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

[arun]

HI,
Modified version of my earlier solution:
res1<-tapply(mydata$Mass,mydata$Sample,FUN=function(x) (x[3]-x[2]))
res2<-data.frame(Sample=names(res1),Gain2_3=res1)
 merge(mydata,res2)

#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
A.K.



----- Original Message -----
From: Julie Lee-Yaw <julleeyaw at yahoo.ca>
To: "r-help at r-project.org" <r-help at r-project.org>
Cc: 
Sent: Monday, September 17, 2012 7:15 PM
Subject: [R] help with calculation from dataframe with multiple entries per sample

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))


  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

  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

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. 

Thanks!
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