学习程序设计语言的原因

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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 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.

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1 .Chapter 1 Preliminaries ISBN 0-321-49362-1

2 .Chapter 1 Topics • Reasons for Studying Concepts of Programming Languages • Programming Domains • Language Evaluation Criteria • Influences on Language Design • Language Categories • Language Design Trade-Offs • Implementation Methods • Programming Environments Copyright © 2015 Pearson. All rights reserved. 1-2

3 .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 Copyright © 2015 Pearson. All rights reserved. 1-3

4 .Programming Domains • Scientific applications – Large numbers of floating point computations; use of arrays – Fortran • Business applications – Produce reports, use decimal numbers and characters – COBOL • 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) Copyright © 2015 Pearson. All rights reserved. 1-4

5 .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 Copyright © 2015 Pearson. All rights reserved. 1-5

6 .Evaluation Criteria: Readability • Overall simplicity – 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 • 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 Copyright © 2015 Pearson. All rights reserved. 1-6

7 .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 Copyright © 2015 Pearson. All rights reserved. 1-7

8 .Evaluation Criteria: Reliability • Type checking – Testing for type errors • Exception handling – Intercept run-time errors and take corrective measures • Aliasing – Presence of two or more distinct referencing methods for the same memory location • Readability and writability – A language that does not support “natural” ways of expressing an algorithm will require the use of “unnatural” approaches, and hence reduced reliability Copyright © 2015 Pearson. All rights reserved. 1-8

9 .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 Copyright © 2015 Pearson. All rights reserved. 1-9

10 .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 Copyright © 2015 Pearson. All rights reserved. 1-10

11 .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 Copyright © 2015 Pearson. All rights reserved. 1-11

12 .Computer Architecture Influence • Well-known computer architecture: Von Neumann • Imperative languages, most dominant, because of von Neumann computers – Data and programs stored in memory – Memory is separate from CPU – Instructions and data are piped from memory to CPU – Basis for imperative languages • Variables model memory cells • Assignment statements model piping • Iteration is efficient Copyright © 2015 Pearson. All rights reserved. 1-12

14 .The von Neumann Architecture • Fetch-execute-cycle (on a von Neumann architecture computer) initialize the program counter repeat forever fetch the instruction pointed by the counter increment the counter decode the instruction execute the instruction end repeat Copyright © 2015 Pearson. All rights reserved. 1-14

15 .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 Copyright © 2015 Pearson. All rights reserved. 1-15

16 .Language Categories • 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# • 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 Copyright © 2015 Pearson. All rights reserved. 1-16

17 .Language Design Trade-Offs • Reliability vs. cost of execution – Example: Java demands all references to array elements be checked for proper indexing, which leads to increased execution costs • Readability vs. writability Example: APL provides many powerful operators (and a large number of new symbols), allowing complex computations to be written in a compact program but at the cost of poor readability • Writability (flexibility) vs. reliability – Example: C++ pointers are powerful and very flexible but are unreliable Copyright © 2015 Pearson. All rights reserved. 1-17

18 .Implementation Methods • 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 • Hybrid Implementation Systems – A compromise between compilers and pure interpreters – Use: Small and medium systems when efficiency is not the first concern Copyright © 2015 Pearson. All rights reserved. 1-18

20 .Compilation • Translate high-level program (source language) into machine code (machine language) • 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 Copyright © 2015 Pearson. All rights reserved. 1-20

22 .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 Copyright © 2015 Pearson. All rights reserved. 1-22

23 .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 Copyright © 2015 Pearson. All rights reserved. 1-23

24 .Pure Interpretation • 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) Copyright © 2015 Pearson. All rights reserved. 1-24

26 .Hybrid Implementation Systems • 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) Copyright © 2015 Pearson. All rights reserved. 1-26

28 .Just-in-Time Implementation Systems • Initially translate programs to an intermediate language • Then compile the intermediate language of the subprograms into machine code when they are called • Machine code version is kept for subsequent calls • JIT systems are widely used for Java programs • .NET languages are implemented with a JIT system • In essence, JIT systems are delayed compilers Copyright © 2015 Pearson. All rights reserved. 1-28

29 .Preprocessors • Preprocessor macros (instructions) are commonly used to specify that code from another file is to be included • A preprocessor processes a program immediately before the program is compiled to expand embedded preprocessor macros • A well-known example: C preprocessor – expands #include, #define, and similar macros Copyright © 2015 Pearson. All rights reserved. 1-29

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