لخّصلي

Objectives
1.1 Reasons for Studying Concepts of Programming Languages 1.2 Programming Domains
1.3 Language Evaluation Criteria
1.4 Influences on Language Design
1.5 Language Categories
1.6 Implementation Methods 1.8 Programming Environments
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Reasons for Studying Concepts of Programming Languages
• Increased ability to express ideas
• Improved background for choosing appropriate
languages
• Increased ability to learn new languages
• Better understanding of significance of implementation
• Better use of languages that are already known
• Overall advancement of computing
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Programming Domains (1 of 2) • Scientific applications
– Large numbers of floating point computations; use of arrays
– Fortran
• Business applications
– Produce reports, use decimal numbers and characters – COBOL
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Programming Domains (2 of 2) • Artificial intelligence
– Symbols rather than numbers manipulated; use of linked lists
– LISP
• Systems programming
– Need efficiency because of continuous use –C
• Web Software
– Eclectic collection of languages: markup (e.g., HTML),
scripting (e.g., PHP), general-purpose (e.g., Java)
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Language Evaluation Criteria
• Readability: the ease with which programs can be read and understood
• Writability: the ease with which a language can be used to create programs
• Reliability: conformance to specifications (i.e., performs to its specifications)
• Cost: the ultimate total cost
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Evaluation Criteria: Readability (1 of 2) • Overall simplicity
The overall simplicity of a programming language strongly affects its readability.
– A manageable set of features and constructs – Minimal feature multiplicity
– Minimal operator overloading
• Orthogonality
– A relatively small set of primitive constructs can be
combined in a relatively small number of ways
– Every possible combination is legal
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Evaluation Criteria: Readability (2 of 2) • Data types
– Adequate predefined data types
• Syntax considerations
– Identifier forms: flexible composition
– Special words and methods of forming compound statements
– Form and meaning: self-descriptive constructs, meaningful keywords
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Evaluation Criteria: Writability • Simplicity and orthogonality
– Few constructs, a small number of primitives, a small set of rules for combining them
• Support for abstraction
– The ability to define and use complex structures or
operations in ways that allow details to be ignored
• Expressivity
– A set of relatively convenient ways of specifying operations
– Strength and number of operators and predefined functions
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Evaluation Criteria: Reliability • Type checking
– Testing for type errors • Exceptionhandling
– Intercept run-time errors and take corrective measures • Aliasing
– Presence of two or more distinct referencing methods for the same memory location
• Readabilityandwritability
– A language that does not support “natural” ways of expressing an algorithm will require the use of “unnatural” approaches, and hence reduced reliability
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Evaluation Criteria: Cost
• Training programmers to use the language
• Writing programs (closeness to particular applications)
• Compiling programs
• Executing programs
• Language implementation system: availability of free compilers
• Reliability: poor reliability leads to high costs
• Maintaining programs
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Evaluation Criteria: Others • Portability
– The ease with which programs can be moved from one implementation to another
• Generality
– The applicability to a wide range of applications
• Well-definedness
– The completeness and precision of the language’s
official definition
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Influences on Language Design
• Computer Architecture
– Languages are developed around the prevalent computer architecture, known as the von Neumann architecture
• Program Design Methodologies
– New software development methodologies (e.g., object-oriented software development) led to new programming paradigms and by extension, new programming languages
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Computer Architecture Influence
• Well-known computer architecture: Von Neumann
• Imperative languages, most dominant, because of von
Neumann computers
– Data and programs stored in the same memory
– Memory is separate from CPU
– Instructions and data must be transmitted from memory to CPU
– Results of operations in the CPU must be moved back to memory
– Basis for imperative languages
▪ Variables model memory cells
▪ Assignment statements are based on the piping operation
▪ Iteration is efficient
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The von Neumann Architecture (1 of 2)
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The von Neumann Architecture (2 of 2)
• The execution of a machine code program on a von Neumann architecture computer occurs in a process called the fetch-execute cycle.
• As stated earlier, programs reside in memory but are executed in the CPU. Each instruction to be executed must be moved from memory to the processor. The address of the next instruction to be executed is maintained in a register called the program counter.
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• The fetch- execute cycle can be simply described by the following algorithm:
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Programming Methodologies Influences
• 1950s and early 1960s: Simple applications; worry about machine efficiency
• Late 1960s: People efficiency became important; readability, better control structures
– structured programming
– top-down design and step-wise refinement
• Late 1970s: Process-oriented to data-oriented – data abstraction
• Middle 1980s: Object-oriented programming
– Data abstraction + inheritance + polymorphism
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Language Categories (1 of 2) • Imperative
– Central features are variables, assignment statements, and iteration
– Include languages that support object-oriented programming
– Include scripting languages
– Include the visual languages
– Examples: C, Java, Perl, JavaScript, Visual BASIC .NET, C++
• Functional
– Main means of making computations is by applying functions to
given parameters
– Examples: LISP, Scheme, ML, F#
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Language Categories (2 of 2)
• Logic
– Rule-based (rules are specified in no particular order) – Example: Prolog
• Markup/programming hybrid
– Markup languages extended to support some
programming
– Examples: JSTL, XSLT
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Implementation Methods
• The two of the primary components of a computer are: its internal memory and its processor.

1. The internal memory is used to store programs and data.


2. The processor is a collection of circuits that provides a realization of a set of primitive operations, or machine instructions, such as those for arithmetic and logic operations.
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• A language implementation system cannot be the only software on a computer. Also required is a large collection of programs, called the operating system, which supplies higher-level primitives than those of the machine language.
• The operating system and language implementations are layered over the machine language interface of a computer. These layers can be thought of as virtual computers, providing interfaces to the user at higher levels
• For example, an operating system and a C compiler provide a virtual C computer
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Layered View of Computer
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Implementation Methods (1 of 2) • Compilation
– Programs are translated into machine language; includes JIT systems
– Use: Large commercial applications • Pure Interpretation
– Programs are interpreted by another program known as an interpreter
– Use: Small programs or when efficiency is not an issue
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Implementation Methods (2 of 2) • Hybrid Implementation Systems
– A compromise between compilers and pure interpreters
– Use: Small and medium systems when efficiency is not the first concern
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1. Compilation
   • Translate high-level program (source language) into machine code (machine language) which can be executed directly on the computer.
   • Slow translation, fast execution
   • Compilation process has several phases:
   – lexical analysis: converts characters in the source program into lexical units
   – syntax analysis: transforms lexical units into parse trees which represent the syntactic structure of program
   – Semantics analysis: generate intermediate code
   – code generation: machine code is generated
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Compilation process has several phases:

1. Lexicalanalyzer:-
   gathers the characters of the source program into lexical units. The lexical analyzer ignores comments in the source program because the compiler has no use for them.


2. Syntax analyzer:-
   takes the lexical units from the lexical analyzer and uses
   them to construct hierarchical structures called parse trees. 3) Intermediate code generator:-
   produces a program in a different language,
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• Semantic analyzer:-
checks for errors, such as type errors, that are difficult, if
not impossible, to detect during syntax analysis.
4) Optimization:-
which improves programs by making them smaller or faster or both. Most optimization is done on the intermediate code.
5) Thecodegenerator:-
translates the optimized intermediate code version of the
program into an equivalent machine language program.
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6. The Symbol Table:-
   serves as a database for the compilation process. This information is placed in the symbol table by the lexical and syntax analyzers and is used by the semantic analyzer and the code generator
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The Compilation Process
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Additional Compilation Terminologies
• Load module (executable image): the user and system code together
• Linking and loading: the process of collecting system program units and linking them to a user program
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Von Neumann Bottleneck
• Connection speed between a computer’s memory and its processor determines the speed of a computer
• Program instructions often can be executed much faster than the speed of the connection; the connection speed thus results in a bottleneck
• Known as the von Neumann bottleneck; it is the primary limiting factor in the speed of computers
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2. Pure Interpretation Process (1 of 2)
   • No translation
   • Easier implementation of programs (run-time errors can easily and immediately be displayed)
   • Slower execution (10 to 100 times slower than compiled programs)
   • Often requires more space
   • Now rare for traditional high-level languages
   • Significant comeback with some Web scripting languages (e.g., JavaScript, PHP)
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Pure Interpretation Process (2 of 2)
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3. Hybrid Implementation Systems (1 of 2) • A compromise between compilers and pure interpreters
   • A high-level language program is translated to an intermediate language that allows easy interpretation
   • Faster than pure interpretation • Examples
   – Perl programs are partially compiled to detect errors before interpretation
   – Initial implementations of Java were hybrid; the intermediate form, byte code, provides portability to any machine that has a byte code interpreter and a run-time system (together, these are called Java Virtual Machine)
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Hybrid Implementation Systems (2 of 2)
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Summary (1 of 2)
• The study of programming languages is valuable for a
number of reasons:
– Increase our capacity to use different constructs – Enable us to choose languages more intelligently – Makes learning new languages easier
• Most important criteria for evaluating programming languages include:
– Readability, writability, reliability, cost
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Summary (2 of 2)
• Major influences on language design have been machine
architecture and software development methodologies
• The major methods of implementing programming languages are: compilation, pure interpretation, and hybrid implementation