10% ers. http://reprog.wordpress.com/2010/04/19/are-you-one-of-the-10-percent/

binary_search.rb

class Array                                                                                                                                                                              
  def self.find_rec(ary,start,fin,num)                                                                                                                                                   
    return if fin < start                                                                                                                                                                
                                                                                                                                                                                         
    center = ((fin + start) / 2)                                                                                                                                                         
                                                                                                                                                                                         
    case ary[center] <=> num                                                                                                                                                             
    when 0 then                                                                                                                                                                          
      return center                                                                                                                                                                      
    when -1 then                                                                                                                                                                         
      find_rec(ary,center + 1, fin, num)                                                                                                                                                 
    when 1 then                                                                                                                                                                          
      find_rec(ary,start, center - 1, num)                                                                                                                                               
    end                                                                                                                                                                                  
  end                                                                                                                                                                                    
  def find(num)                                                                                                                                                                          
    self.class.find_rec(self,0,self.length - 1, num)                                                                                                                                     
  end                                                                                                                                                                                    
end                                                                                                                                                                                      
                                                                                                                                                                                         
                                                                                                                                                                                         
def test_id(ary, num)                                                                                                                                                                    
  str = ary.inspect + '.find ' + num.to_s                                                                                                                                                
  other_str = ary.inspect + '.index ' + num.to_s                                                                                                                                         
  puts " index:#{'% 5s' % eval(other_str).to_s } find:#{'% 5s' % eval(str)} #{str}"                                                                                                      
end                                                                                                                                                                                      
                                                                                                                                                                             
test_id([1,2,3],2) #1                                                                                                                                                                    
test_id([1,2,3],3) #2                                                                                                                                                                    
test_id([1,2,3],1) #0                                                                                                                                                                    
test_id( [1,2,3,4,5], 2) #1                                                                                                                                                              
test_id [0,0,0,0,0,0], 1 #nil                                                                                                                                                            
test_id [0,0,0,0, 1,2], 0 # nil                                                                                                                                                          
test_id [0,0,0,0, 1,2], 1 #4, -2                                                                                                                                                         
test_id [0,0,0,0, 1,2], 2 #5, -1                                                                                                                                                         
test_id [], 0 # nil                                                                                                                                                                      
test_id [1], 0 # nil                                                                                                                                                                     
test_id [1], 1 # 0                                                                                                                                                                       

=begin
ruby binary_search.rb 
 index:    1 find:    1 [1, 2, 3].find 2
 index:    2 find:    2 [1, 2, 3].find 3
 index:    0 find:    0 [1, 2, 3].find 1
 index:    1 find:    1 [1, 2, 3, 4, 5].find 2
 index:      find:      [0, 0, 0, 0, 0, 0].find 1
 index:    0 find:    2 [0, 0, 0, 0, 1, 2].find 0
 index:    4 find:    4 [0, 0, 0, 0, 1, 2].find 1
 index:    5 find:    5 [0, 0, 0, 0, 1, 2].find 2
 index:      find:      [].find 0
 index:      find:      [1].find 0
 index:    0 find:    0 [1].find 1
=end

Whew. Was only off on the equal elements case. I'm gonna call it a win.

This is the third program of moderate complexity I've ever written correctly on the first try.