Object#dupとObject#cloneはオブジェクトをコピーします
obj = ["a","b","c"] #=> ["a", "b", "c"] obj_d = obj.dup => ["a", "b", "c"] obj_d[0] << "PLUS" => "aPLUS" obj => ["aPLUS", "b", "c"] obj_d => ["aPLUS", "b", "c"] obj[1] = "HOGE" => "HOGE" obj => ["aPLUS", "HOGE", "c"] obj_d => ["aPLUS", "b", "c"]
