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ISBN 0-321-33025-0

Chapter 1:Preliminaries

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.ITCS332 by Dr. Abdel Fattah Salman 1-2

Chapter 1 Topics

Reasons for Studying Concepts of ProgrammingLanguages

Programming Domains Language Evaluation Criteria

Influences on Language Design Language Categories

Language Design Trade-Offs Implementation Methods Programming Environments

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Reasons for Studying Concepts of Programming Languages

Benefits of studying programming concepts course: Increased ability to express ideas

Programming languages place limits on the kind of control

structures, data structures, thus the forms of algorithmsconstructed are limited. Language feature can be integrated or simulated . The fact that many features can be simulated in other language does

not lessen the importance of designing languages with best features.Better to use integrated features than simulated.

Improved background for choosing appropriate languages Professional programmers have learned 1 or 2 languages on their

own or through in-house training. When given a choice of languages for a new project, they continue

to use the one they are most familiar even it is not suited to the newproject.

If they were familiar with the features of other languages, theywould be in a better position to make correct choice.

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Increased ability to learn new languages The process of learning a new language can be lengthy and

difficult for one who is comfortable with 1 or 2 languages. When a thorough understanding of the fundamental conceptsof languages is acquired, it becomes easier to see how theseconcepts are incorporated in the language design in general.

For example , one who understand the concepts of OOP willhave it easier to learn Java than those who do not have anyknowledge of OOP.

Better understanding of significance of implementation An understanding of implementation issues leads to an

understanding of why are designed the way they are. This knowledge leads to the ability to use the language more

efficiently.

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We become better programmers by understanding the choicesamong language constructs and consequences of thosechoices.

Certain kinds of bugs can be detected and fixed only byprogrammers who know implementation details. It allows us to visualize how a computer executes various

language constructs. This leads to understanding theefficiency of various constructs. For example , programmerswho know little about how recursion is implemented do notknow that a recursive algorithm is slower than an equivalentiterative algorithm .

Better use of known languages Contemporary languages are large and complex. It is uncommon for programmers to use ALL features of those

languages. Programmers can learn unknown and unused features of

languages

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Overall advancement of computing It is usually possible to determine why a particular language

became familiar.

It might be concluded that a language became widely used notbecause it is the best available, but because those in positionto choose languages were not sufficiently familiar with

programming language concepts. For example , many people believe it would have been better

if ALGOL 60 had replaced FORTRAN, because it was moreelegant and had much better control statements thanFORTRAN. That did not is due software managers of thattime did not clearly understand the conceptual design of ALGOL 60. They found its description difficult to read and to

understand. They did not appreciate the benefits of block structure, recursion, and well-structured control statements.

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Programming DomainsComputers are applied for different areas: from controlling nuclear plants tovideo games and mobile phones . Computer applications are classified into 5categories:

Scientific applications

Scientific applications have simple data structures but require large numberof floating point computations. The most common data structures are counting loops and selections Fortran

Business applications Business applications require producing reports, use decimal numbers andcharacters

COBOL

With the advent of PCs: Spreadsheets and Databases Artificial intelligence AI applications are characterized by use of symbols rather than numbers Symbolic computation is better done with linked lists than arrays.

The first widely used language for AI is LISP. An alternative language for AI applications was used in 1970s: Logicprogramming using Prolog and Scheme .

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Programming Domains Systems programming

System software involves OS and all programming support tools. System software is used continuous and needs to be efficient so it must

have low-level features. Some computer manufacturers had developed special machine-oriented

high-level languages for system software: PL/S from IBM, BLISS fromDigital

UNIX OS was written almost entirely in C

Web Software The Web is supported by a collection of languages: markup (e.g.,

XHTML), scripting (e.g., PHP), general-purpose (e.g., Java). XHTML provides a way of embedding presentation instructions in the

pages of information: text, images, sound, or animation. The dynamic web content requires embedding programming code in an

XHTML document. Such code is in the form of a scripting language suchas JavaScript or PHP.

Alternatively, the XTHML document can request the execution of aseparate program on the Web server to provide dynamic content.

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Language Evaluation Criteria

The purpose of this course: To examine carefully the concepts of the various constructs and

capabilities of PLs. To evaluate these features, focusing on their impact on the softwaredevelopment process including maintenance.

To do this , we need a set of criteria: Readability : means the ease with which programs can be read

and understood

Writability : means the ease with which a language can be usedto create programs Reliability : conformance to specifications (i.e., performs to its

specifications under all conditions) Cost : the ultimate total cost of a programming language

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Evaluation Criteria: ReadabilityThe readability of a PL is affected by the following features: Overall simplicity

A manageable set of features and constructs

Few feature multiplicity (means of doing the same operation) 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

Control statements The presence of well-known control structures (e.g., while statement)

Data types and structures The presence of adequate facilities for defining data structures 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: Readability-Overall simplicity

The following describe features that contribute to the readability of aprogramming language:

Overall simplicity The first complicating feature is a large # of basic constructs is moredifficult to learn than one with a small # of them. A manageable set of

features and constructs. Programmers often learn and use a subset of the large language and

ignore other features. Readability problems occur whenever the programs author used a

different subset from that known to the reader. The second complicating feature is feature multiplicity : means having

more than one way of doing the same operation. a=a+1; a+=1; a++; ++a.

The third complicating feature is operator overloading : a single operatorhas more than one meaning. Some overloadings are useful and simplify the language by reducing

the # of operators: Using + for both integer and real additions.

Some overloadings are harmful and make the construct confusing:using + between 2 vector operands to mean their respective elements.

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Evaluation Criteria: Readability-Orthogonality

Orthogonality means: (Orthogonal vectors are independent of each other) A relatively small set of primitive constructs can be combined in a relatively

small number of ways Every possible combination of primitives is legal and meaningful.

Suppose a L has 4 primitive data types: int, float, double, char and 2 typeoperators: array and pointer . If the type op can be applied to themselvesand the 4 data types, then a large # of data structures can be defined.

The lack of orthogonality leads to the exceptions to the rules of thelanguage. The use of orthogonality can be illustrated by comparing addition

instruction of assembly on IBM mainframes and VAX superminicomputers:Adding 2 32-bit integers that reside in register/memory and replacing one of them with the sum:

A reg1, mem ; on IBMAR reg1, reg2

ADDL op1, op2 ; on VAX The VAX instruction is orthogonal in that a single instruction can use eitherregisters or memory as operands.

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Evaluation Criteria: Readability-Orthogonality

The IBM is not orthogonal because only 2 operand combinations are legalout of 4 possibilities, and the two require 2 different instructions: A and AR .

The IBM is more difficult to learn because of the restrictions and theadditional instructions.

The more orthogonal the design of the language, the fewer exceptions thelanguage rules require. Fewer exceptions mean a higher degree of regularityin the design which makes the language easier to learn, read, andunderstand.

Too much orthogonality can cause problems: every construct in ALGOL 68has a type, there are no restrictions on those types.; most constructs producevalues. This combinational freedom allows complex constructs.

Simplicity in a language is the result of a combination of a relatively small #of primitive constructs and a limited use of orthogonality.

A functional language is one in which computations are made by applyingfunctions to given parameters. They offer the greatest simplicity becausethey can accomplish everything with single construct-function call. Lowefficiency prevent functional languages from becoming more widely used.

An imperative language is one in which computations are usually specifiedwith variables and assignment statements.

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Evaluation Criteria: Readability-Control Statements

Control statements A program that can be read from top to bottom is much easier

to understand than a program with a lots of goto (jumps)statements.

Most recent languages have included sufficient control

statements-so the need for goto statement has been reduced(eliminated).

The presence of well-known control structures allows writing

well-structured and readable programs.

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Evaluation Criteria: Readability-Data types and structures

Data types and structures The presence of adequate facilities for defining data types and

data structures is significant for readability. For example , we use numeric values to indicate flags because

there is no Boolean type in the language:timeout=1 is unclear meaning, whereas

timeout=true is perfectly clear. Languages without record type usually use a collection of parallel arrays : one for each item in the record. Records makeprograms more readable than using parallel arrays .

Character (Len=30) name(100)Integer Age (100)Integer Employee_number(100)Real salary(100)

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Evaluation Criteria: Writability Writability is a measure of how easily a language can be used to

create programs for a chosen problem domain. Most of the language features that affect readability also affect

writability. Writability must be considered in the context of the target

problem domain.

For example , the writabilities of COBOL and FORTRAN aredramatically different in dealing with multi-dimensional arrays-FORTRAN is ideal. The Writability of COBOL is ideal forproducing financial reports.

Writability is influenced by the following factors: Simplicity and orthogonality

Support for abstraction Expressivity

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Evaluation Criteria: Writability

Simplicity and orthogonality Some programmers may not be familiar with all constructs of a large

language. This leads to a misuse of some features and disuse of

others. Few constructs , a small number of primitives, a small set of rules forcombining them is much better than having a large number of primitives.

Too much orthogonality can be detriment to Writability: errors can goundetected when all combinations of primitives are legal.

Expressivity Expressivity means that a language has a set of relatively convenient

ways of specifying operations. a++ is more convenient and shorterthan a=a+1.

Example : the inclusion of for statement in many modern languages

makes writing counting loops easier than with the use of while

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Evaluation Criteria: Writability

Support for abstraction Abstraction means the ability to define and use complex structures

or operations in ways that allow details to be ignored. The degree of abstraction allowed by a L and the naturalness of itsexpression are important to Writability .

PL can support 2 categories of abstractions: process and data . Example of process abstraction is the use of subprogram that

represents a code to be repeated multiple times in a program. Example of data abstraction is a binary tree that stores integers in its

nodes. In languages w/o pointers and dynamic memory managementsuch as Fortran, this can be implemented by 3 parallel arrays. InC++ and Java , trees are implemented by an abstraction of a tree

node as a simple class with 2 pointers (references) and an integer.

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Evaluation Criteria: ReliabilityReliability : a program is said to be reliable if it conforms to itsspecifications under all circumstances.

Reliability is affected by the following factors:

Type checking Testing for type errors during compile or execute times.

The earlier errors are detected, the less expensive making repairs

Compile-time type checking is cheaper and desirable than run-timechecking.

Java requires type checking of all variables and expressions atcompile time.

Exception handling The ability of a program to intercept run-time errors, take corrective

measures and continue is obvious to reliability.

Ada, C++, and Java include extensive capabilities for exceptionhandling, but Fortran not.

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Evaluation Criteria: Reliability

Aliasing Presence of two or more distinct referencing methods or names for

the same memory location. Aliasing is widely accepted as a dangerous feature in PLs .

Readability and writability A language that does not support natural ways of expressing an

algorithm will necessarily use unnatural approaches, and hencereduced reliability.

Unnatural approaches are less likely to be correct for all possiblesituations.

The easier a program to write, the most likely it is to be correct. Readability affects reliability in both writing and maintenance phases

of the life cycle.

Programs that are difficult to read are difficult both to write and tomodify.

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Evaluation Criteria: CostThe total cost of a PL is a function of its features: Training programmers to use language: it is a function of

simplicity and orthogonality of a language and the experience of

programmers. Writing programs (closeness to particular applications): is a

function of writeability

Compiling programs : Quality and speed of used compiler. Executing programs : A language that requires many run-time type

checks will prohibit fast code execution. Optimization is a set of techniques used by compilers to

decrease size and/or increase execution speed of the code theyproduce.

If little or no optimization is done, compilation will be faster . The higher the degree of optimization , the faster the code in

execution.

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Evaluation Criteria: Cost

Language implementation system : availability of free compilers Reliability : poor reliability leads to high costs Maintaining programs :

Because maintenance is often done by individuals other thanoriginal software authors, poor readability makes the task challenging.

Maintenance costs can be as high as 2 to 4 times of developingcosts.

Program development , maintenance , and reliability are themost important contributors to the language cost.

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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 languages officialdefinition Language implementers are concerned with the difficulty of

implementing the constructs and features of the language. Language user are worried about writability first and readability

later. Language designer emphasize elegance and the ability to attract

widespread use.

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Influences on Language Design Computer Architecture

Languages are developed around the prevalent computer architecture,known as the von Neumann architecture.

In a von Neumann computer, both data and instructions are stored in thesame memory. Because of the von Neumann architecture, the central features of imperative

languages are variables- which model memory cells; statements which are

based on piping operation; and the iterative form of repetition. Operands in expressions are piped from memory to CPU, the result is piped

back to memory cell ( L-value ). Iteration is fast on von Neumann computers because instructions are stored

in adjacent memory cells. This efficiency discourages the use of recursionfor repetition although recursion is natural. A functional language is one in which the primary means of computation is

applying functions to given parameters. Programming in FL can be done

without: variable, assignment statements, and iterations. Parallel architectures increases the speed of executing functional programs.

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Influences on Language Design Programming Methodologies

In 1960s-early 70s: Structured programming : software development process and PL

design are motivated by the shift in the major cost of computingfrom hardware to software. Programmers productivity was low. Progressively larger and

more complex problems were being solved by computers.

New software development methodology: top-down design andstepwise refinement . PL deficiencies were discovered: incompleteness of type

checking and inadequacy of control statement.

In late 1970s: a shift from procedure-oriented to data-orientedprogram design methodologies: emphasis on data design and theuse of abstract data types to solve problems.

For data abstraction to be used effectively, it must be supportedby the languages used for implementation. The first language thatsupported data abstraction was SIMULA 67.

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Influences on Language Design The next step in the evolution of data-oriented to object-oriented design.

OO methodology begins with data abstraction, encapsulating processingwith data objects and control access to data, and adding inheritance anddynamic method binding. Inheritance is a powerful concept that greatly enhances the reuse of

existing software which increases software development

productivity. Dynamic (run-time) method binding allows more flexible use of

inheritance.

Languages that support OO methodology: Smalltalk , Ada 95 , Java ,and C++.

In summary : New software development methodologies (e.g., object-oriented software development) led to new language constructs,

programming paradigms and by extension, new programminglanguages.

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The von Neumann Architecture

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Programming Methodologies Influences

1950s and early 1960s : Simple applications; worry about machineefficiency

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

Markup Not a programming language.

It is used to specify the layout of information in Web documents Some programming capabilities has crept into some extensions to

XHTML and XML such as Java Server Pages Standard Taglibrary (JSTL) and eXtensible Stylesheet LanguageTransformations (XSLT).

New: used to specify the layout of information in Web documents Examples: XHTML, XML

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Language Design Trade-Offs Reliability vs. cost of execution - Conflicting criteria

Example : Java demands all references to array elements be checkedfor proper indexing but that leads to increased execution costs

C does not require index range checking - C programs execute fasterthan semantically equivalent Java program although Java are morereliable.

Designers of Java traded execution efficiency for reliability .

Readability vs. writability - Another conflicting criteria APL provides a powerful set of operators for array operands (and a

large number of new symbols), allowing complex computations tobe written in a compact program but at the cost of poor readability .

A huge amount of computation can be specified in a very compactcode. APL programs have very poor readability.

Designers of APL traded readability for writability .

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Language Design Trade-Offs

Writability (flexibility) vs. reliability Another conflicting criteria

C++ pointers are powerful and very flexible but not reliablyused. Pointer are not included in Java

In summary: The task of choosing constructs andfeatures when designing a PL requires manycompromises and trade-offs.

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Implementation Methods Machine language is the only language that most computers understand. A computer can be designed and built to use HLL as its machine

language, but this will be very complex and expensive and highlyinflexible because it would be difficult to use it with other HLLs.

The more practical machine design choice implements in hardware avery low-level language that provides the most commonly neededprimitive operations and requires system software to create an interfaceto programs in HLLs.

In additional to the hardware and the language implementation system,computers need OS.PLs can be implemented by any of 3 methods:

Compilation - Programs are translated into machine language Pure Interpretation Programs are interpreted by another program known as an interpreter

Hybrid Implementation Systems A compromise between compilers and pure interpreters

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Layered View of ComputerThe operating system andlanguage implementationare layered over Machine

interface of a computer.Layers can be thought asvirtual computers providinginterfaces to the user at

higher levels:OS and C compiler provide avirtual C computer.

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Compilation

Translate high-level program ( source language ) into machinecode ( machine language ), which can be executed directly on thecomputer.

Slow translation, fast execution Compilation process has several phases:

Lexical analysis (Scanning) : converts characters in the sourceprogram into lexical units Syntax analysis (Parsing) : 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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Additional Compilation Terminologies

Load module (executable image): the user and system code together. Linking and loading : the process of collecting system program and

linking them to user program. Execution of Machine Code

Fetch-execute-cycle (on a von Neumann architecture) Programs reside in memory but are executed in the CPU.

initialize the program counterrepeat forever

fetch the instruction pointed by the counterincrement the counterdecode the instructionexecute the instruction

end repeat

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Von Neumann Bottleneck

Connection speed between a computers memory and its

processor determines the speed of a computer Program instructions often can be executed a lot faster

than the above connection speed; the connection speed

thus results in a bottleneck Known as von Neumann bottleneck ; it is the primary

limiting factor in the speed of computers.

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Pure Interpretation

Programs are interpreted by another program called interpreter with notranslation.

An interpreter acts as a software simulation of a machine whose fetch-

execute cycle deals with HLL program statements rather than machineinstructions. Easier implementation of programs (run-time errors can easily and

immediately displayed)

Slower execution (10 to 100 times slower than compiled programs). The primary source of slowness is the decoding of HLL statements ,which are more complex that machine language instructions. Loopstatements must be decoded in every iteration.

Bottleneck : Statement decoding rather than CPU-MEM connection. Often requires more space Becoming rare on high-level languages Significant comeback with some Web scripting languages (e.g.,

JavaScript and PHP)

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Pure Interpretation Process

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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.

An intermediate language code is interpreted rather thantranslated.

Faster than pure interpretation : a statement is decoded ONCE .

Perl programs are compiled to detect errors before interpretationand to simplify the interpreter.

Initial implementations of Java were hybrid; Javas intermediate

form- byte code - provides portability to any machine with bytecode interpreter and associated run-time system (Both are calledJava virtual machine). There are new systems that translate bytecode into machine language for faster execution.

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Programming Environments

A programming environment is the collection of tools used insoftware development such as: file system , text editor , compiler ,linker. Several programming environment are:

UNIX An older operating system and tool collection Nowadays often used through a GUI (e.g., CDE, KDE, or

GNOME) that run on top of UNIX Borland JBuilder

Provides an integrated compiler, editor, debugger, and file

system for Java development. Microsoft Visual Studio .NET

A large, complex visual environment

Used to program in 5 .NET languages : J#, Jscript , C#,managed C++, Visual BASIC.NET .

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Summary

The study of programming languages is valuable for a numberof 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 languagesinclude: Readability , writability, reliability , cost

Major influences on language design have been machinearchitecture and software development methodologies The major methods of implementing programming languages

are: compilation , pure interpretation, and hybridimplementation

Documents

332 Ch01