help with calculation from dataframe with multiple entries per sample

HI,
Try this:
?mydata$Gain<-rep(tapply(mydata$Mass,mydata$Sample,FUN=function(x) (x[3]-x[2])),each=length(unique(mydata$Sample)))
?mydata
#? Sample Time Mass Gain
#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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