From mboxrd@z Thu Jan 1 00:00:00 1970 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me X-Spam-Level: * X-Spam-Status: No, score=1.4 required=5.0 tests=BAYES_50,INVALID_MSGID autolearn=no autolearn_force=no version=3.4.4 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: f7be1,be6b7e036aa9236c X-Google-Attributes: gidf7be1,public X-Google-Thread: 11390f,be6b7e036aa9236c X-Google-Attributes: gid11390f,public X-Google-Thread: 101deb,be6b7e036aa9236c X-Google-Attributes: gid101deb,public X-Google-Thread: 1014db,be6b7e036aa9236c X-Google-Attributes: gid1014db,public X-Google-Thread: 1094ba,be6b7e036aa9236c X-Google-Attributes: gid1094ba,public X-Google-Thread: 103376,be6b7e036aa9236c X-Google-Attributes: gid103376,public X-Google-Thread: 1164ba,be6b7e036aa9236c X-Google-Attributes: gid1164ba,public X-Google-Thread: fa0ae,be6b7e036aa9236c X-Google-Attributes: gidfa0ae,public From: jnshapi@argo.ecte.uswc.uswest.com (Jim Shapiro) Subject: Re: Friday 13th, try it yourself (was Language Wars..) Date: 1996/09/26 Message-ID: <52enm9$d53@aeon.ecte.uswc.uswest.com>#1/1 X-Deja-AN: 185528434 distribution: inet references: <51bv60$8d@pheidippides.axion.bt.co.uk> <32406D89.233E@watson.ibm.com> <3242D1EB.3F54@ford.com> <52b2ks$jcb@internet.csl.co.uk> followup-to: comp.lang.ada,comp.lang.apl,comp.lang.basic,comp.lang.c,comp.lang.fortran,comp.lang.perl.misc,comp.lang.pl1,comp.lang.rexx,comp.lang.pascal.misc,comp.lang.smalltalk organization: US WEST Information Technologies TSS newsgroups: comp.lang.ada,comp.lang.apl,comp.lang.basic,comp.lang.c,comp.lang.fortran,comp.lang.perl.misc,comp.lang.pl1,comp.lang.rexx,comp.lang.pascal.misc,comp.lang.smalltalk Date: 1996-09-26T00:00:00+00:00 List-Id: Barrie Walker (bwalker@csl.co.uk) wrote: : In article , mose@ns.ccsn.edu says... : > : >Reposting article removed by rogue canceller. : > : >Clinton Pierce writes: : > : >>See for yourself if the "Friday the 13th" thing is just a UL. If you trust : >>UNIX's 'cal' program, and that it does the Right Thing with Leap Years, the : >>Gregorian/Julian switch etc..etc... This Perl script will show you the : Truth: : > : > I decided to write a script of my own for Solaris 2.4 and perl : >5.003 which calls cal for 1066 to 1996 and rips out the Fridays. The : >following is the number of times a particular day of the month falls : >on a Friday. The results are pretty evenly distributed, i.e. it is an : >Urban Legend. : I think you have caught hold of the wrong end of the stick. : The observation doesn't claim : 1 that 13ths have a monopoly on Fridays - : 6ths, 20ths and 27ths fall on just as many Fridays. : 2 that the uneven distribution is restricted to Fridays - : months are more likely to begin (1sts) on Sundays. : 3 to work with anything but the current inplementation : of the Gregorian calendar. [well written Perl code deleted] I ran your program and got the same results, then I realized that this whole argument is silly. Suppose, for the sake of argument and with no loss in in generality, we look at the day of the week the first of each month falls on. If January starts on say a Monday, the days for the remaining 11 months depend only on the number of days in each month. If all the months were 28 days (or any number that is congruent to 0 (mod 7) then _all_ the firsts would be on Monday! Given the odd ball number of days in each month (a number which even changes with the year, for some years) the firsts of each month will be divided among the days of the week in some _deterministic_ fashion. Over a 400 year interval it does not come out exactly evenly distributed. That is all there is to it. Why does anyone think that firsts or thirteenths or any day should be equally likely to be on any one weekday over any other in the first place? I guess it is a little counterintuitive, sort of like the famous birthday problem, one which I will not introduce to keep the thread length in check. : -- : Barrie : Edinburgh, Scotland http://www.csl.co.uk -- Hope this helps, Jim Shapiro