13 Data Representation

Define UDT [3]

• to create a new data type

• to allow construction of data types not available in a programming language

• constructed by the programmer

Explain Composite data types [3]

• collection of data that consists of elements of the same or different data types grouped under a singular identifier

• can be user-defined or primitive

• includes records, sets, class/object

Explain non-composite data types and give the 2 types [4]

• can be defined w/out referencing another data type

• contains only one data type in their definition (can only hold one kind of data)

• can be user-defined or a primitive data type

• enumerated data types and pointer data types

Explain enumerated data types and give an example

• has an ordered list of possible values

TYPE Tratings = (worst, bad, ok, good, great)

DECLARE RatingOne, RatingTwo: Tratings

RatingOne ← worst

RatingTwo ← RatingOne + 1

Explain pointer data types and give an example [4]

• used to reference a memory location

• stores addresses/ memory locations

• indicates the type of data stored in the memory location

TYPE Tratingpointer = ^Tratings

DECLARE ratingpointer = Tratingpointer

ratingpointer ← ^RatingOne

State the 3 file organisation methods

Serial Files, Sequential Files, Random Filesand Indexed Files.

Explain serial files [3]

• records are stored and accessed one after another

• new records are added in the next available space — records are appended to file

  • when searching — every record needs to be checked until the record is found or all has been checked

Explain sequential files [5]

• files are stored and addressed one after another

• a new version of the file has to be created to update it

• files are stored with ordered records — records are stored in order of the key field

• new records are inserted in the correct position

  • when searching — the key field is compared and every record is checked until it is found, or the key field of a current record is greater than the one being searched for

Explain random fields [3]

• records are stored in no particular order within the file (there is no sequencing)

• there is a relationship between the record key and its location within the file — the location of the record is found using a hashing algorithm

  • updates to the file can be carried out directly

What are the 2 types of file accessing methods and which file organization applies to them

Sequential Access Process and Direct Access

Explain Sequential access process [3]

• records are checked linearly until desired record is found/ EOF reached

• starts searching for records one after the other from the start of file until record found/ EOF reached

  • most suitable when data is stored in a certain order based on a field — eg. bank stored data records in ascending order of account number

Explain Direct file accessing and explain respectively for the 2 types of files it applies to [5]

• most suitable when a record is referenced by a unique address

• allows a record to be found in a file w/out other records being read — records are found by using the key field of the target record (the record’s location is found using hashing algorithm)

sequential files

• an index of all key fields is kept — the index is searched for the address of the file location where the target record is stored

random files

• a hashing algorithm is used on the key field of the record to calculate address of the memory location where the target record is expected to be stored

  • linear probing or search overflow can be used to find a record if it is not a expected location

enumerated data type

self defined data types

pointer data type

used to find addresses of data