-------------------------------基本的な使い方
article1
article2
article3
article4
-------------------------------連想配列の場合
object
article1 => title1
article2 => title2
article3 => title3
article4 => title4
-------------------------------通常配列のkeyを出したい場合
0: article1
1: article2
2: article3
3: article4
-------------------------------連想配列のkeyを出したい場合
0: article1 => title1
1: article2 => title2
2: article3 => title3
3: article4 => title4