add header to file created by "write.csv"

Question

I am trying to automate some data exporting, and I would like to add a header to each file such as "please cite Bob and Jane 2008" ... or even a few lines of specific instructions depending on the context.

I have looked at the write.csv and write.table documentation, but do not see any such feature.

What is the easiest way to achieve this?

Solution

Here are two possible approaches - the solution under EDIT using connections is more flexible and efficient.

---

Using write.table(...,append = T) and cat

• Use append=T within a call to write.table, having cat the header there previously

wrapped in its own function....

write.table_with_header <- function(x, file, header, ...){
  cat(header, '\n',  file = file)
  write.table(x, file, append = T, ...)
}

Note that append is ignored in a write.csv call, so you simply need to call

write.table_with_header(x,file,header,sep=',')

and that will result in a csv file.

---

EDIT

using connections

(Thanks to @flodel whose suggestion is this)

my.write <- function(x, file, header, f = write.csv, ...){
# create and open the file connection
  datafile <- file(file, open = 'wt')
# close on exit
  on.exit(close(datafile))
# if a header is defined, write it to the file (@CarlWitthoft's suggestion)
  if(!missing(header)) writeLines(header,con=datafile)
# write the file using the defined function and required addition arguments  
  f(x, datafile,...)
}

Note that this version allows you to use write.csv or write.table or any function and uses a file connection which (as @flodel points out in the comments) will only open and close the file once, and automatically appends. Therefore it is more efficient!