OpenEuphoria

Re: HELP with 'seek'

• Posted by Al Getz <xaxo at AOL.COM> Aug 18, 2000
• 625 views

On reading files using r, rb seek(), get(), gets() , where() etc.:

I dont know if this helps but, i was working on a project some time
ago that required repeated comparisons of data stored in a file in the form
of text (the file written using printf() statements).  The problem was,
because you never knew where you were going to have to go next in the
file and the file was very large, you couldnt take the time to read
through the whole file time after time just to get one or two items
randomly placed in the file. Since it was all text lines gets() seemed
like a good candidate method for reading back the data without having to
program a get() interface to allow random accessing of  variable
length 'records'.

  The solution came out somewhat simple:

  [A]

  1.  open the file in "r" mode
  2.  while reading all the records through once do:
          do a where() followed by a gets() and
          log all the addresses returned by where() in a sequence.
          If you want you can save the first two letters in the same
          sequence to function as a hash alpha lookup. This would yield
          sub seq's such as: {100,"ab"},{123,"ac"},...
  3. now that you have the address for every text group its simply a
     matter of using seek() followed by gets() to get to the data.

  [B]

  if your data is mixed (not all text) then you simply use an
  alternating series of gets() for text fields and get() for other
  fields.  You only need to log 'where()' addresses once for the
  first field of each 'record'.
  Ultimately, also record the next record address within the file and
  you've got a linked list on the next run. Record the previous address
  also and you can query up and down the list as well.

  [C]

  if your data is doubly mixed (not all records are the same type)
  then simply make the first field the format type identifier.

  [D]

  if your file is so large its not practical to read through
  the whole file at startup, you can start a separate file to record
  where()'s whenever a record IS found during the normal application
  run. Each time the app runs more and more records are located
  making the time to locate data less and less each time.  Of course
  when something is added to the file the location is stored at the
  same time.

  [E]
  ive also had great success with using home made delimiters chosen
  such that an occurance of the delimiter char(s) never or seldom
  occur naturally in the target data, or only in known locations.
  Filenames are a good example as quite a few characters are not
  allowed.


  Hope this helps some, if not perhaps in another project.

  Good luck,
              --Al Getz