Source only part of a file

Question

My R workflow is usually such that I have a file open into which I type R commands, and I’d like to execute those commands in a separately opened R shell.

The easies

Answer 1

Here's another way with just R:

source2 <- function(file, start, end, ...) {
    file.lines <- scan(file, what=character(), skip=start-1, nlines=end-start+1, sep='\n')
    file.lines.collapsed <- paste(file.lines, collapse='\n')
    source(textConnection(file.lines.collapsed), ...)
}

Answer 2

Using the right tool for the job …

As discussed in the comments, the real solution is to use an IDE that allows sourcing specific parts of a file. There are many existing solutions:

• For Vim, there’s Nvim-R.

• For Emacs, there’s ESS.

• And of course there’s the excellent stand-alone RStudio IDE.

As a special point of note, all of the above solutions work both locally and on a server (accessed via an SSH connection, say). R can even be run on an HPC cluster — it can still communicate with the IDEs if set up properly.

… or … not.

If, for whatever reason, none of the solutions above work, here’s a small module[gist] that can do the job. I generally don’t recommend using it, though.¹

#' (Re-)source parts of a file
#'
#' \code{rs} loads, parses and executes parts of a file as if entered into the R
#' console directly (but without implicit echoing).
#'
#' @param filename character string of the filename to read from. If missing,
#' use the last-read filename.
#' @param from first line to parse.
#' @param to last line to parse.
#' @return the value of the last evaluated expression in the source file.
#'
#' @details If both \code{from} and \code{to} are missing, the default is to
#' read the whole file.
rs = local({
    last_file = NULL

    function (filename, from, to = if (missing(from)) -1 else from) {
        if (missing(filename)) filename = last_file

        stopifnot(! is.null(filename))
        stopifnot(is.character(filename))

        force(to)
        if (missing(from)) from = 1

        source_lines = scan(filename, what = character(), sep = '\n',
                            skip = from - 1, n = to - from + 1,
                            encoding = 'UTF-8', quiet = TRUE)
        result = withVisible(eval.parent(parse(text = source_lines)))

        last_file <<- filename # Only save filename once successfully sourced.
        if (result$visible) result$value else invisible(result$value)
    }
})

Usage example:

# Source the whole file:
rs('some_file.r')
# Re-soure everything (same file):
rs()
# Re-source just the fifth line:
rs(from = 5)
# Re-source lines 5–10
rs(from = 5, to = 10)
# Re-source everything up until line 7:
rs(to = 7)

---

¹ Funny story: I recently found myself on a cluster with a messed-up configuration that made it impossible to install the required software, but desperately needing to debug an R workflow due to a looming deadline. I literally had no choice but to copy and paste lines of R code into the console manually. This is a situation in which the above might come in handy. And yes, that actually happened.