x</span><span class="p">)</span>
<span class="nv">guess</span>
<span class="p">(</span><span class="nf">sqrt-iter</span> <span class="p">(</span><span class="nf">improve</span> <span class="nv">guess</span> <span class="nv">x</span><span class="p">)</span>
<span class="nv">x</span><span class="p">)))</span>
<span class="p">(</span><span class="k">define </span><span class="p">(</span><span class="nf">improve</span> <span class="nv">guess</span> <span class="nv">x</span><span class="p">)</span>
<span class="p">(</span><span class="nf">average</span> <span class="nv">guess</span> <span class="p">(</span><span class="nb">/ </span><span class="nv">x</span> <span class="nv">guess</span><span class="p">)))</span>
<span class="p">(</span><span class="k">define </span><span class="p">(</span><span class="nf">average</span> <span class="nv">x</span> <span class="nv">y</span><span class="p">)</span>
<span class="p">(</span><span class="nb">/ </span><span class="p">(</span><span class="nb">+ </span><span class="nv">x</span> <span class="nv">y</span><span class="p">)</span> <span class="mi">2</span><span class="p">))</span>
<span class="p">(</span><span class="k">define </span><span class="p">(</span><span class="nf">square</span> <span class="nv">x</span><span class="p">)</span>
<span class="p">(</span><span class="nb">* </span><span class="nv">x</span> <span class="nv">x</span><span class="p">))</span>
<span class="p">(</span><span class="k">define </span><span class="p">(</span><span class="nf">good-enough?</span> <span class="nv">guess</span> <span class="nv">x</span><span class="p">)</span>
<span class="p">(</span><span class="nb">< </span><span class="p">(</span><span class="nb">abs </span><span class="p">(</span><span class="nb">- </span><span class="p">(</span><span class="nf">square</span> <span class="nv">guess</span><span class="p">)</span> <span class="nv">x</span><span class="p">))</span> <span class="mf">0.001</span><span class="p">))</span>
<span class="p">(</span><span class="k">define </span><span class="p">(</span><span class="nb">sqrt </span><span class="nv">x</span><span class="p">)</span>
<span class="p">(</span><span class="nf">sqrt-iter</span> <span class="mf">1.0</span> <span class="nv">x</span><span class="p">))</span>
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