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How many people in a room?

How many people do you need to gather in a room for there to be an even chance that two of them have the same birthday? Would you believe that it's as few as 23? This is something I've heard said over the years but I wasn't sure if I believed it. Well - here you go:

Day 18, chance is 35.5%
Day 19, chance is 38.9%
Day 20, chance is 42.2%
Day 21, chance is 45.6%
Day 22, chance is 48.9%
Day 23, chance is 52.1%
Day 24, chance is 55.4%
Day 25, chance is 58.5%

Sure ... I used Perl ;-)

#!/usr/bin/perl
$chance = 0;
for ($k=1; $k<25; $k++) {
$chance += ($k/(366 - $k)) * (1 - $chance);
printf "Day %2d, chance is %5.1f%%\n",$k+1,$chance*100;
}


Yes - I DID do a test / sanity check. Ran it up to 365 people and ended up at 100%
(written 2004-08-12, updated 2012-11-04)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
P207 - Perl - File Handling
  [3839] Spraying data from one incoming to series of outgoing files in Perl - (2012-08-15)
  [3830] Traversing a directory in Perl - (2012-08-08)
  [3548] Dark mornings, dog update, and Python and Lua courses before Christmas - (2011-12-10)
  [3326] Finding your big files in Perl - design considerations beyond the course environment - (2011-06-14)
  [2833] Fresh Perl Teaching Examples - part 2 of 3 - (2010-06-27)
  [2821] Chancellor George Osborne inspires Perl Program - (2010-06-22)
  [2818] File open and read in Perl - modernisation - (2010-06-19)
  [2405] But I am reading from a file - no need to prompt (Perl) - (2009-09-14)
  [2233] Transforming data in Perl using lists of lists and hashes of hashes - (2009-06-12)
  [1861] Reactive (dynamic) formatting in Perl - (2008-10-31)
  [1860] Seven new intermediate Perl examples - (2008-10-30)
  [1841] Formatting with a leading + / Lua and Perl - (2008-10-15)
  [1709] There is more that one way - Perl - (2008-07-14)
  [1467] stdout v stderr (Tcl, Perl, Shell) - (2007-12-10)
  [1442] Reading a file multiple times - file pointers - (2007-11-23)
  [1416] Good, steady, simple example - Perl file handling - (2007-10-30)
  [1312] Some one line Perl tips and techniques - (2007-08-21)
  [867] Being sure to be positive in Perl - (2006-09-15)
  [702] Iterators - expressions tha change each time you call them - (2006-04-27)
  [618] Perl - its up to YOU to check your file opened - (2006-02-23)
  [616] printf - a flawed but useful function - (2006-02-22)
  [255] STDIN, STDOUT, STDERR and DATA - Perl file handles - (2005-03-23)
  [114] Relative or absolute milkman - (2004-11-10)


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