Question

I’ve just cleaned up a data frame that I scraped from an excel spreadsheet by amongst other things, removing percentage signs from some of the numbers see, Removing Percentages from a Data Frame.

The data has twenty four rows representing the parameters and results from eight experiments done in triplicate. Eg, what one would get from,

DF1 <- data.frame(X = 1:24, Y = 2 * (1:24), Z = 3 * (1:24))

I want to find the mean of each of the triplicates (which, fortunately are in sequential order) and create a new data frame with eight rows and the same amount of columns.

I tried to do this using,

DF2 <- data.frame(replicate(3,sapply(DF1, mean)))

which gave me the mean of each column as rows three times. I wanted to get a dataframe that would give me,

data.frame(X = c(2,5,8,11,14,17,20,23), Y = c(4,10,16,22,28,34,40,23), Z = c(6,15,24,33,42,51,60,69))

which I worked out by hand; it’s supposed to be the reduced result.

Thanks, …

Any help would be gratefully recieved.

Answer 1

This works:

foo <- matrix(unlist(by(data=DF1,INDICES=rep(1:8,each=3),FUN=colMeans)),
  nrow=8,byrow=TRUE)
colnames(foo) <- colnames(DF1)

Look at ?by.