gistfile1.rb

1.9.3p286 :001 > a = Hash.new([])
 => {} 
1.9.3p286 :002 > a[:foo] << 1
 => [1] 
1.9.3p286 :003 > a[:bar] << 1
 => [1, 1] 
1.9.3p286 :004 > b = Hash.new(){[]}
 => {} 
1.9.3p286 :005 > b[:foo] << 1
 => [1] 
1.9.3p286 :006 > b[:bar] << 1
 => [1] 
1.9.3p286 :007 > b[:bar] << 1
 => [1] 
1.9.3p286 :009 > b[:bar] 
 => [] 
1.9.3p286 :011 > c = Hash.new(){|h,v|h[v] = []}
 => {} 
1.9.3p286 :012 > c[:foo] << 1
 => [1] 
1.9.3p286 :013 > c[:bar] << 1
 => [1] 
1.9.3p286 :014 > c[:bar] << 2
 => [1, 2]