Organisation And Structure Of Data

Question 1

File Headers

Answer 1

Store data within a file, about the file.

.txt in windows has no info. Jpeg files describe how an image should be displayed

Question 2

Fixed Length fields and records
2 points
Advantages & Disadvantages
Example

Answer 2

• same length
• some fields may be ‘padded out’ with spaces to make it the right length
Advantage
• Fast access because the location of each record can be quickly calculated.
• Quck to process all data
Disadvantage
•Usually larger files, need more storage space and are slower to transfer (load/save/transmit down network)
Eg
Information retrieval system
Train time-table program

Question 3

Variable Length Files
1 point
Ad & Dis
Eg

Answer 3

• One or more fields can be differing length
• have terminator byte at end of each field and record
Ad
•Less storage space
• Faster to transmit /load/save
Dis
• Processing will be slower as each start and end point of each field needs to be calculated
Eg
•where download speed would be an issue
Sending list of customer details between 2 company offices

Question 4

Serial File

Answer 4

Records stored one after another in no particular order

Question 5

Sequential File

Answer 5

Records stored one after another and accessed in a key sequence order
Eg register ordered by surname

Question 6

Indexed sequential

Answer 6

Records stored in key order

Index allows data to be accessed directly

Question 7

Random Access

Answer 7

Hashing algorithm is used on a key field to calculate the location for the record in the file

Question 8

Random access file

Answer 8

Consist of fixed length records ordered by their relative address
• can be read in sequential order
• accessed using the relative key. ( represents the location of the record relative to the start address)
• records inserted using relative key. Relative address calculated using relative key
Ad
• fastest access to records
Dis
•if intermediate records are missing, they will occupy space

Question 9

Linear probing

For dealing with person with the same last name as exsisting record.

Answer 9

Next available space is used to store the record of person with the same last name

Question 10

Daisy Chaining

Answer 10

Put record in same memory loaction with a pointer that points to the next person

Question 11

Good hashing algorithm

Answer 11

• produces unique values
• wide spread of values
• fast calculation
• collisions

Question 12

How hashing algorithm deletes records

Answer 12

Logically not physically

Pointers the point to the part of the disk are removed.

Question 13

how hashing algorithm updates records

Answer 13

Calculate location of Harry’s

overwrite file in situ