[R] Average over data sets

[ONKELINX, Thierry]

Here is a solution assuming that all files have the same structure and a
variable TimePoint which contains the time info.

CombinedData <- do.call(rbind, lapply(seq_len(20), function(i){
	fileName <- paste("output", i, ".dat", sep="")
	read.table(fileName, header=TRUE)
}))
library(plyr)
ddply(CombinedData, "TimePoint", colMeans) 


#another option
library(reshape)
recast(CombinedData, TimePoint + variable ~ ., id.var = TimePoint, fun =
mean)

HTH,

Thierry

------------------------------------------------------------------------
----
ir. Thierry Onkelinx
Instituut voor natuur- en bosonderzoek / Research Institute for Nature
and Forest
Cel biometrie, methodologie en kwaliteitszorg / Section biometrics,
methodology and quality assurance
Gaverstraat 4
9500 Geraardsbergen
Belgium
tel. + 32 54/436 185
Thierry.Onkelinx at inbo.be
www.inbo.be

To call in the statistician after the experiment is done may be no more
than asking him to perform a post-mortem examination: he may be able to
say what the experiment died of.
~ Sir Ronald Aylmer Fisher

The plural of anecdote is not data.
~ Roger Brinner

The combination of some data and an aching desire for an answer does not
ensure that a reasonable answer can be extracted from a given body of
data.
~ John Tukey

-----Oorspronkelijk bericht-----
Van: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
Namens Ed Long
Verzonden: woensdag 2 september 2009 14:55
Aan: r-help at r-project.org
Onderwerp: [R] Average over data sets

Hello,

I have a number of files output1.dat, output2.dat, ... , output20.dat,
each of which monitors several variables over a fixed number of
timepoints. From this I want to create a data frame which contains the
mean value between all files, for each timepoint and each variable.

The code below works, but it seems like I should be able to do the
second part without a for loop. I played with sapply(myList, mean), but
that seems to take the mean between time points and files, rather than
just between files.

#Number of files to calculate mean value between numberOfRuns = 20;
myList = list(); for (i in 1:numberOfRuns) {
	#Read in file
	fileName = paste("output", i, ".dat", sep="");
	myData = read.table(fileName, header=TRUE);
	#Append data frame to list
	myList[[i]] = myData;
}

#Create variable to store data means
myAverage = myList[[1]]/numberOfRuns;

for (i in 2:numberOfRuns) {
	myAverage = myAverage + myList[[i]]/numberOfRuns; }

Is a list of data frames a sensible structure to store this or should I
use an array?

Any pointers gratefully received.

Ed Long

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