[R] Two-way/Three-way sum.

[David Winsemius]

On Dec 3, 2009, at 1:50 PM, Peng Cai wrote:

> Hi R Users,
>
> I'm wondering how can I calculate two (or three) way sum of a  
> variable. A
> sample data is:
>
> State Month Year Value
> NC Jan 1996 1
> NC Jan 1996 2
> NC Feb 1997 2
> NC Feb 1997 3
> NC Mar 1998 3
> NC Mar 1998 4
> NY Jan 1996 4
> NY Jan 1996 5
> NY Feb 1997 5
> NY Feb 1997 6
> NY Mar 1998 6
> NY Mar 1998 7
>
> I'm trying to sum up "value" column by State*Month and by  
> State*Month*Year.

?tapply

as in

sum.tbl <- with(dftbl, tapply(Value, list(State, Month), sum) )

> Also, I may need to calculate mean value along with "sum".

Use the mean function in the above formulation to get means, length to  
get counts, or perhaps summary .... or one of the several packages  
that offer a "describe" function. So many functions, so little time.

-- David


>
> Any help would be greatly appreciated,
>
> Thanks,
> Peng
>
> 	[[alternative HTML version deleted]]
>
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David Winsemius, MD
Heritage Laboratories
West Hartford, CT