Iterate over a connection by line

Apply a function line-by-line to a connection, such as an open file connection. This allows simple processing without loading the entire file to memory, something like the common for line in file: ... approach in python.

by_line(connection, FUN, STOP, safe = TRUE, max_lines = 0, ...)

Arguments

connection	an open connection, such as a text connection as created by file
FUN	a function to apply to each line that can be read from the connection
STOP	a function to generate a stop condition for reading lines of the connection
safe	auto-stop iteraction on reaching an empty line in the connection (attempts to prevent infinite loops)
...	extra arguments passed to FUN

Examples

not_run({
  file <- file("myfile.txt", "r")

  # use with for automatic connection closing
  with(file, {
    # read and print each line sequentially
    by_line(file, print)
  })

  # stop reading after finding the word 'stop'
  connex <- file("logfile.txt", "r")
  by_line(connex, cat, function(line) grepl(line, "stop"))
  close(connex)
})