[R] help with calculation from dataframe with multiple entries per sample

[David Winsemius]

On Sep 17, 2012, at 4:15 PM, Julie Lee-Yaw wrote:

> Hi 
> 
> I have a dataframe similar to:
> 
>> Sample<-c(1,1,1,2,2,2,3,3,3)
> 
>> Time<-c(1,2,3,1,2,3,1,2,3)
> 
>> Mass<-c(3,3.1,3.4,4,4.3,4.4,3,3.2,3.5)
> 
>> mydata<-as.data.frame(cbind(Sample,Time,Mass))
> 
Please tell me where you learned that as.data.frame(cbind(.)) construction.

> (
>   Sample Time Mass
> 1      1    1  3.0
> 2      1    2  3.1
> 3      1    3  3.4
> 4      2    1  4.0
> 5      2    2  4.3
> 6      2    3  4.4
> 7      3    1  3.0
> 8      3    2  3.2
> 9      3    3  3.5
> 
> where for each sample, I've measured mass at different points in time. 
> 
> I now want to calculate the difference between Mass at Time 2 and 3 for each unique Sample and store this as a new variable called "Gain2-3". So in my example three values of 0.3,0.1,0.3 would be calculated for my three unique samples and these values would be repeated in the table according to Sample. I am thus expecting:
> 
>> mydata #after adding new variable

mydata$gain2.3 <- with( mydata, ave( Mass , Time, FUN=function(x) diff(x[2],x[3]) ) )
> 
>   Sample Time MassGain2-3
> 1      1    1  3.00.3
> 2      1    2  3.1 0.3
> 3      1    3  3.4 0.3
> 4      2    1  4.0 0.1
> 5      2    2  4.3 0.1
> 6      2    3  4.4 0.1
> 7      3    1  3.0 0.3
> 8      3    2  3.2 0.3
> 9      3    3  3.5 0.3
> 

> mydata$gain2.3 <- with( mydata, ave( Mass , Sample, FUN=function(x) (x[3]-x[2]) ) )
> mydata
  Sample Time Mass gain2.3
1      1    1  3.0     0.3
2      1    2  3.1     0.3
3      1    3  3.4     0.3
4      2    1  4.0     0.1
5      2    2  4.3     0.1
6      2    3  4.4     0.1
7      3    1  3.0     0.3
8      3    2  3.2     0.3
9      3    3  3.5     0.3

> Does anyone have any suggestions as to how to do this? I've looked at the various apply functions but I can't seem to make anything work. I'm fairly new to R and would appreciate specific suggestions. 

-- 
David Winsemius, MD
Alameda, CA, USA