Re: help on Indexing contents in file

"Giovanni Dicanio" <>
Wed, 10 Oct 2007 10:33:59 +0200

I would define a simple data structure like this, assuming that categories
and items can be identified by integer numbers.


struct ItemData
    DWORD Category;
    DWORD Item;
    DWORD Rank;


I would reserve also a special value for Rank, to mean not-ranked (e.g. 0).
If Rank can be taken from a small basket of values, I would define an enum
for Ranks, e.g.


enum ItemRank
  ItemRankUnrated = 0,


Then I would use a binary format for the file, considering that it should be
made up by *fixed-size* records (each record is an instance of ItemData
structure; in the above representation, we have 3 DWORDs, i.e. 3 * 4 bytes =
12 bytes/record). Using binary format and fixed-size records give you *very
fast* access and easy management of the data.

On startup, I would load the entire file content into memory, and use a
memory representation of it, something like std::vector< ItemData >.
Then I would do the modifications into the in-memory representation, and
before exiting (or when user presses Save button, I don't know about your
program user-interface...) I would save the in-memory representation back to

Both loading and saving are very easy, because all records are fixed size
(you may use the simple C file function APIs like fopen, fclose, fread and

e.g. assuming that the file handler for your data is "fileDB" (of type FILE
*), to read a record you can do:


  // Will be filled from data read from file
  ItemData itemData;

  // Read one record from file
  size_t result = fread(
      &itemData, // where to put the read data
      sizeof(ItemData), // size in bytes of one record
      1, // read one record
      fileDB // file handler

  // ...check result


Then you can add your read record into a std::vector container, something
like this:


  // In-memory copy of the DB
  typedef std::vector< ItemData > ItemContainer;
  ItemContainer items;

  // Add the new item into DB
  items.push_back( itemData );


You can access items into the vector using both operator[] (e.g. items[0],
items[1], ...) or std::vector::at method (,,...). The
difference is that at() method does a bounds-checking on the index, and
throws an exception if the index is out of vector bounds.

If you want to write a record to a file, you can use fwrite, something like


  // Contains item data to write to file
  ItemData itemData;

  // Write one record to file
  size_t result = fwrite(
      &itemData, // pointer to source data
      sizeof(ItemData), // size in bytes of one record
      1, // write one record
      fileDB // file handler

  // ...check result


I would also prefix the database file with:

1 - A signature (e.g. just a sequence of bytes chosen by you), so that you
can identify the file as having a valid format

2 - The number of records stored in the file (so you can read this
information and prealloc the in-memory shadow of the DB, e.g. using
std::vector::reserve or std::vector::resize methods).

You could also store strings into your record, and if you want to continue
with the fast-and-easy-management approach, you can use fixed-size strings,

  struct ItemData
      wchar_t Category[ 60 ];
      wchar_t Item[ 80 ];
      DWORD Rank;

(Note that I use Unicode wchar_t, because IMHO using ANSI these days is a
Of course, when you load the item record from file into memory, you can use
a more robust CString (or std::wstring) class than raw-C arrays in the
in-memory representation of the record.

This is just one of many possible and valid solutions to your problem.

Another option could be to use /ad hoc/ MFC serialization classes, and
derive your ItemData from CObject, making it serializable, and use MFC
CArchive, operator<< and >> and other MFC serialization facilities.
For that path, you may find the following MSDN link to be interesting:



"Nash" <> ha scritto nel messaggio

Sorry for the delayed response... The file should be text/binary file
which can be parsed easily using MFC classes. The categories i
mentioned is pre defined it cannot be changed. But the user can
update(add/remove) the items in each category. None of the item will
be common across category. The file which iam going to maintain should
have only the ratings for the items. Only one process i going to acces
the data.. I will give an example to make it clear

Cat A Item A1 Rank 0
Cat A Item A2 Rank 1
Cat A Item A3 Rank 0

Cat B Item B1 Rank 2

Cat C Item C2 Rank 1

even though cat b has some more items user didnt rate it yet so the
idea is the rating file should have only the items which are rated.
Now the user want to see the items under Cat B we will display all the
items under Cat B in a list but the Item B1 will be displayed with
ranking. So while displaying we need to parse the rating file to see
which items are ranked and what is the ranking. Now from the list the
user can choose any item and can add a rating now we need to update
the ranking file.

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