Question

Hi I have a quick question about converting a list of data.frames into an array (I prefer the latter format for plotting and manipulation).

I have the following code which just about works but I am not getting the loop right and wondered if someone could point me to what I am doing wrong.

What I want to do is fill an array with the values of one of the columns from each of the different data.frames in my list of data.frames.

# list all the files in a directory
data.dir = "/data/output"

# list all the files to plot
files <- list.files(data.dir, full=TRUE, pattern="Fields_grid*")

# cat them together into one data frame
data <- lapply( files,  read.csv, header=FALSE, skip=26)

# calc no. of files
nfiles <- length(files)

# set up an empty array to fill with data
z <- array( NA, dim=c(length(x), length(y), nfiles ))

# loop through all the data.frames/files 
for (i in 1:length(nfiles)) {

# calc index's of lists
x <- sort(unique( data[[i]]$V3))
y <- sort(unique( data[[i]]$V4))
indx <- match( data[[i]]$V3, x)
indy <- match( data[[i]]$V4, y)

# fill array
z[ cbind( indx, indy, i)] <- data[[i]]$V5 }

I know this is a terrible question as I can not reproduce the result I get, but I hope you can understand my issue. When I look at z[,,1] this contains data but z[,,2] to z[,,nfiles] contains NA (i.e. my loop has not worked). I guess its something to do with the indexing but I really can’t see what I have done wrong.

Again apologies for an opaque question and thanks for having a look!

Answer 1

The problem is with your loop:

for (i in 1:length(nfiles)) { [...]

Here, length(nfiles) is 1 so your loop is only run for i equals 1.

Instead, you meant to do

for (i in 1:nfiles) { [...]

or

for (i in seq_len(nfiles)) { [...]

or

for (i in seq_along(files)) { [...]