Chapter 13.1: User-defined Data Types

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Purpose of user-defined data types:

• To create new data type (from existing data types)
• To allow data types not available in a programming language to be constructed // To extend the flexibility of programming language

Why user-defined data types are necessary:

• No suitable data type is provided by the language used
• The programmer needs to specify a new data type
• … that meets the requirement of the application

State what is meant by user-defined data types:

• Derived from one or more existing data types
• Used to extend the built-in data types
• Creates data types specific to applications

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Non-composite Data Types

Define non-composite data types

• A single data type that does not involve a reference to another type / usually built in to a programming language

Give examples of non-composite data type:

• Integer
  • Stores as a whole number
• Boolean
  • Stores true or false
• Real/double/float/decimal
  • Stores decimal numbers
• String
  • Stores zero or more characters
• Char
  • Stores a single character
• Pointer
  - Whole number used to reference a memory location

Enumerated

This is a data type used to store constant values. It is a list of possible values.

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⚠: Enumerated 里面不用加引号'' 或双引号""

⚠:The values defined in an enumerated data type are ordinal. This mean that enumerated data types have an implied order or values.

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Pointer☝

A user-defined non-composite data type referencing a memory location is called a pointer

• Define: TYPE <pointer> = ^<Typename>
• Declaration: DECLARE pointerVar : <pointer>
• Reference: pointerVar <- ^variable
• Dereference: pointerVar^

The pointer should be defined as follows:

TYPE <pointer> = ^<Typename>
//For example:
TYPE IntPointer = ^INTEGER
DECLARE number: INTEGER
DECLARE numberLocation: IntPointer
numberLocation <- ^number //reference

Combining enumerated data type and pointer data type:

TYPE season = (Spring,Summer,Autumn,Winter)
TYPE seasonPointer = ^Season
DECLARE currentSeason: Season
currentSeason <- Spring
DECLARE mySeason: seasonPointer
mySeason <- ^currentSeason //reference
OUTPUT mySeason^ //dereference: output will be Spring

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Composite Data Types

Define composite data type:

• Data type constructed from other data types

Give examples of composite data types:

• Array
  • Indexed collection of items with the same data type
• List
  • Indexed collection of items that can have different data type.
• Record
  • Collection of related items which may have different data types
• Set
  • Stores a finite numbers of different values that have no order // Support mathematical operation
• Class
• Stack
• Queue
• Linked List
• Dictionary

Set🔄

A data type to create sets and apply the mathematical operations defined in set theory

• Contains a collection of data values
• There is no organization of data values within the set
• Duplicate values are now allowed

TYPE letter = SET OF CHAR
DECLARE myname ('s','a','m','u','e','l'): letter

Supported operations:

• Check if a value exists in a set
• Adding a new data in set
• Delete a data in set
• Union two sets

Record⏺

TYPE TbookRecord
  DECLARE title: STRING
  DECLARE author: STRING
  DECLARE publisher: STRING
  DECLARE noPages: STRING
  DECLARE isFiction: TRUE
ENDTYPE

Class/object🏛

A template from which all objects are based

Gives the properties and methods for an object

• Attributes: Properties/fields of an object
• Methods: Modules attached to objects to allow it to perform certain actions

Use of User-defined Data Types

Past paper questions

Write pseudocode to create an enumerated type called Parts to include these parts sold in a computer shop:

Monitor, CPU, SSD, HDD, LaserPrinter, Keyboard, Mouse

TYPE Parts = (Monitor, CPU, SSD, HDD, LaserPrinter, Keyboard, Mouse)

Write pseudocode to create a pointer type called SelectParts that will reference the memory location in which the current part name is stored.

TYPE SelectParts = ^Parts