[R] Question on summing rows within nested variable

[David Winsemius]

DF2 <- read.table(textConnection("sid        pid        slope
+ 1.1        1.1        2
+ 1.1        4.1        3
+ 1.1        5.1        2
+ 2.1        5.1        3
+ 3.2        1.2        2
+ 3.2        1.7        3"), header = TRUE)


 > tapply(DF2$slope, as.factor(DF2$pid), mean)
1.1 1.2 1.7 4.1 5.1
2.0 2.0 3.0 3.0 2.5

Could also wrap it in with(DF2, ....)  to make it more readable and  
compact. As testing shows that the as.factor() is not needed.

-- 
David Winsemius

On Mar 13, 2009, at 6:43 PM, Vedula, Satyanarayana wrote:

> Hi,
>
> I was hoping someone could help figure out how to write code for R  
> to do the below.
>
> I have data that looks like below. Variables, sid and pid are  
> strings, slope is numeric. I need R to get me the mean of slopes for  
> all pid's nested within each sid if there are more than one pid's  
> nested within sid.
>
> If there is only pid for a sid, like for 2.1 below, I want R to  
> write a 0.
>
> In the data below, I want to get the mean of slopes for pid (1.1;  
> 4.1; and 5.1) because they are nested within sid 1.1 and so on.
>
> Thanks in advance for any suggestions.
> Swaroop
>
> sid        pid        slope
> 1.1        1.1        2
> 1.1        4.1        3
> 1.1        5.1        2
> 2.1        5.1        3
> 3.2        1.2        2
> 3.2        1.7        3
>
>
>
>
> 	[[alternative HTML version deleted]]
>
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David Winsemius, MD
Heritage Laboratories
West Hartford, CT