Programming Domains

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Programming Domains

• Scientific Applications
• Typically, scientific applications have simple
  data structures but require large numbers of
  floating-point arithmetic computations.
• For some scientific applications where
  efficiency is the primary concern, like those
  that were common in the 1950s and 1960s, no
  subsequent language is significantly better than
  FORTRAN.
• .

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Programming Domains (cond)

• Business Applications
• The use of computers for business applications
  began in the 1950s.
• The first successful high-level language for
  business was COBOL which appeared in 1960.
• Business languages are characterized, according
  to the needs of the application, by elaborate
  input and output facilities and decimal data
  types.
• With the advent of microcomputers came new ways
  of businesses, especially small businesses, to
  use computers. Two specific tools, spreadsheet
  systems and database systems, were developed for
  business and now are widely used.

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Programming Domains (cond)

• Artificial Intelligence
• AI is a broad area of computer applications
  characterized by the absence of exact algorithms
  and the use of symbolic computations rather than
  numeric computation.
• Symbolic computation means that symbols,
  consisting of names rather than numbers, are
  manipulated.
• The first widely used programming language
  developed for AI applications was the functional
  language LISP (Scheme) which appeared in 1959.
• An alternative approach to these applications
  appeared in the early 1970s logic programming
  using Prolog language

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Programming Domains (cond)

• Systems Programming Languages
• The operating system and all of the programming
  support tools of a computer system are
  collectively known as its systems software.
• Systems software is used almost continuously and
  therefore must have execution efficiency.
• A language for this domain must have low-level
  features that allow the software to external
  devices to be written.
• In the 1960s and 1970s, some computer
  manufacturers, such as IBM, Digital, and
  Burroughs (now UNISYS) developed special
  machine-oriented high level languages for systems
  software on their machines. For IBM mainframe
  computers, the language was PL/S, a dialect of
  PL/I for Digital, it Is BLISS, a language at a
  level just above assembly language for
  Burroughs, it was Extended Algol.
• The UNIX operating system is written almost
  entirely in C, which was made it relatively easy
  to port, or move, to different machines.

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Programming Domains (cond)

• Very High-level Languages (VHLLs)
• The languages in the category called very
  high-level have evolved slowly over the past 25
  years.
• The various scripting languages for UNIX are
  examples of VHLLs. A scripting language is one
  that is used by putting a list of commands,
  called script, in a file to be executed.
• The first of these languages, named shell, began
  as a small collection of commands that were
  interpreted to be calls to system subprograms
  that performed utility functions, such as file
  management and simple file filtering.
• Other VHLLs are awk, for report generation, tcl
  combined with tk, which provide a method of
  building X Windows applications. The perl is a
  combination of shell and awk.

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Programming Domains (cond)

• Special-Purpose Languages
• A host of special-purpose languages have appeared
  over the past 40 years.
• They range from RPG, which is used to produce
  business reports, to APT, which is used for
  instructing programmable machine tools, to GPSS,
  which is used for systems simulation.

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Language Evaluation Criteria

• Readability
• Writability
• Reliability
• Cost

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Readability

• The ease with which programs can be read and
  understood.
• Overall Simplicity A language that has a large
  number of basic components is more difficult to
  learn than one with a small number of basic
  componenets.
• examples C
• count count 1
• count 1
• count
• count
• Orthogonality in a programming language means
  that a relatively small set of primitive
  constructs can be combines in a relatively small
  number of ways to build the control and data
  structures of the language
• Orthogonality is closely related to simplicity.
  The more orthogonal the design of a language, the
  fewer exceptions the language rules require.
  Fewer exceptions means a higher degree of
  regularity in the design, which makes the
  language easier to learn.

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Readability

• Control Structures. The structured programming
  revolution of the 1970s was a reaction to the
  poor readability caused by the limited control
  statements of some of the languages of the 1950s
  and 1960s.
• Examples
• Goto statements vs Loops

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Readability

• Data Types and Structures.
• The presence of adequate facilities for defining
  data types and data structures in a language is
  another significant aid to readability.

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Readability

• Syntax Considerations
• The syntax, or form, of the elements of a
  language has a significant effect on the
  readability of programs.
• Examples
• Identifier Forms (length, case sensitivity)
• Special Words ( )
• Form and Meaning (Appearance does not always
  suggest their function?)

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Writability

• Writability is a measure of how easily a language
  can be used to create programs for a chosen
  problem domain.

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Writability

• Simplicity and Orthogonality
• Support for Abstraction
• Abstraction means the ability to define and then
  use complicated structures or operations in ways
  that allow many of the details to be ignored.
• Process abstraction
• Data abstraction

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Writability

• Expressivity
• Expressivity in a language means that there are
  very powerful operators that allow a great deal
  of computation to be accomplished with a very
  small program.

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Reliability

• Type checking
• Type checking is simply testing for type errors
  in a given program, either by the compiler or
  during program execution
• Exception Handling
• Aliasing
• Readability and Writability

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Cost

• The ultimate total cost of a programming language
  is a function of many of its characteristics.
• Cost of training programmers
• Cost of writing programs in the language
• Cost of compiling programs
• Cost of executing programs
• Cost of compilers
• Cost of poor reliability

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Influences on Language Design

• Computer Architecture
• - the basic architecture of computers has a
  crucial effect on language design. Most of the
  popular languages of the past 35 years have been
  designed around the prevalent computer
  architecture, called the von Neumann
  architecture. These languages are called
  imperative languages.

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Influences on Language Design

• Computer Architecture (cond)
• In a von Neumann computer, both data and
  programs are stored in the same memory. The
  central processing unit (CPU), which actually
  executes instructions is separate from the
  memory. Therefore, instructions and data must be
  piped, from the memory to the CPU. Results of
  operations in the CPU must be moved back to
  memory.

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The von Neumann computer Architecture
Memory (stores both instructions and data)
Instructions Data
Results of operations
Input Output Device
Arithmetic and Logic Unit
Control Unit
Central Processing Unit
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Influences on Language Designs

• Programming Methodologies
• (1960s) Structured Programming
• Process-Oriented
• (1970s) Object-Oriented Programming
• Data Abstraction
• Encapsulation
• Inheritance
• Polymorphism

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Implementation Methods

• Programming languages can be implemented by any
  of the three general methods.
• 1. Compilation programs are translated to
  machine language which can then be executed
  directly on the computer.
• Advantage(s)
• - very fast program execution
• Disadvantage(s)
• - programs should be error-free before
  execution.

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Implementation Methods

• 2. Pure Interpretation The interpreter program
  acts as a software simulation of a machine whose
  fetch-execute cycle deals with high-level
  language program statements rather than machine
  instructions. This software simulation obviously
  provides a virtual-machine for the language.
• Advantage
• - Allows easy implementation of many
  source-level debugging operations.
• Disadvantage
• - Requires more space

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Influences on Language Designs

• 3. Hybrid Implementation Systems
• A compromise between compilers and pure
  interpreters.

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Evolution of Programming Languages

• (read Zuses Plankalkul) link

Z1 computer
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Names, Bindings, Type Checking, and Scopes

• Name. A name is a string of characters used to
  identify some entity in a program.
• Design Issues
• What is the maximum length of a name?
• Can connector characters be used in names?
• Are names case sensitive?
• Are special words reserved words or keywords?

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Names (cond)

• Special Words
• Special words in programming languages are used
  to make programs more readable by naming actions
  to be performed. They also are used to separate
  the syntactic entities of programs. In most
  languages, these words are classified as reserved
  words, but in some they are only keywords.

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Names (cond)

• Keyword is a word of a programming language
  that is special only in certain contexts.
• Example (FORTRAN) Fortran is one of the languages
  whose special words are keywords.
• REAL APPLE
• REAL 3.4

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Names (cond)

• Reserved Word is a special word of a programming
  language that cannot be used as a name. As a
  language design choice, reserved words are better
  than keywords because the ability to redefine
  keywords can lead to readability problems

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Variables

• A program variable is nothing more than an
  abstraction of a computer memory cell or
  collection of cells. Programmer often think of
  variable as names for memory locations, although
  there is much more to a variable than just a
  name.

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Variables (cond)

• Attributes
• Name
• Address
• Aliases
• Type
• Value

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Binding

• Binding is an association, such as between an
  attribute and an entity or between an operation
  and a symbol. The time at which a binding takes
  place is called binding time.

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Binding (cond)

• Example Consider the following C assignment
  statement, whose variable count has been defined
  as shown
• int count
• .
• count count 5

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Binding (cond)

• Some of the bindings and their binding times for
  the parts of this assignment statement are as
  follows
• Set of possible types for count bound at
  language design time
• Type of count bound at compile time.
• Set of possible values of count bound at
  compiler design time.
• Value of count bound at execution time with this
  statement.
• Set of possible meanings for the operator symbol
  bound at language definition time
• Meaning of the operator symbol bound at compile
  time
• Internal representation of the literal 5 bound
  at compiler design time.

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Binding(cond)

• Binding of Attributes to Variables
• A binding is static if it occurs before run time
  and remains unchanged throughout the program
  execution. If it occurs during run time or can
  change in the course of program execution, it is
  called dynamic.

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Bindings(cond)

• Type Bindings
• Explicit declaration is a statement in a
  program that lists variable names and declares
  them to be of a particular type.
• Implicit declaration is a means of associating
  variables with types through default conventions
  instead of declaration statements.
• Type inference

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Bindings(cond)

• Static Variables
• Are those that are bound to memory cells before
  program execution begins and remain bound to
  those same memory cells until program execution
  terminates.

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Next Chapter 5 Data Types

• P 191
• Concepts of Programming Languages, 3rd Edition
• Robert W. Sebesta

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DATA TYPES

• Primitive Data Types are types which are not
  defined in terms of other types.
• Examples in C
• int
• char
• float
• double
• Void

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DATA TYPES

• Examples in Pascal
• Char
• Boolean
• Integer
• Real
• String

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DATA TYPES

• Numeric Types
• Integer is the most common primitive numeric data
  type is integer. Many computers now support
  several sizes of integers.
• An integer value is represented in a computer by
  a string of bits, with one of the bits, typically
  the leftmost, representing the sign.
• Most computers now use a notation called twos
  complement to store negative integers, which is
  convenient for addition and subtraction.

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DATA TYPES

• Floating-Point data types model real numbers, but
  the representations are only approximations for
  most real values.
• Floating point values are represented as
  fractions and exponents.
• Floating point types have values ranges that are
  defined in terms of precision and range.

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DATA TYPES

• Decimal (quite similar to Float?)
• Most larger computers that are designed to
  support business systems applications have
  hardware support for decimal data types.
• Decimal data types store a fixed number of
  decimal digits, with the decimal point at a fixed
  position in the value. These are the primary
  data types for business data processing and are
  there fore essential to COBOL.

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DATA TYPES

• Boolean types are perhaps the simplest of all
  types. Their range of values has only two
  elements, one for true and one for false. They
  were introduced in ALGOL 60 and have been
  included in most general-purpose languages
  designed since 1960.

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DATA TYPES

• Character Type are stored in computers as numeric
  codings. The most commonly used coding is ASCII
  (Americal Standard Code for Information
  Interchange)

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Next Character String Type

• P. 197 Concepts of PLs 3rd ed. Robert W. Sebesta.

46
Character String Types

• A character string type is one in which the
  objects consist of sequences characters.
• Design Issues
• should strings be a primitive or simply a special
  kind of character array?
• Should strings have static or dynamic length?

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Character String Types

• Strings and Their Operations
• If strings are not defined as a primitive type,
  string data is usually stored in arrays of single
  characters.

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Character String Types

• String Length Options there are several design
  choices regarding the length of string objects.
• First, the length can be static and specified in
  the declaration. (static length string)
• The second option is to allow strings to have
  varying length up to a declared and fixed maximum
  set by the variables definition, as exemplified
  by the strings in C and C. (limited dynamic
  length)
• The third option is to allow strings to have
  varying length with no maximum. (dynamic length
  strings)

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Character String Types

• Evaluation
• String types are important to the writability of
  a language. Dealing with strings as arrays can
  be more cumbersome than dealing with a primitive
  string type. The addition of strings as a
  primitive type to a language is not costly, in
  terms of either language or compiler complexity.
  Therefore, it is difficult to justify the
  omission of primitive string types in some
  contemporary languages. Of course, the
  availability of standard libraries of string
  manipulation subprograms can remove this
  deficiency when strings are not included as a
  primitive type.

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Array Types

• What is an array?

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Arrays

• An array is a homogeneous aggregate of data
  elements in which the individual element is
  identified by its position in the aggregate,
  relative to the first element. The individual
  data elements of an array are of some previously
  defined type, either primitive or otherwise.

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Arrays

• Design Issues
• The primary design issues specific to arrays are
  the following
• What types are legal to 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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Record Types

• A record is a possibly heterogeneous aggregate of
  data elements in which the individual elements
  are identified by names.