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Chapter Four Data Types

Chapter Four Data Types. Pratt. Data Objects. A run-time grouping of one or more pieces of data in a virtual machine a container for data it can be system defined programmer defined. Attributes and Bindings. Type Location Value Name. Data Types.

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Chapter Four Data Types

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  1. Chapter Four Data Types Pratt

  2. Data Objects • A run-time grouping of one or more pieces of data in a virtual machine • a container for data • it can be • system defined • programmer defined

  3. Attributes and Bindings • Type • Location • Value • Name

  4. Data Types • A data type is a class of data objects together with a set of operations for creating and manipulating them. • Specification of a data type: • attributes • valid values • valid operations • example: specification of an array

  5. Data Types • Implementation of a data type • storage representation of data object • algorithms of valid operations • Syntactic representation

  6. Elementary Data Types • Elementary data object contains a single data value. • A class of such data objects and the valid operations: elementary data type.

  7. Operations • Signature of an operation: op name: arg type * arg type * … *arg type --> result type

  8. Operations as Mathematical Functions • Undefined for certain inputs. • Underflow , overflow • Implicit arguments. • Side effects (implicit results). • Self-modification (history sensitive)

  9. Implementation • Storage representation. • Attributes: • not stored in the runtime storage representation • run time descriptor • implementation of operations

  10. Declarations • Choice of storage representation • Storage management • Polymorphic operations • Type checking

  11. Type Checking • Checking that each operation executed by a program receives the proper number of arguments of the proper data type. • Dynamic type checking: run-time (type tags for data objects) • Static type checking: compile-time

  12. Dynamic Type Checking • Advantage: Flexibility • Disadvantages: • difficult debugging, some paths never checked. • Extra storage for type information during program execution. • Software simulated type checking, reducing speed.

  13. Static Type Checking • Information required: • For each operation, the number, order, and data types of its arguments and results. • For each variable, the type of data object named. • Always A has the same type (a formal parameter). • The type of each constant data object.

  14. Strong Typing. • Detect all type errors statically. • A function f ,with signature f : S --> R , is type safe if execution of f cannot generate a value outside of R . • Type inference. • ML (p.124)

  15. Type Conversion and Coercion • A type mismatch can cause : • error • coercion (implicit type conversion) • type conversion: • conversion-op : type1 --> type2 • coercions if no loss of information. • Widening or promotion • Narrowing

  16. What about Coercion • for dynamic type checking? • for static type checking? ( Code inserted during compilation) (p. 126)

  17. Two Opposed Philosophies • No coercions (Pascal, Ada) • Coercion as a rule (C)

  18. Assignment • Assignment is the basic operation for changing the binding of a value to a data object. • In Pascal: • assignment: integer * integer --> void • In C: • assignment:integer * integer-->integer (p 127)

  19. Initialization • An uninitialized variable: an l-value with no corresponding r-value. • A serious source of programming errors. • Explicit , implicit.

  20. Elementary Data Types • Numeric Data Types • Integers • Subranges • Floating-point Real Numbers • Fixed-point Real Numbers • Enumerations (one of a small number of symbolic values) • Booleans • Characters

  21. Internationalization • Sorting • Case • Scanning direction • Country-specific data format • Country-specific time format

  22. Structured Data Objects and Data Types • Structured data object or data structure: a data object that is constructed as an aggregate of other data objects, called components. • Particular aspects of structured data types: • how to indicate the component data objects of a data structure and their relationships. • storage management.

  23. Specification of data structure types • Number of components. • Type of each component. • Names to be used for selecting components. • Maximum number of components. • Organization of the components.

  24. Number of Components • Fixed size. • Arrays, records , character strings. • Variable size. • Stacks, lists, sets, tables, files, character strings. • Use a pointer data type. • Insert and delete operations.

  25. Type of Each Component • Homogeneous. • Arrays, character strings, sets, files. • Heterogeneous. • Records, lists.

  26. Names to be used for selecting components • Array: an integer subscript or a sequence of subscripts. • Record: a programmer defined identifier. • Stacks and files: ?

  27. Maximum number of components • For a variable size data structure.

  28. Organization of the components • Simple linear sequence . • Vectors, records, strings, stacks, lists, files. • Multidimensional. • Arrays, record, lists.

  29. Operations on Data Structures • Component selection operations. • Random selection • Sequential selection. How you select a component? • Whole-data-structure operations. • Addition(arrays), assignment(records), union(sets). • Insertion/deletion of components. • Creation/deletion of data structures.

  30. Implementation of Data Structure Types Storage Representation : • affected by • efficient selection of components. • efficient overall storage management. • Includes • storage for the components, • an optional descriptor (for the attributes).

  31. Storage Representation • Sequential representation. • Descriptor and components. • Fixed size. • Linked representation. • By pointers. • Variable size.

  32. Implementation of Operations • Sequential representation • base-address-plus-offset using an accessing formula. (p. 146) • Linked representation • following a chain of pointers

  33. Storage Management • Access path : its name, a pointer. • Life time of a data object: binding to a storage location. Two problems: • garbage • dangling references

  34. garbage: all access paths to a data object are destroyed but the data object continues to exist (the binding of data object to storage location has not been broken), • dangling references: an access path that continues to exist after the lifetime of the associated data object. (p. 149)

  35. Type Checking • Existence of a selected component. • Type of a selected component.

  36. Data Structures • Vectors and Arrays • Records • Variant Records • Lists • Character Strings • Pointers • Sets • Files

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