C编程语言基础教程

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"《C编程语言》是由Kernighan & Ritchie编写的经典教材，深入浅出地介绍了C语言的基础知识和编程技巧。" 在《C编程语言》这本书中，作者通过清晰的结构和易懂的例子，为读者提供了一个学习C语言的全面教程。书中的章节覆盖了C语言的核心概念和语法，是初学者和有经验的程序员的宝贵资源。首先，书中第一章是"教程介绍"，它以简单易懂的方式引导读者进入C语言的世界。1.1节"开始"介绍了如何设置编程环境并编写第一个简单的C程序。1.2节"变量与算术表达式"讲解了变量的概念和基本的算术运算。1.3节"for语句"介绍了C语言中常用的循环结构。1.4节"符号常量"让读者了解如何定义不可修改的常量。1.5节涉及字符输入输出，包括文件复制、字符计数、行计数和单词计数的实例。1.6节"数组"介绍了如何存储和操作一组数据。1.7节"函数"讲解了如何定义和调用函数来组织代码。1.8节"按值传递参数"解释了函数调用时参数传递的工作原理。1.9节"字符数组"涵盖了字符串的处理。1.10节"外部变量和作用域"讨论了变量在不同作用域内的可见性和生命周期。第二章"类型、运算符和表达式"更深入地探讨了C语言的语法。2.1节"变量名"规定了变量命名的规则。2.2节"数据类型和大小"列出了C语言的基本数据类型及其内存占用。2.3节"常量"介绍了如何声明和使用常量。2.4节"声明"讲解了变量和类型的声明方式。2.5至2.11节分别介绍了算术、关系、逻辑、类型转换、增量和减量、位运算以及赋值运算符。2.12节"条件表达式"和2.13节"运算符优先级和求值顺序"帮助读者理解复杂的表达式如何被解析和执行。第三章"控制流"讲述了程序的流程控制结构。3.1节"语句和块"定义了语句的组成。3.2至3.4节分别讲解了"if-else"、"else-if"和"switch"语句用于条件判断。3.5至3.6节介绍了"while"和"for"循环以及"do-while"循环。3.7和3.8节"break"和"continue"关键字以及"goto"和标签则提供了控制流程的其他手段。第四章"函数和程序结构"进一步扩展了对函数的理解，讨论了如何设计和组织大型程序，包括函数的重用和模块化编程。通过这些章节的学习，读者将能够掌握C语言的基本语法和编程思想，为后续的C语言开发和进阶学习打下坚实基础。

{

intfahr;

for(fahr=0;fahr<=300;fahr=fahr+20)

printf("%3d%6.1f\n",fahr,(5.0/9.0)*(fahr-32));

}

This produces the same answers, but it certainly looks different. One major change is the

elimination of most of the variables; only

fahr

remains, and we have made it an

int

. The

lower and upper limits and the step size appear only as constants in the

for

statement, itself a

new construction, and the expression that computes the Celsius temperature now appears as

thethirdargumentof

printf

insteadofaseparateassignmentstatement.

This last change is an instance of a general rule - in any context where it is permissible to use

the value of some type, you can use a more complicated expression of that type. Since the

third argument of

printf

must be a floating-point value to match the

%6.1f

, any floating-

pointexpressioncanoccurhere.

The

for

statement is a loop, a generalization of the

while

. If you compare it to the earlier

while

, its operation should be clear. Within the parentheses, there are three parts, separated

bysemicolons.Thefirstpart,theinitialization

fahr=0

isdoneonce,beforetheloopproperisentered.Thesecondpartisthe

testorconditionthatcontrolstheloop:

fahr<=300

This condition is evaluated; if it is true, the body of the loop (here a single

ptintf

) is

executed.Thentheincrementstep

fahr=fahr+20

is executed, and the condition re-evaluated. The loop terminates if the condition has become

false. As with the

while

, the body of the loop can be a single statement or a group of

statements enclosed in braces. The initialization, condition and increment can be any

expressions.

The choice between

while

and

for

is arbitrary, based on which seems clearer. The

for

usually appropriate for loops in which the initialization and increment are single statements

and logically related, since it is more compact than

while

and it keeps the loop control

statementstogetherinoneplace.

Exercise1-5.Modifythetemperatureconversionprogramtoprintthetableinreverseorder,

thatis,from300degreesto0.

1.4SymbolicConstants

A final observation before we leave temperature conversion forever. It's bad practice to bury

``magic numbers''like 300 and 20 in a program; they convey little information to someone

who might have to read the program later, and they are hard to change in a systematic way.

One way to deal with magic numbers is to give them meaningful names. A

#define

line

definesasymbolicnameorsymbolicconstanttobeaparticularstringofcharacters:



#define

namereplacementlist

Thereafter, any occurrence of name (not in quotes and not part of another name) will be

replaced by the corresponding replacement text. The name has the same form as a variable

name: a sequence of letters and digits that begins with a letter. The replacement text can be

anysequenceofcharacters;itisnotlimitedtonumbers.

#include<stdio.h>

#defineLOWER0/*lowerlimitoftable*/

#defineUPPER300/*upperlimit*/

#defineSTEP20/*stepsize*/

/*printFahrenheit-Celsiustable*/

main()

{

intfahr;

for(fahr=LOWER;fahr<=UPPER;fahr=fahr+STEP)

printf("%3d%6.1f\n",fahr,(5.0/9.0)*(fahr-32));

}

The quantities

LOWER

UPPER

and

STEP

are symbolic constants, not variables, so they do not

appear in declarations. Symbolic constant names are conventionally written in upper case so

they can ber readily distinguished from lower case variable names. Notice that there is no

semicolonattheendofa

#define

line.

1.5CharacterInputandOutput

We are going to consider a family of related programs for processing character data. You will

findthatmanyprogramsarejustexpandedversionsoftheprototypesthatwediscusshere.

The model of input and output supported by the standard library is very simple. Text input or

output, regardless of where it originates or where it goes to, is dealt with as streams of

characters. A text stream is a sequence of characters divided into lines; each line consists of

zero or more characters followed by a newline character. It is the responsibility of the library

to make each input or output stream confirm this model; the C programmer using the library

neednotworryabouthowlinesarerepresentedoutsidetheprogram.

The standard library provides several functions for reading or writing one character at a time,

of which

getchar

and

putchar

are the simplest. Each time it is called,

getchar

reads the

nextinputcharacterfromatextstreamandreturnsthatasitsvalue.Thatis,after

c=getchar();

the variable

contains the next character of input. The characters normally come from the

keyboard;inputfromfilesisdiscussedinChapter7.

Thefunction

putchar

printsacharactereachtimeitiscalled:

putchar(c);

prints the contents of the integer variable

as a character, usually on the screen. Calls to

putchar

and

printf

may be interleaved; the output will appear in the order in which the

callsaremade.

1.5.1FileCopying

Given

getchar

and

putchar

, you can write a surprising amount of useful code without

knowing anything more about input and output. The simplest example is a program that

copiesitsinputtoitsoutputonecharacteratatime:

readacharacter

while(charaterisnotend-of-fileindicator)

outputthecharacterjustread

readacharacter

ConvertingthisintoCgives:

#include<stdio.h>

/*copyinputtooutput;1stversion*/

main()

{

intc;

c=getchar();

while(c!=EOF){

putchar(c);

c=getchar();

}

}

Therelationaloperator

means``notequalto''.

What appears to be a character on the keyboard or screen is of course, like everything else,

stored internally just as a bit pattern. The type

char

is specifically meant for storing such

character data, but any integer type can be used. We used

int

for a subtle but important

reason.

The problem is distinguishing the end of input from valid data. The solution is that

getchar

returns a distinctive value when there is no more input, a value that cannot be confused with

any real character. This value is called

EOF

, for ``end of file''. We must declare

to be a type

big enough to hold any value that

getchar

returns. We can't use

char

since

must be big

enoughtohold

EOF

inadditiontoanypossible

char

.Thereforeweuse

int

.

EOF

is an integer defined in <stdio.h>, but the specific numeric value doesn't matter as long as

it is not the same as any

char

value. By using the symbolic constant, we are assured that

nothingintheprogramdependsonthespecificnumericvalue.

TheprogramforcopyingwouldbewrittenmoreconciselybyexperiencedCprogrammers.In

C,anyassignment,suchas

c=getchar();

is an expression and has a value, which is the value of the left hand side after the assignment.

This means that a assignment can appear as part of a larger expression. If the assignment of a

character to

is put inside the test part of a

while

loop, the copy program can be written this

way:

#include<stdio.h>

/*copyinputtooutput;2ndversion*/

main()

{

intc;

while((c=getchar())!=EOF)

putchar(c);

}

The

while

gets a character, assigns it to

, and then tests whether the character was the end-

of-file signal. If it was not, the body of the

while

is executed, printing the character. The

while

then repeats. When the end of the input is finally reached, the

while

terminates and so

does

main

.

This version centralizes the input - there is now only one reference to

getchar

- and shrinks

the program. The resulting program is more compact, and, once the idiom is mastered, easier

to read. You'll see this style often. (It's possible to get carried away and create impenetrable

code,however,atendencythatwewilltrytocurb.)

The parentheses around the assignment, within the condition are necessary. The precedence

is higher than that of

, which means that in the absence of parentheses the relational

test

wouldbedonebeforetheassignment

.Sothestatement

c=getchar()!=EOF

isequivalentto

c=(getchar()!=EOF)

This has the undesired effect of setting

to 0 or 1, depending on whether or not the call of

getchar

returnedendoffile.(MoreonthisinChapter2.)

Exercsise1-6.Verifythattheexpression

getchar()!=EOF

is0or1.

Exercise1-7.Writeaprogramtoprintthevalueof

EOF

.

1.5.2CharacterCounting

Thenextprogramcountscharacters;itissimilartothecopyprogram.

#include<stdio.h>

/*countcharactersininput;1stversion*/

main()

{

longnc;

nc=0;

while(getchar()!=EOF)

++nc;

printf("%ld\n",nc);

}

Thestatement

++nc;

presents a new operator,

, which means increment by one. You could instead write

nc =

nc + 1

but

++nc

is more concise and often more efficient. There is a corresponding operator

todecrementby1.Theoperators

and

canbeeitherprefixoperators(

++nc

)orpostfix

operators (

nc++

); these two forms have different values in expressions, as will be shown in

Chapter2, but

++nc

and

nc++

both increment

. For the moment we will will stick to the

prefixform.

The character counting program accumulates its count in a

long

variable instead of an int.

long

integers are at least 32 bits. Although on some machines,

int

and

long

are the same

size, on others an

int

is 16 bits, with a maximum value of 32767, and it would take relatively

little input to overflow an

int

counter. The conversion specification

%ld

tells

printf

that the

correspondingargumentisa

long

integer.

It may be possible to cope with even bigger numbers by using a

double

(double precision

float

). We will also use a

for

statement instead of a

while

, to illustrate another way to write

theloop.

#include<stdio.h>

/*countcharactersininput;2ndversion*/

main()

{

doublenc;

for(nc=0;gechar()!=EOF;++nc)

;

printf("%.0f\n",nc);

}

printf

uses

for both

float

and

double

;

%.0f

suppresses the printing of the decimal point

andthefractionpart,whichiszero.

The body of this

for

loop is empty, because all the work is done in the test and increment

parts. But the grammatical rules of C require that a

for

statement have a body. The isolated

semicolon, called a null statement, is there to satisfy that requirement. We put it on a separate

linetomakeitvisible.

Before we leave the character counting program, observe that if the input contains no

characters, the

while

for

test fails on the very first call to

getchar

, and the program

produces zero, the right answer. This is important. One of the nice things about

while

and

for

is that they test at the top of the loop, before proceeding with the body. If there is nothing

to do, nothing is done, even if that means never going through the loop body. Programs

should act intelligently when given zero-length input. The

while

and

for

statements help

ensurethatprogramsdoreasonablethingswithboundaryconditions.

1.5.3LineCounting

The next program counts input lines. As we mentioned above, the standard library ensures

that an input text stream appears as a sequence of lines, each terminated by a newline. Hence,

countinglinesisjustcountingnewlines:

#include<stdio.h>

/*countlinesininput*/

main()

{

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