Re: help on Indexing contents in file

Hi,

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

<code>

struct ItemData
{
    DWORD Category;
    DWORD Item;
    DWORD Rank;
};

</code>

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.

<code>

enum ItemRank
{
  ItemRankUnrated = 0,
  ItemRankLow,
  ItemRankMedium,
  ItemRankHigh,
  ItemRankSuper
};

</code>

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
file.

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
fwrite).

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

<code>

  // 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

</code>

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

<code>

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

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

</code>

You can access items into the vector using both operator[] (e.g. items[0],
items[1], ...) or std::vector::at method (items.at(0), items.at(1),...). 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
this:

<code>

  // 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

</code>

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,
e.g.

  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
non-sense.)
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:

http://msdn2.microsoft.com/en-us/library/6bz744w8(VS.80).aspx

Giovanni

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"Nash" <jeevs007@gmail.com> ha scritto nel messaggio
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