Classification and distinctive features of programming languages

1
Classification and distinctive features of
programming languages

• procedural
• declarative
• object-oriented
• visual
• query

2
Imperative language

• In an imperative language, an expression is
  evaluated and the result is stored in a memory
  location, which is represented as a variable in a
  program.

3
Functional language

• It provides a set of primitive functions, a set
  of functional forms to construct complex
  functions from those primitive functions, a
  function application operation, and some
  structure or structures for storing data.
• The objective of the design of a functional
  programming language is to mimic mathematical
  functions to the greatest extent possible.

4
Purely functional language

• A purely functional programming language does not
  use variables or assignment statements. This free
  the programmer from concerns about the memory
  cells of the computer on which the program is
  executed.
• Programs are function definitions and function
  application specifications, and executions
  consist of evaluating the function applications.

5
Procedural

• Programming in both imperative and functional
  languages is primarily procedural, which means
  that the programmer knows what is to be
  accomplished by a program and instructs the
  computer on exactly how the computation is to be
  done.

6
Declarative Language

• Programming that uses a form of symbolic logic as
  a programming language is often called logic
  programming.
• Languages based on symbolic logic are called
  logic programming languages or declarative
  languages.
• e.g. Prolog

7
Object-oriented programming (1)

• Data-oriented programming focuses on abstract
  data types.
• An abstract data type is defined as a data
  structure in which the data and its operations
  are defined together in a single syntactic unit.

8
Object-oriented programming (2)

• Focusing on data is the opposite from the
  traditional procedural programming approach that
  focuses on processes and their implementation in
  subprograms.

9
Object-oriented programming (3)

• One restriction of abstract data types is that,
  once defined, they cannot be conveniently
  modified for slightly different applications.
• There is no way to collect the common
  characteristics of families of closely related
  types.

10
Object-oriented programming (4)

• Objects have local memory, inherent processing
  ability, the capability to communicate with other
  objects, and the ability to inherit
  characteristics from ancestors.
• Objects do not have names they are anonymous and
  can be referenced only by pointers.

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Object-oriented programming (5)

• Objects communicate with other objects by sending
  messages.
• A message is used by an object to request an
  operation that another object (or itself)
  provides.

12
Object-oriented programming (6)

• The operation definitions in objects are called
  methods.
• A method specifies the reaction of the object
  when it receives a message that is directed to
  that method.
• The binding of a message to a method is dynamic.

13
Object-oriented programming (7)

• The common characteristics of a category of
  objects is captured by a class definition.
• Instances of the class can be created and are the
  objects of the program.
• Each object has its own local data and represents
  a different instance of its class.
• The only difference between two objects of the
  same class is the state of their local variables.

14
Procedural languages

• (C, Pascal)

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Data Types

• Nearly all programming languages provide a set of
  primitive data types. Some of the primitive types
  are merely reflections of the hardware for
  example, integer types two's complement
  floating-point types format of IEEE single or
  double precisions. Before a variable can be
  referenced in a program, it must be bound to a
  data type, for example, a 2-bytes integer types
  bind a variable to a range between -32768 and
  32767.

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Simple Data Type (1)

• Nearly all programming languages provide a set of
  simple data type.
• Some of these data types are merely reflections
  of the hardware (e.g. integer type).
• Others require only a little support for their
  implementation. (e.g. IEEE floating point format).

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Simple Data Type (2)

• single items (numbers, characters etc) that are
  associated with single identifiers on a
  one-to-one basis.
• examples are integer, character, boolean,
  floating-point numbers.

18
User-Defined Data Type

• An ordinal type is one in which the range of
  possible values can be easily associated with a
  set of positive integers.
• In many languages, users can define two kinds of
  ordinal types enumeration and subrange.

19
Enumeration Types

• An enumeration type is one in which all of the
  possible values, which are symbolic constants,
  are enumerated in the definition.
• Enumeration types provide greater readability in
  a very direct way.
• Named values are easily recognized, whereas coded
  values are not.
• Coding are meaningless to everyone except the
  program author.

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Pascal Example

• type colortype (red, blue, green, yellow)
• var color colortype
• .
• color blue
• if (color gt red)
• ...

21
In Pascal,

• a literal constant is not allowed to be used in
  more than one enumeration type definition in a
  given referencing environment.
• Enumeration type variables can be used as array
  subscripts, for loop variables, and case selector
  expressions, but can be neither input nor output.
• Two enumeration type variables and / or literal
  can be compared with the relational operators,
  with their relative positions in the declaration
  determining the result.

22
Subrange Types (1)

• A subrange type is a contiguous sub-sequence of
  an ordinal type.
• e.g. 12..18 is a subrange of integer type.
• Pascal Example
• type
• uppercase A..Z
• index 1..100

23
Subrange Types (2)

• Subrange types enhance readability by making it
  clear to readers that variables of subtypes can
  store only certain ranges of values.
• Reliability is increased with subrange types,
  because assigning a value to a subrange variable
  that is outside the specified range is detected
  as an error by the run-time system.

24
Structured Data Type (1)

• multiple data items are related to one another in
  some specified manner.
• each group of data items is associated with a
  particular identifier.
• individual data items within each group can also
  be associated with a particular identifier. e.g.
  index using in array field name using in record.

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Structured Data Type (2)

• Array Types
• Record Types
• Union Types
• Set Types
• Pointer Types

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Array Types (1)

• An array is a homogeneous aggregate of data
  elements in which an individual element is
  identified by its position in the aggregate,
  relative to the first element.
• A majority of computer programs need to model
  collections of objects, in which the objects are
  of the same type and must be processed in the
  same way.

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Array Types (2)

• Design issues specific to arrays are the
  following
• What types are legal for subscripts ?
• When are subscript ranges bound ?
• When does array allocation take place ?
• How many subscripts are allowed ?
• Can arrays be initialized when they have their
  storage allocated ?

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Array Type (3)

• C allows initialization of its static arrays.
• e.g. int list 4 , 5 , 7 , 83
• The compiler sets the length of the array.
• Character strings in C are implemented as arrays
  of char.
• e.g. char name freddie

29
Record Types

• A record is a possibly heterogeneous aggregate of
  data elements in which the individual elements
  are identified by names.
• The record elements (fields) are named with
  identifiers, and references are made using these
  identifiers.

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Union Types

• A union is a type that is allowed to store
  different type values at different times during
  program execution.
• The same location (a table field) could store a
  value of different types.
• If type checking is required, it must be dynamic.

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Pascal Union Types (1)

• In Pascal, it is called record variant.
• e.g.
• type shape (circle, triangle, rectangle)
• object record
• case form shape of
• circle (diameter real)
• triangle (leftside
  integer rigthside integer angle real)
• rectangle (side1
  integer side2 integer)
• end
• var thing object

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Pascal Union Types (2)

• case thing.form of
• circle writeln(It is a circle its
  diameter , thing.diameter)
• triangle begin
• writeln(It is a
  triangle)
• writeln(its sides are,
  thing.leftside, thing.rightside)
• writeln(angle between the
  sides is, thing.angle)
• end
• rectangle begin
• writeln(It is a
  rectangle)
• writeln(its sides are
  , thing.side1, thing.side2)
• end
• end

33
Pascal Union Types (3)
34
Set Types (1)

• A set type is one whose variables can store
  unordered collections of distinct values from
  some ordinal type called its base type.
• Set types are often used to model mathematical
  sets.
• e.g. text analysis often requires that small sets
  of characters, such as punctuation characters or
  vowels, be stored and conveniently searched.

35
Set Types (2)

• A variable of a set type can store any subset of
  the range of the base type.
• The set of all subsets of a given base set is
  often called the powerset of that base set.

36
Sets in Pascal (1)

• The maximum size of Pascal base sets is
  implementation dependent.
• Many implementations severely restrict it, often
  to much less than 100.
• They do so because sets and their operations are
  most efficiently implemented by representing set
  variables as bit strings that fit into a single
  machine word.

37
Sets in Pascal (2)

• Pascal allows the following set operations
• assignment of compatible set types
• set union
• set intersection
• - set difference
• set equality
• lt gt set inequality
• lt set included in (subset)
• gt set includes (superset)
• in element inclusion in a set

38
Sets in Pascal (3)

• e.g.
• type colors (red, blue, green, yellow, orange,
  white)
• colorset set of colors
• var set1, set2, set3 colorset
• set1 red, blue, yellow, white
• set2 blue, yellow

39
Sets in Pascal (4)

• set3 set1 set2 set3 is now red, blue,
  yellow, white
• set3 set1 set2 set3 is now blue, yellow
  
• set3 set1 - set2 set2 is now red, white
• set1 set2 is false
• set1 lt gt set2 is true
• set1 lt set2 is false
• set1 gt set2 is true
• red in set1 is true
• red in set2 is false

40
Pointer Types (1)

• A pointer type is one in which the variables have
  a range of values that consists of memory
  addresses and a special value, nil.
• The value nil is not a valid address and is used
  to indicate that a pointer cannot currently be
  used to reference another object.

41
Pointer Types (2)

• Two distinct kinds of uses.
• Pointers provide some of the power of indirect
  addressing
• Pointers provide a method of dynamic storage
  management.
• A pointer access to a location in the area where
  storage is dynamically allocated is called a heap.

42
Heap

• Complex organization occur because of randomly
  ordered dynamic allocations and deallocations.
• Variables are dynamically allocated from heap.
• No identifier associated with and can be
  referenced only by pointer variables called
  anonymous variables.

43
Design Issues of Pointer

• What are the scope and lifetime of a pointer
  variable?
• What is the lifetime of a dynamic variable?
• Are pointers restricted as to the type of object
  to which they can point?

44
Variables

• an abstraction of a computer memory cell, or a
  collection of cells.
• translator converts the names of variables to
  actual addresses.
• attributes of variables name, address, value,
  type, lifetime, scope.

45
Keywords

• Special words in programming languages are used
  to make programs more readable by naming actions
  to be performed.
• A keyword is a word of a programming language
  that is special only in certain contexts.
• A reserved word is a special word of a
  programming language that cannot be used as a
  name.

46
Aliases

• Multiple identifiers reference the same address,
  the names are called aliases.
• Aliasing is a hindrance to readability because it
  allows a variable to have its value changed by an
  assignment to a different variable.
• Aliases can be created through subprogram
  parameters, or using union (record variant).
• Two pointer variables are aliases when they point
  to the same memory location.

47
Explicit And Implicit Declarations

• An explicit declaration is a statement in a
  program that lists variable names and declares
  them to be of a particular type.
• An implicit declaration is a means of associating
  variables with types through default conventions
  instead of declaration statements.

48

• Although they are a minor convenience to
  programmers, implicit declarations can be
  detrimental to reliability, because they prevent
  the compilation process from detecting some
  programmer errors. Variables that are
  accidentally left undeclared by the programmer
  are given default types and unexpected
  attributes, which could cause subtle errors that
  are difficult to diagnose.

49
Blocks (1)

• Some languages allow local variables to be
  defined in the midst of executable code.
• Such variables have their storage allocated when
  the section is entered and deallocated when the
  section is exited.
• Such a section of code is called a block.
• C allows any compound statement (a statement
  sequence surrounded by matched braces) to have
  declarations

50
Blocks (2)

• For example, if list were an integer array, one
  could write
• if (listi lt listj)
• int temp
• temp listi
• listi listj
• listj temp

51
Named Constants

• A named constant is bound to a value only at the
  time it is bound to storage.
• Its value cannot be changed by assignment or by
  an input statement.
• Named constants are useful as aids to readability
  and program reliability.
• Named constants can aid modifiability.
• Static binding of values cannot be changed
  during execution.
• e.g. define PI 22/7 assigns PI the value during
  compilation.

52
Initialization (1)

• Initialization of variables and binding a value
  to a named constant is the same process, except
  that named constant is permanent.
• The binding of a variable to a value at the time
  it is bound to storage is called initialization.
• If the variable is statically bound to storage,
  binding and initialization occur before run time.
  If the storage binding is dynamic, initialization
  is also dynamic.

53
Initialization (2)

• In BASIC, all numeric variables are initialized
  to be zero during compile time.
• In C, initial values of variables can be
  specified in the declaration statement.
• Pascal provides a way to initialize variables,
  except at run time with assignment statements.
• Initialization occurs only once for static
  variables, but it occurs with every allocation
  for dynamically allocated variables.

54
Expressions And Assignment Statements

• The purpose of assignment statements is to change
  the values of variables.
• To understand expression evaluation, it is
  necessary to be familiar with the orders of
  operator and operand evaluation.
• The operator evaluation order of expressions is
  governed by the associativity and precedence
  rules of the language.

55
Operator Evaluation Order (1)

• It includes the operator precedence rules and the
  associativity rules of the language.
• The operator precedence rules for expression
  evaluation define the order in which the
  operators of different precedence levels are
  evaluated.
• The operator precedence rules for expressions are
  based on the hierarchy of operator priorities of
  the language.

56
Operator Evaluation Order (2)

• When an expression contains two adjacent
  occurrences of operators with the same level of
  precedence, the question of which operator is
  evaluated first is answered by the associativity
  rules of the language.
• An operator can either have left or right
  associativity, meaning that the leftmost
  occurrence is evaluated first or the rightmost
  occurrence is evaluated first, respectively.

57
Mixed Mode Expressions (1)

• An operator of an expression can have operands of
  different types.
• Langauges that do allow such expressions, called
  mixed-mode expressions, must define conventions
  for implicit operand type conversions, called
  coercions, because computers usually do not have
  operations that use operands of different types.

58
Mixed Mode Expressions (2)

• A coercion is an implicit type conversion that is
  initiated by the compiler.
• Type conversions explicitly requested by the
  programmer as explicit conversions, or casts, not
  coercions.

59
Relational And Boolean Expressions (1)

• A relational operator is an operator that
  compares the values of its two operands.
• A relational expression has two operands and one
  relational operator.
• The value of a relational expression is Boolean,
  except when Boolean is not a type in the language.

60
Relational And Boolean Expressions (2)

• Boolean expressions consist of Boolean variables,
  Boolean constants, relational expressions and
  Logical operators.
• Logical operators usually include those for the
  AND, OR, and NOT operations, and sometimes for
  exclusive OR and equivalence.
• In C, no Boolean type and thus no Boolean values.

61
Relational And Boolean Expressions (3)

• Instead, numeric values are used to represent
  Boolean values.
• With zero considered false and all nonzero values
  considered true.
• the result of evaluating such an expression is an
  integer, with the value 0 if false and 1 if true.
• One odd result of C's design is that the
  expression a gt b gt c is legal.
• Left associative.

62
Multiple Assignments

• e.g. In C language, the statement
• nl nw nc 0
• sets all three variables to zero.
• Right associativity.

63
Conditional Assignments (1)

• In C, a conditional expression has the form
• expression_1 ? expression_2 expression_3
• where expression_1 is interpreted as a Boolean
  expression.
• If expression_1 evaluates to true, the value of
  the whole expression is the value of
  expression_2
• otherwise it is the value of expression_3.

64
Conditional Assignments (2)

• e.g. 1
• z (a lt b) ? a b / z min(a, b) /
• set z to the minimum of a and b.
• e.g. 2
• average (count 0) ? 0 sum / count
• prevents from divide by zero runtime error

65
Assigning Operators (1)

• Compound assignment operators is a shorthand
  method of specifying commonly needed forms of
  assignments.
• The form of assignment that can be abbreviated
  with this technique has the destination variable
  also appearing as the first operand in the
  expression on the right side.

66
Assigning Operators (2)

• e.g. In C, compound assignment operators is the
  catenation of the desired binary operator to the
  operator.
• sum value
• Unary arithmetic operators combine increment and
  decrement operations with assignment.
• Right associativity
• - count is equivalent to - (count )

67
Assigning Operators (3)

• Assignment statements as operands
• In C, the assignment statement produces a result,
  which is the value assigned to the target.
• It can therefore be used as an operand in
  expressions.
• e.g.
• while ((ch getchar( )) ! EOF)
• .....
• The result, or value assigned, is then compared
  with the constant EOF.

68
Unary Operators

• The operators can be unary, meaning they have a
  single operand,
• or binary, menaing they have two operands.

69
Mixed Mode Assignments

• If the type of the expression is not the same as
  the type of the variable being assigned, coercion
  can be applied.

70
Control Structure

• A control structure is a control statement and
  the collection of statements whose execution it
  controls.

71
Control Statements And Their Structures

• Control statements are statements that provide
  the capabilities of choosing among alternative
  control flow paths (of statement execution) and
  of causing the repeated execution of certain
  collections of statements.

72
Single And Two Way Selection

• if (expression)
• ....
• else
• ....

73
Nesting Selectors

• An interesting problem arises when two-way
  selection constructs can be nested.
• e.g. In C,
• if (a gt b)
• if (b gt c)
• z b
• else
• z c
• this construct can be interpreted in two
  different ways, depending on whether the else
  clause is matched with the first then clause or
  the second.

74
Multiple Selectors

• This structure is encapsulated and has single
  entry.
• Implicit branches to a single point at the end of
  the whole construct are also provided for each
  selectable statement or compound statement.
• This restricts the control flow through the
  structure to a single selectable segment.

75
Repetitive Constructs

• An iterative statement is one that causes a
  statement or collection of statements to be
  executed zero, one or more times.

76
Problems With Unconditional Branching

• The unconditional branch, or goto, is the most
  powerful statement for controlling the flow of
  execution of a program's statements.
• Without restrictions on use, imposed either by
  language design or programming standards, goto
  statements can make programs virtually unreadable
  and, as a result, highly unreliable and difficult
  to maintain.

77
Subprograms

• Subprograms are the fundamental building blocks
  of programs.
• Each subprogram has a single entry point.
• The calling program unit is suspended during the
  execution of the called subprogram, which implies
  that there is only one subprogram in execution at
  any given time.
• Control always returns to the caller when the
  subprogram execution terminates.

78
Process Abstraction (1)

• A subprogram definition describes the actions of
  the subprogram abstraction.
• A subprogram call is the explicit request that
  the subprogram be executed.
• An active subprogram after having been called,
  has begun execution but has not yet completed
  that execution.
• A subprogram header is the first line of the
  definition.
• The parameters in the subprogram header are
  called formal parameters.

79
Process Abstraction (2)

• Subprogram call statements must include the name
  of the subprogram and a list of parameters to be
  bound to the formal parameters of the subprogram.
  These parameters are called actual parameters.
• The first actual parameter is bound to the first
  formal parameter and so forth. Such parameters
  are called positional parameters.
• In keyword parameters, the name of the formal
  parameter to which an actual parameter is to be
  bound is specified with the actual parameter.

80
Local Referencing (1)

• Subprograms are generally allowed to declare
  their own variables, thereby defining local
  referencing environments.
• Variables that are declared inside subprograms
  are called local variables.
• Access to local variables is usually restricted
  to the subprogram in which they are declared.
• Local variables can be either statically or
  dynamically bound to storage.

81
Local Referencing (2)

• If local variables are semidynamic, they are
  bound to storage when the subprogram begins
  execution and unbound from storage when that
  execution terminates.
• If local variables are static, they are bound to
  storage when the subprogram first begins
  execution.

82
Local Referencing (3)

• Advantages of static local variables is that they
  are very efficient they usually can be accessed
  faster because there is no indirection and no
  run-time overhead of allocation and deallocation.
• They allow subprograms to be history sensitive.
• Disadvantage of static local variables is the
  inability to support recursion.

83
Parameter-Passing Methods

• Parameter-passing methods are the ways in which
  parameters are transmitted to and / or from
  called subprograms.
• Formal parameters are characterized by one of
  three distinct semantics models
• They can receive data from the corresponding
  actual parameter,
• they can transmit data to the actual parameter,
• they can do both.

84
Parameter-Passing

• These three semantics models are called in mode,
  out mode and inout mode respectively.

85
Value

• the value of the actual parameter is used to
  initialize the corresponding formal parameter,
• which then acts as a local variable in the
  subprogram,
• thus providing in-mode semantics.
• actual parameter can be an expression.
• Disadvantages are additional storage is required
• physically movement is required
• The storage and the move operations can be costly
  if the parameter is a large object.

86
Result

• out-mode parameters.
• no value is transmitted to the subprogram
• the corresponding formal parameter acts as a
  local variable.
• but just before control is transferred back to
  the caller,
• its value is passed back to the caller's actual
  parameter
• actual parameter must be a variable.

87
Value-Result

• inout-mode parameters.
• pass-by-copy.
• value of actual parameter is used to initialize
  the corresponding formal parameter
• acts as a local variable
• at subprogram termination, the value of the
  formal parameter is transmitted back to the
  actual parameter.

88
Reference

• transmits an access path, usually just an
  address, to the called subprogram.