How to add a index by set of data when using rbindlist?

Question

I have several different csv files with the same structure. I read them into R using fread, and then union them into a bigger dataset using rbindlist().

Answer 1

You are only missing the idcol argument from rbindlist(). Run:

x2csv <- rbindlist(lapply(files, fread, stringsAsFactors = FALSE), fill = TRUE, idcol = TRUE )

Answer 2

This is an enhanced version of Nicolás' answer which adds the file names instead of numbers:

x2csv <- rbindlist(lapply(files, fread), idcol = "origin")
x2csv[, origin := factor(origin, labels = basename(files))]

• fread() uses stringsAsFactors = FALSE by default so we can save some keystrokes
• Also fill = TRUE is only required if we want to read files with differing structure, e.g., differing position, name, or number of columns
• The id col can be named (the default is .id) and is populated with the sequence number of the list element.
• Then, this number is converted into a factor whose levels are labeled with the file names. A file name might be easier to remember than just a mere number. basename() strips the path off the file name.