On Sep 17, 2012, at 5:00 PM, David Winsemius wrote:
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$gain2.3 <- with( mydata, ave( Mass , Time, FUN=function(x) diff(x[2],x[3]) ) )
OOOPpps .... the code above was a failed attempt.
Sample Time MassGain2-3
1 1 1 3.00.3
2 1 2 3.1 0.3
3 1 3 3.4 0.3
4 2 1 4.0 0.1
5 2 2 4.3 0.1
6 2 3 4.4 0.1
7 3 1 3.0 0.3
8 3 2 3.2 0.3
9 3 3 3.5 0.3
... the code below should "work".
mydata$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