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<html><!-- Mirrored from c-faq.com/misc/bitsets.970425.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 08:02:51 GMT --><head><title></title></head><body><p>[Someone asked how to implement sets using arrays of bits.This is an excerpt from the additional explanation I posted.]<p>Newsgroups: comp.lang.c<br>From: scs@eskimo.com (Steve Summit)<br>Subject: Re: pascal-like set implementation<br>Message-ID: <E97GrL.F33@eskimo.com><br>Date: Fri, 25 Apr 1997 18:02:08 GMT<p>If the repertoire of elements which might be contained in the set(what was that buzzword from set theory? ``Universe''?) is known,then the thing to do is set up a correspondence between thoseelements and the 0-based integers. (That is, if the universe isthe elements ``apple'', ``pear'', and ``orange'', set apple = 0,pear = 1, and orange = 2. Hmm, you might be able to slickly usean enum for this.) Then, each set you want to manipulate is anarray of <TT>BITNSLOTS(n)</TT> elements. To add an element to a set,turn on the corresponding bit. To remove an element from a set,turn off the corresponding bit. To compute the union of twosets, run the loop<p><pre> for(i = 0; i < BITNSLOTS(nb); i++) set3[i] = set1[i] | set2[i];</pre><p>Computing the intersection is left as an exercise for the reader.<p>Printing a set out will obviously require iterating over all thebits, using <TT>BITTEST()</TT> to discover whether each bit is turned on,and then somehow going from the bit index back to the name of theelement, if necessary. (We had a thread a week or two ago abouthow to print enum values symbolically. Here, since the indicesare known to be 0-based integers, a simple lookup table, in theform of an array of strings, would suffice.)<p><address><a href="http://www.eskimo.com/~scs/">Steve Summit</a><br><a href="mailto:scs@eskimo.com">scs@eskimo.com</a></address></body><!-- Mirrored from c-faq.com/misc/bitsets.970425.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 08:02:51 GMT --></html>⌨️ 快捷键说明
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