On Sep 17, 2012, at 7:28 PM, arun wrote: > 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.
That is going to fail as soon as there are an uneven number of rows for one value of Sample. > Sample<-c(1,1,1,2,2,2,3,3,3,3) > Time<-c(1,2,3,1,2,3,1,2,3, 4) > Mass<-c(3,3.1,3.4,4,4.3,4.4,3,3.2,3.5, 3.7) > mydata<-as.data.frame(cbind(Sample,Time,Mass)) > mydata$Gain<-rep(tapply(mydata$Mass,mydata$Sample,FUN=function(x) > (x[3]-x[2])),each=length(unique(mydata$Sample))) Error in `$<-.data.frame`(`*tmp*`, "Gain", value = c(0.3, 0.3, 0.3, 0.100000000000001, : replacement has 9 rows, data has 10 > > > > > ----- Original Message ----- > From: Julie Lee-Yaw <jullee...@yahoo.ca> > To: "r-help@r-project.org" <r-help@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! > [[alternative HTML version deleted]] > > > ______________________________________________ > R-help@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. > David Winsemius, MD Alameda, CA, USA ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
