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From: lambert@spectrum.cs.unsw.oz.au (Tim Lambert)
Newsgroups: comp.lang.prolog,comp.lang.functional
Subject: Re: shuffling a list
Message-ID: <LAMBERT.90Sep6231606@nankeen.spectrum.cs.unsw.oz.au>
Date: 6 Sep 90 13:16:06 GMT
References: <5056@daffy.cs.wisc.edu>
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In-reply-to: quale@saavik.cs.wisc.edu's message of 5 Sep 90 16:58:26 GMT

>>>>> On 5 Sep 90 16:58:26 GMT, quale@saavik.cs.wisc.edu (Douglas E. Quale) said:

> What is the best way to shuffle a list in a functional language?

In Miranda:

Riffle shuffle (as in a pack of cards) a list

> riffle:: [*] -> [*]
> riffle xs = riffle_merge (take half xs) (drop half xs)
>             where half = #xs div 2

Riffle merge two lists.  Take elements alternately from each
list to form the new list.

> riffle_merge (x:xs) (y:ys) = x:y:riffle_merge xs ys
> riffle_merge [] ys =  ys
> riffle_merge (x:xs) [] = x:xs

Now you can evaluate "iterate riffle [1..52]" and discover that eight
riffle shuffles put a deck back the way it started.

:-)

Tim