Re: [R] extracting row means from a list

From: Sundar Dorai-Raj <sundar.dorai-raj_at_pdf.com>
Date: Wed 20 Jul 2005 - 01:35:11 EST

Andy Bunn wrote:
> Hello: I'm reading in a series of text files (100 files that are each 2000
> rows by 6 columns). I wish to combine the columns (6) of each file (100) and
> get the row mean. I'd like to end up with a data.frame of 2000 rows by 6
> columns.
>
> foo <- list()
> for(i in 1:10){
> # The real data are read in from a series of numbered text files
> foo[[i]] <- data.frame(x1 = rnorm(100), x2 = rnorm(100), x3 =
> rnorm(100),
> x4 = rnorm(100), x5 = rnorm(100), x6 =
> rnorm(100))
>
> }
>
> str(foo)
> # by hand
> mean.x1 <-
> rowMeans(cbind(foo[[1]][,1],foo[[2]][,1],foo[[3]][,1],foo[[4]][,1],foo[[5]][
> ,1]),
> foo[[6]][,1],foo[[7]][,1],foo[[8]][,1],foo[[9]][,1
> ],foo[[10]][,1]))
> mean.x2 <-
> rowMeans(cbind(foo[[1]][,2],foo[[2]][,2],foo[[3]][,2],foo[[4]][,2],foo[[5]][
> ,2]),
> foo[[6]][,2],foo[[7]][,2],foo[[8]][,2],foo[[9]][,2
> ],foo[[10]][,2]))
> # and so on to column 6
> mean.x6 <-
> rowMeans(cbind(foo[[1]][,6],foo[[2]][,6],foo[[3]][,6],foo[[4]][,6],foo[[5]][
> ,6]),
> foo[[6]][,6],foo[[7]][,6],foo[[8]][,6],foo[[9]][,6
> ],foo[[10]][,6]))
>
>
> I've implemented this with nested loops that create temporary variables and
> calc the mean, but the approach is clunky. E.g.,
>
> # nested loops
> for(i in 1:ncol(foo[[1]])){
> for(j in 1:length(foo)){
> # etc ...
> }
> }
>
> Is there a way to build a better mouse trap?
>
> TIA, Andy

I don't know of a way of getting around at least one for loop, but the following might be want you need:

r <- matrix(, NROW(foo[[1]]), length(foo)) for(i in 1:NCOL(foo[[1]]))

   r[, i] <- rowMeans(do.call("cbind", lapply(foo, "[", i))) dim(r)

This assumes each element of foo has identical dimensions. Otherwise you'll get an error.

HTH, --sundar



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https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Wed Jul 20 01:40:38 2005

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