R help - Apr 2006 - Conditional Row Sum

Hi,
   
  How can I accomplish this in R. Example:
   
  R1  R2
  3     101
  4     102
  3     102
  18    102
  11    101
   
  I want to find Sum(101) =  14 - i.e SUM(R1) where R2 = 101
                  Sum(102) = 25    - SUM(R2) where R2 = 102
   
  TIA
  Sachin

		
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	[[alternative HTML version deleted]]

Using the built in data.frame iris sum each of the first 4 columns for
each value of the 5th column.

rowsum(iris[,-5], iris[,5])


On 4/20/06, Sachin J <sachinj.2006 at yahoo.com>
wrote:
> Hi,
>
>  How can I accomplish this in R. Example:
>
>  R1  R2
>  3     101
>  4     102
>  3     102
>  18    102
>  11    101
>
>  I want to find Sum(101) =  14 - i.e SUM(R1) where R2 = 101
>                  Sum(102) = 25    - SUM(R2) where R2 = 102
>
>  TIA
>  Sachin
>
>
> ---------------------------------
>
>        [[alternative HTML version deleted]]
>
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>

On Thu, 2006-04-20 at 11:46 -0700, Sachin J wrote:
> Hi,
>    
>   How can I accomplish this in R. Example:
>    
>   R1  R2
>   3     101
>   4     102
>   3     102
>   18    102
>   11    101
>    
>   I want to find Sum(101) =  14 - i.e SUM(R1) where R2 = 101
>                   Sum(102) = 25    - SUM(R2) where R2 = 102
>    
>   TIA
>   Sachin
Presuming that your data is in a data frame called DF:
> DF
  R1  R2
1  3 101
2  4 102
3  3 102
4 18 102
5 11 101

At least three options:
> with(DF, tapply(R1, R2, sum))
101 102
 14  25

> aggregate(DF$R1, list(R2 = DF$R2), sum)
   R2  x
1 101 14
2 102 25

> by(DF$R1, DF$R2, sum)
INDICES: 101
[1] 14
------------------------------------------------------
INDICES: 102
[1] 25


See ?by, ?aggregate and ?tapply and ?with.

HTH,

Marc Schwartz