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<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN"><!-- This collection of hypertext pages is Copyright 1995-2005 by Steve Summit. --><!-- Content from the book "C Programming FAQs: Frequently Asked Questions" --><!-- (Addison-Wesley, 1995, ISBN 0-201-84519-9) is made available here by --><!-- permission of the author and the publisher as a service to the community. --><!-- It is intended to complement the use of the published text --><!-- and is protected by international copyright laws. --><!-- The on-line content may be accessed freely for personal use --><!-- but may not be published or retransmitted without explicit permission. --><!-- --><!-- this page built Sat Dec 24 21:47:47 2005 by faqproc version 2.7 --><!-- from source file algorithms.sgml dated Sat Feb 14 14:35:07 2004 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/misc/zeller.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:59:05 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 20.31</title><link href="gaussian.html" rev=precedes><link href="leapyear.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="gaussian.html" rev=precedes><img src="../images/buttonleft.gif" alt="prev"></a><a href="index.html" rev=subdocument><img src="../images/buttonup.gif" alt="up"></a><a href="leapyear.html" rel=precedes><img src="../images/buttonright.gif" alt="next"></a> <a href="../index-2.html"><img src="../images/buttontop.gif" alt="top/contents"></a><a href="../search.html"><img src="../images/buttonsrch.gif" alt="search"></a><hr><p><!-- qbegin --><h1>comp.lang.c FAQ list<font color=blue>·</font><!-- qtag -->Question 20.31</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>How can I find the day of the week given the date?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>Here are three methods:<OL><li>Use<TT>mktime</TT>or <TT>localtime</TT>(see question<a href="../lib/mktime.html">13.13</a>).Here isa code fragment which computes the day of the week forFebruary 29, 2000:<pre>#include <stdio.h>#include <time.h>char *wday[] = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"};struct tm tm;tm.tm_mon = 2 - 1;tm.tm_mday = 29;tm.tm_year = 2000 - 1900;tm.tm_hour = tm.tm_min = tm.tm_sec = 0;tm.tm_isdst = -1;if(mktime(&tm) != -1) printf("%s\n", wday[tm.tm_wday]);</pre>When using <TT>mktime</TT> like this,it's usually important to set <TT>tm_isdst</TT> to -1,as shown(especially if <TT>tm_hour</TT> is 0),otherwise a daylight saving timecorrection could push the time past midnight into another day.<li>UseZeller's congruence,which says that if<br><br><pre> <I>J</I> is the number of the century [i.e. the year <TT>/</TT> 100], <I>K</I> the year within the century [i.e. the year <TT>%</TT> 100], <I>m</I> the month, <I>q</I> the day of the month, <I>h</I> the day of the week [where 1 is Sunday];</pre><br><br>and if January and February are taken as months 13 and 14 of the previous year[affecting both <I>J</I> and <I>K</I>];then <I>h</I>for the Gregorian calendaris the remainder when the sum<br><br><pre> <I>q</I> + 26(<I>m</I> + 1) / 10 + <I>K</I> + <I>K</I>/4 + <I>J</I>/4 - 2<I>J</I></pre><br><br>is divided by 7,and where all intermediate remainders are discarded.<a href="zeller2.html" rel=subdocument>[footnote]</a>The translation into C is straightforward:<pre> h = (q + 26 * (m + 1) / 10 + K + K/4 + J/4 + 5*J) % 7;</pre>(wherewe use <TT>+5*J</TT> instead of <TT>-2*J</TT>to make sure thatboth operands of the modulus operator <TT>%</TT> arepositive;this bias totalling <TT>7*J</TT>will obviously not change the final value of <TT>h</TT>, modulo 7).<li>Usethis elegant codebyTomohiko Sakamoto:<pre>int dayofweek(int y, int m, int d) /* 0 = Sunday */{ static int t[] = {0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4}; y -= m < 3; return (y + y/4 - y/100 + y/400 + t[m-1] + d) % 7;}</pre></OL></p><p>See also questions <a href="../lib/calendar.html">13.14</a>and <a href="leapyear.html">20.32</a>.</p><p>References:ISO Sec. 7.12.2.3<br>Chr. Zeller, ``Kalender-Formeln''<br></p><!-- aend --><p><hr><a href="gaussian.html" rev=precedes><img src="../images/buttonleft.gif" alt="prev"></a><a href="index.html" rev=subdocument><img src="../images/buttonup.gif" alt="up"></a><a href="leapyear.html" rel=precedes><img src="../images/buttonright.gif" alt="next"></a> <a href="../questions.html"><img src="../images/buttontop.gif" alt="contents"></a><a href="../search.html"><img src="../images/buttonsrch.gif" alt="search"></a><br><!-- lastfooter --><a href="../about.html">about this FAQ list</a> <a href="../eskimo.html">about eskimo</a> <a href="../search.html">search</a> <a href="../feedback.html">feedback</a> <a href="copyright.html">copyright</a><p>Hosted by<a href="http://www.eskimo.com/"><img src="../../www.eskimo.com/img/link/eskitiny.gif" alt="Eskimo North"></a></body><!-- Mirrored from c-faq.com/misc/zeller.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:59:05 GMT --></html>⌨️ 快捷键说明
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