C语言指针详解入门示例与数据结构实现

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"《Pointers on C 英文版》是一本深入浅出的C语言指南，专为理解C语言而设计。该书详尽剖析了指针的概念，包括其语法、有效使用的技巧以及在编程中的常见模式。它通过对比不同的方法来实现常见的抽象数据结构，使读者能够掌握这个关键概念在实际编程中的应用。章节一开始就强调了介绍编程语言的挑战性，因为很多细节只有在具备足够的背景知识后才有意义。作者试图通过分析一个示例程序来展示C语言的大致框架，程序的功能是读取标准输入的文本，进行修改后输出到标准输出。首先，程序读取一串列号，这些是成对出现的，表示输入行中需要处理的列范围，以一个负数作为结束标记。后续的输入行则被逐行读取并打印，但对特定列的操作则是由那些由指针支持的逻辑完成。指针在C语言中扮演着至关重要的角色，它们是内存地址的引用，允许程序员直接操作内存中的数据，从而提高代码的灵活性和效率。书中详细讲解了如何声明、初始化和使用指针，以及不同类型指针（如整型、字符型、数组指针等）的用法。此外，还包括了指针算术、指针与数组的关系、动态内存分配等核心概念。对于初学者而言，章节还介绍了如何通过指针实现数据结构，如链表、树和图等，这些都是C语言编程中常见的抽象数据结构实现方式。通过这些实例，读者不仅能理解指针的工作原理，还能学习到如何利用指针优化算法和解决问题。《Pointers on C 英文版》不仅提供了C语言的基础知识，还深入探讨了指针这一复杂但强大的工具，使读者能够逐步掌握编写高效、灵活的C程序所需的技能。通过阅读这本书，新手程序员可以迅速进入C语言的世界，并逐渐提升他们的编程能力。"

1.1 Introduction

Theȱ nextȱ &&ȱ makesȱ sureȱ thatȱ theȱ numberȱ isȱ testedȱ forȱ aȱ negativeȱ valueȱ onlyȱ ifȱ

scanfȱwasȱsuccessfulȱinȱreadingȱit.ȱTheȱstatementȱ

num += 1;

addsȱ1ȱtoȱtheȱvariableȱnum.ȱItȱisȱequivalentȱtoȱtheȱstatementȱ

num = num + 1;

IȱdiscussȱlaterȱwhyȱCȱprovidesȱtwoȱdifferentȱwaysȱtoȱincrementȱaȱvariable

.ȱ

** Make sure we have an even number of inputs, as they are

** supposed to be paired.

if( num % 2 != 0 ){

puts( "Last column number is not paired." );

exit( EXIT_FAILURE );

}

Thisȱ testȱ checksȱ thatȱ anȱ evenȱ numberȱ ofȱ integersȱ wereȱ entered,ȱ whichȱ isȱ requiredȱ

becauseȱtheȱnumbersȱareȱsupposedȱtoȱbeȱinȱpairs.ȱTheȱ% operatorȱperformsȱanȱintegerȱ

division,ȱ butȱ itȱ givesȱ theȱ remainderȱ ratherȱ thanȱ theȱ quotient.ȱ Ifȱ numȱ isȱ notȱ anȱ evenȱ

number,ȱtheȱremainderȱofȱdividingȱitȱbyȱtwoȱwillȱbeȱnonzero.ȱ

Theȱputsȱfunctionȱisȱtheȱoutputȱversionȱofȱgets;ȱitȱwritesȱtheȱspecifiedȱstringȱtoȱ

theȱstandardȱoutputȱandȱappendsȱaȱnewlineȱcharacterȱtoȱit.ȱTheȱprogramȱthenȱcallsȱtheȱ

exit;ȱfunction,ȱwhichȱterminatesȱitsȱexecution.ȱTheȱvalueȱEXIT_FAILUREȱisȱpassedȱbackȱ

toȱtheȱoperatingȱsystemȱtoȱindicateȱthatȱsomethingȱwasȱwrong.ȱ

** Discard the rest of the line that contained the final

** number.

while( (ch = getchar()) != EOF && ch != '\n' )

;

scanfȱonlyȱreadsȱasȱfarȱasȱitȱhasȱtoȱwhenȱconvertingȱinputȱvalues.ȱTherefore,ȱtheȱ

remainderȱ ofȱ theȱ Lineȱ thatȱ containedȱ theȱ lastȱ valueȱ willȱ stillȱ beȱ outȱ there,ȱ waitingȱ to

ȱWithȱtheȱprefixȱandȱpostfixȱ++ȱoperators,ȱthereȱareȱactuallyȱfourȱwaysȱtoȱincrementȱaȱvariableȱ

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Chapter 1 A Quick Start

16ȱ

beȱread.ȱItȱmayȱcontainȱjustȱtheȱterminatingȱnewline,ȱorȱitȱmayȱcontainȱotherȱcharactersȱ

too.ȱRegardless,ȱthisȱwhileȱloopȱreadsȱandȱdiscardsȱtheȱremainingȱcharactersȱtoȱpreventȱ

themȱfromȱbeingȱinterpretedȱasȱtheȱfirstȱlineȱofȱdata.ȱ

Theȱexpressionȱ

(ch = getchar()) != EOF && ch != '\n'

meritsȱ someȱ discussion.ȱ First,ȱ theȱ functionȱ getcharȱ readsȱ aȱ singleȱ characterȱ fromȱ theȱ

standardȱinputȱandȱ returnsȱitsȱvalue.ȱIfȱthereȱareȱnoȱ moreȱcharactersȱinȱtheȱinput,ȱtheȱ

constantȱEOFȱ(whichȱisȱdefinedȱinȱstdio.h)ȱisȱrammedȱinsteadȱtoȱsignalȱendȬofȬline.ȱ

Theȱ valueȱ returnedȱ byȱ getcharȱ isȱ assignedȱ toȱ theȱ variableȱ ch,ȱ whichȱ isȱ thenȱ

comparedȱ toȱ EOF.ȱ Theȱ parenthesesȱ enclosingȱ theȱ assignmentȱ ensure ȱ thatȱ itȱ isȱ doneȱ

beforeȱtheȱcomparison.ȱIfȱchȱisȱequalȱtoȱEOF,ȱtheȱexpressionȱisȱfalseȱandȱtheȱloopȱstops.ȱ

Otherwise,ȱchȱisȱcomparedȱtoȱ aȱ newline;ȱ again,ȱ theȱ loopȱ stopsȱ ifȱtheyȱ areȱ foundȱ toȱ beȱ

equal.ȱ Thus,ȱ theȱ expressionȱ isȱ trueȱ {causingȱ theȱloopȱ toȱ runȱagain)ȱ onlyȱ ifȱendȱ ofȱ lineȱ

wasȱnotȱreachedȱandȱtheȱcharacterȱreadȱwasȱnotȱaȱnewline.ȱThus,ȱtheȱloopȱdiscardsȱtheȱ

remainingȱcharactersȱonȱtheȱcurrentȱinputȱline.ȱ

Nowȱlet’sȱmoveȱonȱtoȱtheȱinterestingȱpart.ȱInȱmostȱotherȱlanguages,ȱweȱ wouldȱ

haveȱwrittenȱtheȱloopȱlikeȱthis:ȱ

ch = getchar();

while( ch != EOF && ch != '\n' )

ch = getchar();

Getȱ aȱ character,ȱ thereȱ ifȱ weȇveȱ notȱ yetȱ reachedȱ endȱ ofȱ tileȱ orȱ gottenȱ aȱ newline,ȱ

getȱanotherȱcharacter.ȱNoteȱthatȱthereȱareȱtwoȱcopiesȱofȱtheȱstatement.ȱȱ

ȱ ch = getchar();

TheȱabilityȱtoȱembedȱtheȱassignmentȱinȱtheȱwhileȱstatementȱallowsȱtheȱCȱprogrammerȱ

toȱeliminateȱthisȱredundantȱstatement.ȱ

Theȱ loopȱ inȱ theȱ sampleȱ programȱ hasȱtheȱ sameȱ functionalityȱ asȱ theȱ oneȱ shownȱ above,ȱ

butȱ itȱ containsȱ oneȱ fewerȱ statement. ȱ Itȱ isȱ admittedlyȱ harderȱ toȱ road,ȱ andȱ oneȱ couldȱ

makeȱaȱconvincingȱargumentȱthatȱthisȱcodingȱtechniqueȱshouldȱbeȱavoidedȱforȱjustȱthatȱ

reason.ȱ However,ȱ most,ȱ ofȱ theȱ difficultyȱ inȱ readingȱ isȱ dueȱ coȱ inexperienceȱ withȱ theȱ

languageȱ andȱ itsȱ idioms;ȱ experiencedȱ Cȱ programmersȱ haveȱ noȱ troubleȱ readingȱ (andȱ

writing)ȱ statementsȱ suchȱ asȱ thisȱ one.ȱ Youȱ shouldȱ avoidȱ makingȱ codeȱ harderȱ toȱ readȱ

whenȱthereȱisȱnoȱtangibleȱbenefitȱtoȱbeȱgainedȱfromȱit,ȱbutȱtheȱmaintenanceȱadvantageȱ

inȱnotȱhavingȱmultipleȱcopiesȱofȱcodeȱmoreȱthanȱjustifiesȱthisȱcommonȱcodingȱidiom.ȱ

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1.1 Introduction

Aȱ questionȱ frequentlyȱ askedȱ isȱ whyȱ chȱ isȱ declaredȱ asȱ anȱ integerȱ whenȱ weȱ areȱ

usingȱ itȱ toȱ readȱ characters?ȱ Theȱ answerȱ isȱ thatȱ EOFȱ isȱ anȱ integerȱ valueȱ thatȱ requiresȱ

moreȱbitsȱthanȱareȱavailableȱinȱaȱcharacterȱvariable;ȱthisȱfactȱpreventsȱaȱcharacterȱinȱthe ȱ

inputȱ fromȱ accidentallyȱ beingȱ interpretedȱ asȱ EOF.ȱ Butȱ itȱ alsoȱ meansȱ thatȱ ch,ȱ whichȱ isȱ

receivingȱ theȱ characters,ȱ mustȱ beȱ largeȱ enoughȱ toȱ holdȱ EOFȱ 100,ȱ whichȱ isȱ whyȱ anȱ

integerȱisȱused.ȱAsȱdiscussedȱinȱChapterȱ3,ȱcharactersȱareȱjustȱtinyȱintegersȱanyway,ȱsoȱ

usingȱanȱintegerȱvariableȱtoȱholdȱcharacterȱvaluesȱcausesȱnoȱproblems.ȱ

Oneȱ finalȱ commentȱ onȱ thisȱ fragmentȱ ofȱ theȱ program:ȱ thereȱ areȱ noȱ statementsȱ inȱ theȱ

bodyȱ ofȱ theȱ whileȱ statement.ȱ Itȱ turnsȱ outȱ thatȱ theȱ workȱ doneȱ toȱ evaluateȱ theȱ whileȱ

expressionȱisȱallȱthatȱisȱneeded,ȱsoȱthereȱisȱnothingȱleft forȱtheȱbodyȱofȱtheȱloopȱtoȱdo.ȱ

Youȱ willȱ encounterȱ suchȱ loopsȱ occasionally,ȱ andȱ handlingȱ themȱ isȱ noȱ problem.ȱ Theȱ

solitaryȱsemicolonȱafterȱtheȱwhileȱstatementȱisȱcalledȱtheȱemptyȱstatement,ȱandȱitȱisȱusedȱ

inȱsituationsȱlikeȱthisȱoneȱwhereȱtheȱsyntaxȱrequiresȱaȱstatementȱbutȱthereȱisȱnoȱworkȱtoȱ

beȱ done.ȱ Theȱ semicolonȱ isȱ onȱ aȱ lineȱ byȱ itselfȱ inȱ orderȱ toȱ preventȱ theȱ readerȱ fromȱ

mistakenlyȱassumingȱthatȱtheȱnextȱstatementȱisȱmeȱbodyȱofȱtheȱloop.ȱ

return num;

}

Theȱreturnȱ statementȱisȱhowȱ aȱfunctionȱreturnsȱ aȱvalueȱtoȱtheȱexpressionȱfromȱ

whichȱitȱwasȱcalled.ȱInȱthisȱcase,ȱtheȱvalueȱofȱtheȱvariableȱnumȱisȱreturnedȱtoȱtheȱcallingȱ

program,ȱwhereȱitȱisȱassignedȱtoȱtheȱmainȱprogramȇsȱvariableȱn_columns.ȱ

1.1.5 The rearrange Function

** Process a line of input by concatenating the characters from

** the indicated columns. The output line is the NUL terminated,

void

rearrange( char *output, char const *input,

in n_columns, int const columns[] )

{

int col; /* subscript for columns array */

int output_col; /* output column counter */

int len; /* length of input line */

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Chapter 1 A Quick Start

18ȱ

Theseȱstatementsȱdefineȱtheȱrearrangeȱfunctionȱandȱdeclareȱsomeȱlocalȱvariablesȱ

forȱit.ȱTheȱmostȱ interestingȱ pointȱ hereȱ isȱ thatȱtheȱfirstȱtwoȱ parametersȱ areȱ declaredȱ asȱ

pointersȱbutȱarrayȱnamesȱareȱpassedȱasȱargumentsȱwhenȱtheȱfunctionȱisȱcalled.ȱWhenȱ

anȱ arrayȱ nameȱ isȱusedȱasȱ anȱ argument,ȱ whatȱ isȱ passedȱ toȱ theȱ functionȱ isȱ aȱ pointerȱtoȱ

theȱ beginningȱ ofȱ theȱ array,ȱ whichȱ isȱ actuallyȱ theȱ addressȱ whereȱ theȱ arrayȱ residesȱ inȱ

memory.ȱTheȱfactȱthatȱaȱpointerȱisȱpassedȱratherȱthanȱatȱcopyȱofȱtheȱarrayȱisȱwhatȱgivesȱ

arraysȱtheirȱcallȱbyȱreferenceȱsemantics.ȱTheȱfunctionȱcanȱmanipulateȱtheȱargumentȱasȱ

aȱpointer,ȱorȱitȱcanȱuseȱaȱsubscriptȱwithȱtheȱargumentȱjustȱasȱwithȱanȱarrayȱname.ȱTheseȱ

techniquesȱareȱdescribedȱinȱmoreȱdetailȱinȱChapterȱ8.ȱ

Becauseȱ ofȱ theȱ callȱ byȱ referenceȱ semantics,ȱ though,ȱ ifȱ theȱ functionȱ modifiesȱ

elementsȱofȱtheȱparameterȱarray,ȱitȱactuallyȱmodifiesȱtheȱcorrespondingȱelementsȱof ȱtheȱ

argumentȱ array.ȱ Thus,ȱ declaringȱ columnsȱ toȱ beȱ constȱ isȱ usefulȱ inȱ twoȱ ways.ȱ First,ȱ itȱ

statesȱ thatȱ theȱ intentionȱ ofȱ theȱ functionȇsȱ authorȱ isȱ thatȱ thisȱ parameterȱ isȱ notȱ toȱ beȱ

modified.ȱ Second,ȱ itȱ causesȱ theȱ compilerȱ toȱ verifyȱ thatȱ thisȱ intentionȱ isȱ notȱ violated.ȱ

Thus,ȱ callersȱ ofȱ thisȱ functionȱ needȱ notȱ worryȱ aboutȱ theȱ possibilityȱ ofȱ elementsȱ ofȱ meȱ

arrayȱpassedȱasȱtheȱfourthȱargumentȱbeingȱchanged.ȱ

len = strlen( input );

output_col = 0;

** Process each pair of column numbers.

for( col = 0; col < n_columns; col += 2 ){

Theȱ realȱ workȱ ofȱ theȱ functionȱ beginsȱ here.ȱ We ȱ firstȱ getȱ theȱ lengthȱ ofȱ theȱ inputȱ

string,ȱsoȱweȱcanȱskipȱcolumnȱnumbersȱthatȱareȱbeyondȱtheȱendȱofȱtheȱinput.ȱTheȱforȱ

statementȱinȱCȱisȱnotȱquiteȱlikeȱotherȱlanguages;ȱitȱisȱmoreȱofȱatȱshorthandȱnotationȱforȱ

aȱ commonlyȱ usedȱ styleȱ ofȱ whileȱ statement.ȱ Theȱ forȱ statementȱ containsȱ threeȱ

expressionsȱ (allȱ ofȱ whichȱ areȱ optional,ȱ byȱ theȱ way).ȱ Theȱ firstȱ expressionȱ isȱ theȱ

initializationȱandȱisȱevaluatedȱonceȱbeforeȱtheȱloopȱbegins.ȱTheȱsecondȱisȱtheȱtestȱandȱisȱ

evaluatedȱbeforeȱeachȱiterationȱofȱtheȱloop;ȱifȱtheȱresultȱisȱfalseȱtheȱloopȱterminates.ȱTheȱ

thirdȱ

expression,ȱ isȱ theȱ adjustmentȱwhichȱisȱevaluatedȱ atȱ theȱ endȱ ofȱ eachȱ iterationȱ justȱ

beforeȱ theȱ testȱ isȱ evaluated.ȱ Toȱ illustrate,ȱ theȱ

forȱ loopȱ thatȱ beginsȱ aboveȱ couldȱ beȱ

rewrittenȱasȱaȱwhileȱloop:ȱ

col = 0;

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