st: Summing variables over all observations by identifier

["Amber Katz" <academicgirl@gmail.com>]

Hi,

I'm looking for a function that will sum a variable across all
observations by identifier.

For example, if I have

ID  A
1   1
1   0
2   1
2   2

I would like a function that would give me:
SumByID=function(A,ID)

SumbyID would be equal to 1 for all observations with ID=1 and 3 for
all observations with ID=2 .

Thanks very much,
Rachel

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