2019-03-31 11:00:21 +00:00
---
title: 自分でmaxを定義する（ ）
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" > < =< / 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" > > =< / 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" > > =< / 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" > < =< / 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" > > =< / 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" > < < / 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 > ：
< / p >
2019-04-02 16:06:15 +00:00
< / div >
