[R] how to calculate another vector based on the data from a combination of two factors

[jeffc]

Hi, 

I have a data set similar to the following

State	Gender	Quantity
TX	Male	1
NY	Female	2
TX	Male	3
NY	Female	4


I need to calculate cumulative sum of the quantity by State and Gender. The
expected output is 
State	Gender	Quantity	CumQuantity
TX	Male	1	1
TX	Male	3	4
NY	Female	2	2
NY	Female	4	6

I highly appreciate if someone can give me some hints on solving that in R.

Hao

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