File organization

Def: Refers to the data is stored in a file

Text file VS binary form

• Text file: Data is stored in a file as a sequence of characters.

• Binary file: The binary equivalent of data is stored as a sequence od bytes

Record stored in a text file

• The number of data items per line must be known and the number of characters per item must be known.

OR

• Item seperator characters must be used

• repeating lines defined be end-of-line character or characters

Drect access

• Random access files

• Can be any a

Serial files:

• The records are put into the file chronologically, ie., according to when each record is produced. The one produced first is put into the file first, then the next one

• Each new record is simply appended to the file so that the only
  ordering in the file is the time order of data entry.

• Typical example:

• For a bank to record transactions involving customer accounts

Sequential files

• A sequential file have records that are ordered!

• In order to allow a sequential file to be ordered, there has to be a key for which the values are unique and sequential but not necessarily consecutive.

• When a new record is to be added what need to do?

• Read the file sequentially and each record is written to a new file.

• This is continued until the appropriated position for the new record is
  reached.

• The new record is then written to the new file before the remain
  records in the old file are copied in.

Direct-access files

• Random-access files

• Access can be to any record in the file without sequential reading of the file

• Direct access can be achieved with a sequential file.

• A separated index file is created which has two fields per record

• The first field has the key field and the second field has a value for the position of this key field value in the main file.

• The alternative is to use a hashing algorithm

• What is hashing algorithm?

One simple hashing algorithm

If there is a numeric key field in each record

• Choose a suitable number

• Divide this number by the value in the key field

• The remainder from this division then identifies the address in the file for storage of that record

• The suitable number works best if it is a prime number of a similar size to the expected size of the file

• collision: when the same address is calculated for different field values, it is usually referred to as collision.

• Ways to handle collisions
  -Use a sequential search to look for a vacant address following the calculated one.
  -Keep a number of overflow addresses at the end of the file.
  -Have a linked list accessible from each address