1.9.2p320 :122 > person = {'name'=>'jack','age'=>25,'city'=>'abc'}
 => {"name"=>"jack", "age"=>25, "city"=>"abc"}
1.9.2p320 :123 > person.to_a
 => [["name", "jack"], ["age", 25], ["city", "abc"]]
1.9.2p320 :124 > person['age']
 => 25
1.9.2p320 :125 > person['name']
 => "jack"
1.9.2p320 :126 > person.length
 => 3
1.9.2p320 :127 > person[1]
 => nil
1.9.2p320 :128 >

 => nil
1.9.2p320 :128 >