AS6

Question 1

Purpose and Structure of Files

Answer 1

• Files enable data to persist in storage between executions of a program.
• One file contains many records.
• One record is structured as multiple fields.

Question 2

Fixed Length Record

Answer 2

• Every record has to be the same size, so the programmer needs to define the size at the start. (Think about max number of characters, not average).

Question 3

Variable Records

Answer 3

• Has a different number of bytes in each record.
• More difficult to calculate how much space required.
• Slower to process by a computer as start/end locations need to be calculated at read/write time.
• Saves storage space as no blank spaces are left.
• Each field can extend to accommodate characters.

Question 4

Estimating File Size

Answer 4

• Singular record: Adding together the size of the individual fields.
• Final size: Multiply by number of records + 10%. (for metadata)

Question 5

Sizes of Types

Answer 5

• Byte = 1B
• Short = 2B
• Int = 4B
• Long = 8B
• Float = 4B
• Double = 8B
• Boolean = 1B
• Single ASCII character = 1B
• Single Unicode character = 2B/4B
• N characters = NB (ASCII)/2NB(Unicode)

Question 6

File Access

Answer 6

• Normally provided by the core language or imported libraries.
• Files are processed by being read line by line. The file handler will contain a pointer to tell the handler where the last read/write operation occurred up until it reached an End Of File designator.
• Normal practice to close files once finished with them.

Question 7

Serial Files

Answer 7

• New records being appended to back of file is easiest storing method. Important that file is opened in append mode.
• Normally stored in chronological order.
• Serial files are used when records are not required to be in order, the number of records is small, or the data must stored in the order it arrived.

Question 8

Sequential File Access

Answer 8

• Order records based on primary key field.
• One field will determine if order is ascending or descending. Makes searching for a record easier.
• A requirement for implementing an index.
• Inserting into a sequential file is more complex than appending to a serial file. Necessary to find correct position in the file to insert it.
• Not possible to move records to make room for a new one, a temp file copies all records before, the new record, and then records after. Replaces original file.

Question 9

Indexed Sequential File Access

Answer 9

• Contains an index that makes accessing record groups faster.
• Locates records in logical chunks e.g. starting with same letter.
• Speeds up searches, slows down insertion/deletion. When a record is added, the start and finish points of the index will change the size of one record.
• Access can still be slow for large amounts of records.

Question 10

Direct (random) File Access

Answer 10

• Jumping to a record without needed to perform a search/use an index.
• Can be implemented using a hashing algorithm (primary key taken and used to produce file position).
• DA files are split into blocks which contain a fixed number of records. The file will take up the same amount of space no matter amount of stored records.

Question 11

Overflow

Answer 11

• A collision is when a hashing algorithm returns the same block position for two keys.
• Each block has max record count, so it can fill up.
• An overflow file can allow for excess records.
• Creating a new file containing more blocks and re-hashing every record the the new file fixes this.

Question 12

Overflow v. File Re-Organisation

Answer 12

• Overflow area is lower in cost than a new file, can slow down searches.
• When a search reaches the end of the block, overflow will also be checked. Because both areas work in the same way, the file will be less efficient.
• Creating a new file and re-hashing could be expensive. This is used when the load factor is too high.
• Selecting a larger block size reduces the need.

Question 13

Generations of Files

Answer 13

• File backups can be created in a structure to allow a series of steps to be taken so that the system will be restored in steps depending on when the loss occurred.

Question 14

Transaction Logs

Answer 14

• Maintain the security of the system by giving accountability/ allowing file changes to be tracked.

Question 15

Master and Transaction Files

Question 16

Field

Answer 16

• To store data, it needs to be organised in fields.
• Each field represents a piece of data.