User-Defined Types (Cambridge International A-Level Computer Science)

What are user-defined data types?

• A user-defined data type is a custom data type created by the programmer, built from existing types to match the specific needs of a program
• It is built using primitive data types such as INTEGER, STRING, BOOLEAN, etc

Why are user-defined types necessary?

• Primitive types alone cannot model many real-world entities (e.g. a customer, a date, a linked list)

• User-defined types let you:

  • Group related data under a single identifier (record)
  • Restrict a variable to a fixed set of valid values (enumerated)
  • Build dynamic data structures such as linked lists (pointer)
  • Make code more readable, reusable and easier to validate

Non-composite

• A non-composite data type is defined without referencing any other data type
• It can be a primitive type available in the language, or a user-defined type (enumerated or pointer)

What is an enumerated data type?

• An enumerated data type is a non-composite, user-defined type consisting of a list of all possible values the variable can take
• The values are ordered: they have an implied sequence based on the order they are declared, so comparisons work (e.g. Monday comes before Tuesday)
• Use an enumerated type to restrict a variable to a known, fixed list, improving readability and reducing invalid values

Example code

• Today can only hold one of the seven ordered values in DayType

What is a pointer data type?

• A pointer is a non-composite, user-defined type that stores a memory address only, rather than the data itself
• Its definition also indicates the type of data stored at that memory location
• Pointers are used to build dynamic data structures such as linked lists

• The ^ shows the type is a pointer
• <data type> is the type of data stored at the address

Example code

Composite

• A composite data type is a collection of data made up of multiple elements, of the same or different data types, grouped under a single identifier
• It references other data types in its definition (primitive or other user-defined types)
• The three composite user-defined types you need are record, set and class

What is a record?

• A record is a composite type that groups together a fixed number of related items (fields), which may be of different data types, to form a single new type
• Access a field using dot notation: RecordName.FieldName

Example code

What is a set?

• A set is a composite type holding a list of unordered elements
• All elements must be of the same data type, and the set definition includes the data type it uses
• Set theory operations such as union and intersection can be applied to the elements

Example code

What is a class?

• A class is a composite type that groups together both data (attributes) and the methods that operate on that data, under a single identifier
• It is used as a template to create objects in object-oriented programming
• Full detail on defining and using classes is covered in Object-Oriented Programming

Choosing a user-defined data type

• Exam questions may ask you to choose and design an appropriate type for a problem
• Match the type to what the data looks like:

Exam Hint

• Composite vs non-composite" is about whether the type references other types, not whether it holds one value or many. State this precisely
• For enumerated types, remember to say the values are ordered, that is a mark point
• For a "describe the set/record data type" question, include that it is composite, that it references/uses other data types, and the specific detail (set = unordered, same type, supports union/intersection; record = fixed number of related items)