User-defined data types

Built-in data types are the data types that a programming language already has. They can be used straight away in a program, without the programmer having to create them. Every programming language has its own set of built-in data types.

A user-defined data type is a data type that the programmer designs for use within a program, as opposed to a built-in data type. It is built from other data types, which can be built-in or user-defined themselves.

User-defined data types come in two kinds:

• A non-composite data type is defined without reference to another data type. Enumerated and pointer types are non-composite.
• A composite data type is built from other data types: it holds several values, of the same or different types, under one declaration. Record data types, set data types and classes are composite.

An enumerated data type is a non-composite data type defined from an ordered list of values. Variables can be declared with this data type and assigned one of the values in the list.

This pseudocode declares an enumerated data type for months:

TYPE TMonth = (Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec)
// The TMonth data type is declared

DECLARE ThisMonth : TMonth
// A variable is declared with the TMonth enumerated data type

ThisMonth ← May
// The variable ThisMonth is given the value May

ThisMonth > Feb
// This is TRUE, as May is later in the list than Feb

Note The values in an enumerated data type are not strings, so do not put quotation marks around them.

A pointer data type is used to reference a memory location. It has to state the type of data that it points to.

This pseudocode declares a pointer data type and a pointer, and then uses the pointer:

TYPE TMyPointer = ^INTEGER
// TMyPointer is a pointer data type that points to integers

DECLARE IntegerPointer : TMyPointer
// A variable of the TMyPointer data type

DECLARE MyNum, YourNum : INTEGER
MyNum ← 23
// Two integer variables; one of them is given a value

IntegerPointer ← @MyNum
// IntegerPointer now holds the address of MyNum

YourNum ← IntegerPointer^
// Reads the value stored at the address IntegerPointer points to.
// This is called dereferencing.

Pointers are used to build more complex data structures, such as linked lists and binary trees.

A record data type stores a collection of related data about one thing, like a record in a database. It is made of several fields, and each field has its own data type, so a record is a composite data type.

This pseudocode defines a record type for a student and then uses it:

TYPE TStudentRecord
   DECLARE FirstName : STRING
   DECLARE LastName  : STRING
   DECLARE Absences  : INTEGER
   DECLARE Class     : STRING
ENDTYPE

DECLARE Student1 : TStudentRecord
// The variable is declared as a record

Student1.FirstName ← "Zain"
Student1.LastName  ← "Merchant"
Student1.Absences  ← 0
Student1.Class     ← "A2 Level"
// Each field is reached using dot notation

A set data type is a composite data type that lets a programmer use set operations on data. The values in a set have no order and no repeats. The operations usually include:

• union
• difference
• intersection
• including an element
• excluding an element
• checking whether an element is in a set

TYPE LetterSet = SET OF CHAR
// LetterSet is a set data type that holds characters

DEFINE Vowels ('A', 'E', 'I', 'O', 'U') : LetterSet
// Vowels is a set of type LetterSet with these values

An object (or class) data type is a composite data type used in object-oriented programming. A class describes the data and the procedures that act on that data; an object is one instance of a class. In a way, objects are records with their own functions attached.

An exam question may describe a situation and ask you to choose a suitable data type, or to write the pseudocode that declares it. First list the data that has to be stored, then match each part to a type:

• Enumerated: a small, fixed, ordered list of named values, such as the days of the week.
• Record: several related items of different data types that belong together, such as the details of one book.
• Pointer: stores the memory address of another variable or data item, for example to link one node of a linked list to the next.
• Set: a collection of values with no order and no repeats, where you mainly need to check whether a value is in it.