Dear Minghua Yao, I suggest you looking up the archives of the mailing list. The answers to the following message made things clear to me:
Wayne Jones <[EMAIL PROTECTED]>
Címzett:
R-Help <[EMAIL PROTECTED]>
Tárgy:
[R] Sorting a data frame
Dátum:
Wed, 16 Jul 2003 14:42:09 +0100
Gábor
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