Open file, get first line, delete first line close file

Discussion in 'Ruby' started by Richard Schneeman, Aug 23, 2008.

  1. Hey, i'm trying to open a file, get the first line of the file, delete
    that line from the file, and then close the file. Using ruby 1.8.6.

    I've tried using

    The_File = IO.readlines("public/languages/chinese/practice.txt")


    The_File = open('public/languages/chinese/practice.txt','r+')

    but i can't figure out the correct syntax for how to delete a line in a
    file, and then save that file.
    Richard Schneeman, Aug 23, 2008
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  2. Richard Schneeman

    Daniel Bush Guest

    Simplest thing is to get it into memory like you did with readlines
    above and write out the altered contents to a new file and then move it.
    (I prefer File.readlines - although this is the same method either way).
    Large files might require different treatment rather than slurping into
    memory like that.

    That being said, I'd be curious to hear how people use r+ mode.

    Daniel Bush, Aug 24, 2008
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  3. That is one option...the reason i need to be able to delete lines, is
    that i am running a rake task on a server, that i cannot easily modify
    files, and I can't predict how long the rake task will time out. The
    task takes entries from text based dictionaries and adds it to my DB,
    the thing is, if i didn't delete the lines i've already added, everytime
    i ran the task, (it has to be run multiple times due to timeouts) i
    would only re-add the same lines. The deletion acts as a place holder of
    sorts. I'll play around with your suggestion, and i'll let you know. If
    anyone else has an alternate method...i'm all ears
    Richard Schneeman, Aug 24, 2008
  4. Its gross but it works

    active_dictionary =

    open('public/languages/chinese/practice.txt', 'w') do |file|
    file.puts active_dictionary[1,active_dictionary.size]

    Once again, i'm interested in different approaches, or something, not
    quite so processor intensive
    Richard Schneeman, Aug 24, 2008
  5. Richard Schneeman

    Daniel Bush Guest

    Richard Schneeman wrote:
    I don't know if it's all that gross. I have feeling this is
    standard way to do it for apps and editors ie write out entire altered
    after working with file in-memory using whatever scheme.
    Different story if you're a database I guess.

    Here is one scheme some text editors use:
    although it doesn't discuss file system/persistence issues.
    Presumably when you hit the save button, the system writes
    out to a new file (the gap buffer is not playing around with
    the old file stream).

    If you're just replacing stuff character for character, then
    it seems ok to use the file stream (in r+ mode) or if you're
    appending (or both); but deleting or inserting content seems
    problematic - not sure it's possible let alone standardized.
    Anyone want to weigh in here?

    If the file is really large, you can perhaps just move through the
    stream till you get to the point where you want to start
    then commence writing from the old stream to the new file stream.
    May be ways to optimise it.

    Sparse files and fixed line lengths ? :)

    Maybe I've said enough wrong things to provoke a reacion
    from someone else.

    Daniel Bush, Aug 24, 2008
  6. tail +2 the_file > the_new_file

    Not all problems are best solved with ruby :)
    Erik Hollensbe, Aug 24, 2008
  7. Richard Schneeman

    Dave Bass Guest

    And sometimes the problems are not with the program but with the data
    structure -- maybe a flat file isn't the right way to do things?

    And... it's a lot easier to delete the *last* line of a file than the

    Just some thoughts. :)
    Dave Bass, Aug 24, 2008
  8. I don't think this is actually true, can you explain further?

    Erik Hollensbe, Aug 25, 2008

  9. You can just truncate the file size. You don't have any subsequent
    lines (bytes) to move into a new position within the file.


    Rob Biedenharn
    Rob Biedenharn, Aug 25, 2008
  10. How do you know which line is the last line?

    Unless there's something I don't know, that involves reading the whole
    file, or a combination of seek/read from the end until you find the last
    newline, which is essentially what tail +2 does, but starts at the
    beginning of the file.

    "Moving" data in a file is the worst possible scenario for I/O at all.
    You can do both of these operations in a single pass read of the file
    without shoving the whole thing into memory at once. It just involves
    writing to one file and reading from another, is all.
    Erik Hollensbe, Aug 25, 2008
  11. Well, if you want/need the last line(s) of a file (presumably text or
    how would you define a "line"), you can take a look at the File::Tail

    gem install file-tail

    I had some Perl code (lifted from some forum or article) that would
    cut initial lines out of a log file using sysread/syswrite with a
    truncate to reset the end-of-file. I don't recall if it used a single
    file descriptor or two separate ones, but the idea is the same -- move
    bytes "backward" across the gap that you want to eliminate. I agree
    with your "worst possible scenario for I/O" assessment.


    Rob Biedenharn
    Rob Biedenharn, Aug 25, 2008
  12. You are talking about `tail -f`. This is different.

    (And if you ever need to find that again, perldoc -q tail).

    Erik Hollensbe, Aug 25, 2008
  13. Richard Schneeman

    Dave Bass Guest

    My Ruby isn't up to coding it, but in principle I'd seek to the end of
    the file, then backtrack until I found the appropriate newline. Then I'd
    truncate the file.
    Dave Bass, Aug 25, 2008
  14. Richard Schneeman

    Mark Thomas Guest

    the thing is, if i didn't delete the lines i've already added, everytime
    I suggest exploring a distributed worker system like Rinda,
    Backgroundrb, AP4R, or Sparrow. You can prepare a master list of
    dictionary words, and then worker processes can take one at a time and
    add them to your database. Having a timeout won't slow things down,
    nor will it cause you to have to re-read your wordlist.
    Mark Thomas, Aug 25, 2008
  15. I've set it up with a rake task and a cron job, that re-runs every 5hrs
    (my timeout window)

    i this is the code that i've already uploaded, and is currently

    namespace :chinese do

    desc "adds all chinese files to database"
    task :create => :environment do

    active_dictionary =
    count = 0
    for @element in active_dictionary
    count += 1
    open('public/languages/chinese/practice.txt', 'w') do




    where process_chinese contains all my proprietary code, i played around
    with only writing to the file every time i've processed ten entries, but
    it only cut my process time down by a trivial amount of time, so i just
    let it write over the file after every line. As we've seen here, there
    are quite a few ways this can be accomplished, this ended up
    working...and didn't kill my processor (the writes to the DB are
    infinitely more expensive than opening and writing to this file).

    Thanks for the help!!
    Richard Schneeman, Aug 25, 2008
  16. If the file is exceptionally large, you can save a lot of memory (and
    processing time, likely), by doing something like this:"my_file") do |f|
    f.readline"my_file.tmp", 'w') do |f2|
    f2 <<
    end"my_file.tmp", "my_file")

    The point here is that almost all the work is done on the file
    descriptors instead of in memory. I don't know if ruby has a sendfile()
    implementation, but that would be the most ideal, as it'd instruct the
    OS to do the copy.
    Erik Hollensbe, Aug 25, 2008
  17. Richard Schneeman

    Daniel Bush Guest

    Just on the "f2 <<" part, isn't this still reading the rest of
    the file into ruby?
    I was thinking more of reading stuff into a fixed buffer and then
    writing it.
    while # bytes
    f2.write buf # or f2 << buf
    which will result in a bazillion more calls to IO#read and IO#write on a
    large file but doesn't read the whole thing into memory. I'm not
    recommending this or anything - just wanted to clarify.

    Daniel Bush, Aug 27, 2008
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