Programming Language Ch III' Data Types

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Programming LanguageCh III. Data Types

• Internet Management Technology Lab.

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Contents

• Introduction
• Basic Concepts for data
• Elementary Data Type
• Structure Data Type

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Introduction

• Three elementary components in a programming
  language
• Data
• Basic information unit to hold values to
  determine the state of a system
• Operations
• Actions to manipulate data or sequencing
• Control
• A mechanism provided to control the sequence of
  instructions

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Basic Concepts for Data

• Data storage of an actual computer
• Memory, register or external media
• Usually have simple structure as sequence of bits
  comprising bytes or words
• Data storage of a virtual computer
• Arrays, stacks, numbers, character strings
• Usually have more complex organization

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Basic Concepts for Data

• Data Object
• Run-time grouping of one or more pieces of data
  in a virtual computer
• Some are defined by programmer (variables,
  constants, arrays) and others are defined by
  system for house keeping (activation records)
• Container for a data value (a single number,
  character or a memory location)
• Classified as elementary object (manipulated as a
  single unit( or structured object (aggregated)
• Each data object has a lifetime (extent)

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Basic Concepts for Data

• Attributes associated with a data object
• Name
• Represent the object and is referred during
  execution time
• Type
• Specify the data values that the object may
  contain
• Location
• Address of the storage where the object is
  located
• Value
• Actual value that the object contains
• Component
• The binding of a data object to one or more data
  objects
• Operations
• Mechanisms to manipulate the object

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Basic Concepts for Data

• Variable
• A data object that is defined and named by
  programmer explicitly in a program
• The content (value) of a variable can be changed
  during its life time
• Simple variable an elementary data object with
  a name
• Complex variable group of variables
• Constant
• A data object with a name which is bound to a
  value permanently during its lifetime
• Types of constants
• Literal its representation is just the written
  representation of the value
• Programmer defined constant its name is chosen
  by the programmer

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Basic Concepts for Data

• Data type
• A class of data objects together with a set of
  operations for creation and manipulation
• Types are array, integer, file, etc.
• Classes of data type
• Primitive data type built into the language
• User-defined data type defined with the
  facilities that the language provides
• Self-modifying types the content of a data
  object is modified
• Basic elements of a data type include
  specification and implementation
• Specification specifying the defined data type
  as an object with attributes, values and
  operations
• Implementation simulation of that part of the
  virtual computer with the basic elements

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Basic Concepts for Data

• Data type (contd)
• Specification
• Attributes that distinguish data objects of that
  type
• Number of dimensions, subject range, data type of
  components
• Values that data objects of that type may have
• Set of numbers
• Operations that define the possible manipulations
  of data objects that type
• e.g. consider array data type
• Subscribing to select a component, create array,
  change shape, access attributes, and perform
  arithmetic on a pair of array
• Implementation
• Storage representation used to represent the
  data object of that type
• Manner in which the operations are defined to
  manipulate the data object (algorithm or
  procedures)

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Elementary Data Type

• What is the Elementary Data Type?
• It contains a single data value, with various
  operations
• Integer, Real, Character, Boolean, Enumeration,
  Pointer
• Specification of Elementary Data Types
• Attributes
• Basic attributes (name, data, type) are invariant
  during its life time
• Attribute information could be stored in dove
  vector (descriptor) or only used to determine
  storage size
• Value of an attribute of a data object
• Not equals to the value of a data object contains
• Values
• Type of an object determines the set of possible
  data values
• Usually closely related to the values that the
  underlying hardware provides
• The set of values for a data type is usually an
  ordered set

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Elementary Data Type

• Specification of Elementary Data Types (contd)
• Operations
• Determine how data objects of that type may be
  manipulated
• Primitive operation part of language definition
• Programmer defined operation form of
  subprograms, or method declaration
• Elements of an operation
• Domain a set of possible input values
• Range a set of possible result values
• Action determines results from any given set of
  input values
• Signature (called prototype in C)
• Is used to specify an operations elements
• opname arg_type x arg_type x x arg_type ?
  result_type
• Ex) integer x integer ?integer (binary
  operation)
• SQRT real ? real (unary operation)

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Elementary Data Type

• Specification of Elementary Data Types (contd)
• Four main factors that combine to obscure the
  definitions of operations
• Operations undefined for certain inputs
• Implicit arguments use of global variable
• Side effects (implicit results) change global
  variable
• Self-modification history sensitive actions
  like counter, random number generator, LISP
  allows self modification through code change
• Subtype
• A data type which is a part of a larger class
  (superclass)
• Ex) type smallInt 1..20

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Elementary Data Type

• Implementation of elementary data types
• Storage representation for data objects and
  values of that type
• Strongly influenced on underlying hardware
  (efficient)
• Sometimes it is software simulated (inefficient)
• Data attributes are determined by compiler in
  many languages or
• Sometimes data attributes are stored in a
  descriptor vector (e.g. LISP)
• Set of algorithms or procedures that defines the
  operations of the type
• Directly implemented by hardware or
• As a procedure or function subprogram
• As in-line code sequence
• abs(x) if x lt 0 then -x else x

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Elementary Data Type

• Declarations
• Definition
• Statements that specify information about the
  name, type of data object, and lifetime of each
  object
• Explicit declaration vs. implicit (default)
  declaration
• Declaration of operations can be done by the
  signature of each operation
• Explicit declaration of arguments and result type
  of programmer defined operations
• Purpose of declaration
• Choice of storage representation for translator
• Storage management by specifying the life time of
  variables
• Polymorphic operations
• Type checking

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Elementary Data Type

• Type Checking
• Checking the proper number of arguments of the
  proper data type of each operations
• Dynamic type checking
• Difficult to debug
• Need storage to keep type information
• Slow execution speed for type checking
• Limit the compiler optimization due to unknown
  factors
• flexibility
• Static type checking
• Operation checking number and types of
  arguments and results
• Variable checking type of object under the name
• Type of constant syntactic form of literals
• Strong typing
• Detect all type errors statically

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Elementary Data Type

• Type safe system
• A function cannot generate result with a type
  outside of the signature
• Type inference
• Types can be resolved from the program by how
  they are used
• Ex) fun area (lengthint, widthint)int length
  width in ML
• Only illegal example fun area (length, width)
  length width
• Type conversion
• Takes one type and produce the corresponding type
• Explicit type conversion by using a set of
  built-in functions
• Implicit type conversion (type coercion) as
  specified in the language
• Basic principle is not to lose information
  called widening, or promotion

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Elementary Data Type

• Polymorphic operations
• Polymorphic function a subprogram or function
  that can assume more than one type
• Ex) f(x) x in PASCAL
• function f(xint) int
• function f(xboolean) boolean
• function f(xreal) real
• Ad hoc polymorphism
• Different code for different manifestations of
  the operators
• Overloading
• Implicit coercion
• Universal polymorphism
• A function name selects on a variety of
  implementations depending on the types of its
  arguments

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Elementary Data Type

• Assignment
• Basic operation for changing the binding of a
  value to a data object
• Forms of Assignment Statements
• Schizophrenic Representation
• lvalue (location attribute) and rvalue (value
  attribute)
• Ex) X X 1

integer x integer -gt void (PASCAL)
integer x integer -gt integer
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Elementary Data Type

• Assignment
• Variation on assignment
• The updating assignment
• Multiple target assignment
• Multiple assignment statement

L1, L2, L3, ..., Ln E ALGOL60, PL/1
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Elementary Data Type

• Storage models
• Value semantics assigned to immutable value
• Pointer semantics assign the location
• Storage insecurities
• Use before initialization
• Dangling reference

A 2 B 3 A B B 5 print A
var p, q T Begin new(p) q p
dispose(p) end
var p integer procedure q var i
integer Begin p ADDR(i) end access using
p
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Elementary Data Type

• Initialization
• Must be explicit
• Must be done at object creation
• Initialized by default
• Equality and Equivalence

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Elementary Data Type

• Integers
• Specification
• Attribute type attribute integer only
• Value ordered and finite subset of integer
  values
• Operations
• Arithmetic operations BinaryOp (, - , x, /,
  mod), UnaryOp (-, , abs)
• Relational operations (equal, not equal,
  less-then...)
• Assignment operations ( , )
• Bit operations (, , ltlt, gtgt)
• Implementation
• Use complete memory word
• Three possible storage representation
• No runtime descriptor
• Descriptor stored in separate words (LISP)
• Descriptor stored in the same word

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Elementary Data Type

• Subranges
• Specification
• Attribute subtype of a type
• Value a sequence of integer values within some
  restricted values
• Operations same operations with the integer
• Implementation
• Same as the base type but has a few effects
• Smaller storage requirements
• Range 110 requires only 4 bits
• Better type checking

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Elementary Data Type

• Floating point real numbers
• Specification
• Attribute type attribute real
• Value hardware determined numbers from min to
  max, and not every distributed
• Operations same operations with integer except
  boolean that has some restrictions()
• Built-in functions
• sin real ? real
• max real x real ? real
• Implementation
• Split the storage location into mantissa and
  exponent

ltlt Excess 127 notation gtgt
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Elementary Data Type

• Fixed-point real numbers
• Specification
• Digit sequence of fixed length
• Avoid round-off errors (e.g. dollar and cents)
• Ex) in COBOL X PICTURE 999V99.
• Implementation
• Either supported by hardware or simulated by
  software
• Ex)
• Calculation is done after converting the numbers
  in the same format

Scale factor
Y(SF2) X(SF3) Y(SF2) ((X(SF3) (10
Y)(SF3)) /10)(SF2)
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Elementary Data Type

• Enumeration
• Specification
• Allow a programmer to define and manipulate
  subranged variables more directly
• Ex)
• Implementation
• Allocated within the minimum number of bits
  needed
• Each entry is numbered with 1,2,3,..and simple
  operations on the index numbers are used
• Use primitive operations of the superset

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Elementary Data Type

• Booleans
• Specification
• Attribute type Boolean false, true in
  PASCAL, ADA
• And, or, and not operations
• Implementation
• Single bit of storage
• Could be word or byte due to the addressing
  problem
• A designated bit is used and the rest are ignored
• Entire word (or byte) has 0 for false, else for
  true

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Elementary Data Type

• Characters
• Sequence of characters are often processes as a
  unit
• Specification
• Value set of possible character values that are
  language-defined enumeration, and supported by
  underlying hardware
• Operation assign, relational, test special type
  of characters (letter, digit, special characters)
• Implementation
• Directly supported by underlying hardware
• Converted from input-output system to the
  character-set representation

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Structured data types

• Structured data Objects
• Aggregate data object
• Component
• Elementary object
• Another data structure
• Issues about data structures
• Maintaining specification of the structural
  information
• Indicating the component data objects
• Managing the storage

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Structured data types

• Specification of structured data types
• Major attributes
• Number of components
• Fixed size array, record
• Variable size lists, stacks, files, sets
• Type of each component
• Homogeneous - array
• Heterogeneous - record
• Names to be used for selecting components
• Homogeneous type may use integer subscript or
  sequence of subscripts
• Heterogeneous type use programmer defined
  identifier
• Maximum number of components
• Only for the variable size data structure
• Organization of the components
• Simple linear sequence is the most common
  organization
• Multi-dimension forms are used to extend linear
  sequences

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Structured data types

• Specification of structured data types
  (continued)
• New classes of operations on data structures
• Component selection operations
• Random selection
• Sequential selection
• Whole-data-structure operations
• Addition of two arrays
• Assignment of one records to another
• Union operation on sets
• Insertion/Deletion
• Storage representation and management problems
  related
• Creation/destruction of data structure
• Storage management problems related

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Structured data types

• Implementation of data structure types
• Storage representation
• Storage for data optional descriptor
• Usually S/W simulated
• basic representation
• Sequential representation
• linked representation
• Implementation of operation on data structures
• Sequential representation
• Using accessing formula to locate the component
• Location base address offset
• e.g. A10 lvalue(A) 10 sizeof(A0)
  (homogeneous)
• Linked representation
• Following a chain of pointers

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Structured data types

• Implementation of data structure types
• Type checking issues
• Existence of a selected component
• Type of a selected components
• Name equivalence VS structural equivalence
• Name equivalence
• Same name
• structural equivalence
• Same structure

e.g. in Ada type BLACK is INTEGER type WHITE is
INTEGER B BLCAK WWHITE IINTEGER begin
W5 BW I B end
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Structured data types

• Vectors
• linear arrays
• Composed of a finite number of homogeneous
  components
• Selected by using its subscript
• Attributes of a vector
• Number of components
• Data type
• Subscripts to be used to select each component
• Could be an integer value or enumerated value

float A10 /in C / V
array 1 .. 10 of real / in PASCAL / type
class (Freshman Sophomore, Junior, Senior) var
ClassAverage arrayclass of real / in
Pascal /
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Structured data types

• Vectors (continued)
• Operations of vectors
• Subscripting
• Creation
• Destroying
• Assigning a value to a component
• Arithmetic operation on two vectors
• implementation
• For efficiency
• Homogeneity
• Fixed size

Insertion or deletion are not available
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Structured data types

• Vectors (continued)
• Implementation (continued)

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Structured data types

• Vectors (continued)
• Implementation (continued)
• Accessing formula
• Packed storage representation
• Create issues like boundary crossing
• Whole vector operation

Where a is the base location If subscript does
not start from 0, then compute the virtual
origin(may not exist) VO (a LB E)
Lvalue(ai) a (I LB) E (a LB E)
(I E)
Struct a char str3 int
on_boundary
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Structured data types

• Multi-dimensional arrays
• Specification
• Extension of vector array with multiple
  subscripts
• Implementation
• A vector of vectors
• Storage representation

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Structured data types

• Multi-dimensional arrays
• Implementation (continued)
• Row-major order
• Column-major order

lvalue(AI,J) a (I - LB1) x S (J - LB2) x
E Where, a base address S length of a row
(UB2 - LB2 1) x E VO a - LB1 x S - LB2 x
E lvalue(AI,J VO I x S J x E
lvalue(AI,J VO' I x E' J x S Where, VO
a - LB1 x E' - LB2 x S
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Structured data types

• Slice
• Specification
• A substructure of an array that it itself an
  array
• Implementation
• 3 by 4 array

e.g. in FORTRAN77 Pass A(1,3) to B(1) A(1,3)
B(1), A(2,3) B(2), A(3,3) B(3) and so on.
lvalue(AI,J)   a I x 3 J x 1
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Structured data types

• Records
• A data structure of a fixed number of components
  of different typed
• Specification
• The attribute of components
• The number of components
• The data type of each component
• A name typed literally to select a component
• Operations on entire structure is not common

/ records in C / struct EmployeeType int
ID int Age float Salary char Dept

• Array is a subtype of Record
• Record may have heterogeneous type of
  components
• Components may be distinguished by a
  name(identifier) rather than subscripts

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Structured data types

• Records (continued)
• Implementation
• Mapping into an offset
• the distance from the base
• Padding occurs for data alignment

struct Class int ClassName char Class
int classID
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Structured data types

• Variant Records
• Union type in C
• Implementation
• Allocate largest storage for the variant record
• Dynamic checking is provided for range error

type PayClass (Salaried, Hourly) Var
HourlyEmployee record ID integer Dept
array1..3 of char Age integer case
PayClass PayType of Salaried
(MonthlyRate real StartDAte
integer) Hourly (HourlyRate real
Reg integer Overtime integer) end
/ records in C / Union EmployeeTemp int
ID int Age float Salary char Dept
PayType is called tag in PASCAL Or discriminant
in Ada
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Structured data types

• Lists
• Similar to vectors except
• Variable length
• Heterogeneous components
• Usually implicit declaration is used
• Operation
• Selector, Insert, Append, Delete, etc.
• Variation of lists
• Stack, queue, tree, etc.

e.g. in LISP (cons '(a b c) '(d e f)) ((a b c)
d e f) - each element is called an atom
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Structured data types

• Character strings
• Specification
• Operation
• Concatenation
• Relational operations on string
• Substring selection
• I/O formatting
• Implementation
• S/W simulated for the part that H/W does not
  support
• Implementation manner
• Fixed declared length
• Variable length to a declared bound
• Unbounded length
• C uses a null terminator to indicate the end of a
  string

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Structured data types

• Pointers
• A object that contains the location of another
  data object
• Its rvalue is lvalue of another object
• Specification
• Pointers may reference data object of a single
  type
• Pointers may reference data object of any type
• Operations
• Creation
• Dereferencing
• Implementation
• Storage representation
• Absolute address
• Relative address
• Issues
• Garbage
• dangling reference
• optimization

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Structured data types

• Sets
• Containing unordered collection of distinct
  values
• Operations
• Membership
• Insertion/deletion
• Union
• Intersection
• And/or
• Implementation
• Bit string representation
• Hash-coded representation