C语言编程入门：经典教程第二版

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"《C语言编程-The.C.Programming.Language.Second.Edition》是C语言的经典教材，由国外专家撰写，深入浅出地介绍了C语言的核心概念和技术。本书旨在帮助读者掌握C语言的基础和高级特性，是学习和工作的理想参考资料。" 在书中，作者首先通过【 Preface】和【Prefacetothefirstedition】为读者提供了对本书的背景和目标的了解，强调了C语言的重要性及其在编程领域的地位。第一章【1-ATutorialIntroduction】是C语言的入门教程，包括以下几个关键知识点： 1.1 【GettingStarted】引导读者开始学习C语言，讲解如何设置开发环境和编写第一个程序。 1.2 【VariablesandArithmeticExpressions】介绍了变量和算术表达式的基本概念，这是理解任何编程语言的基础。 1.3 【Theforstatement】详细讲解了C语言中的for循环，这是最常见的循环结构之一。 1.4 【SymbolicConstants】提到了符号常量的使用，它们可以提高代码的可读性。 1.5 【CharacterInputandOutput】涵盖了字符输入输出，包括： - 1.5.1【FileCopying】教授如何复制文件，这是基本的I/O操作示例。 - 1.5.2【CharacterCounting】介绍字符计数功能，展示了简单的统计方法。 - 1.5.3【LineCounting】讲解行计数，与字符计数相关。 - 1.5.4【WordCounting】解释词数统计，进一步拓展了文本处理能力。 1.6 【Arrays】讨论数组，它是存储和处理多个数据项的重要工具。 1.7 【Functions】介绍了函数的概念，它是代码模块化的基础。 1.8 【Arguments-CallbyValue】解释了参数传递的值传递方式。 1.9 【CharacterArrays】特别讨论了字符数组，如字符串。 1.10 【ExternalVariablesandScope】涵盖了外部变量和作用域，这是理解变量可见性和生命周期的关键。第二章【2-Types,OperatorsandExpressions】深入到C语言的数据类型、运算符和表达式： 2.1 【VariableNames】规定了变量命名的规则和约定。 2.2 【DataTypesandSizes】详细介绍了C语言的数据类型及其内存占用。 2.3 【Constants】讲解常量的定义和使用。 2.4 【Declarations】阐述了声明的语法和意义。 2.5 至2.11详细探讨了各种运算符，包括： - 2.5 【ArithmeticOperators】算术运算符：加、减、乘、除等。 - 2.6 【RelationalandLogicalOperators】关系和逻辑运算符：比较和布尔逻辑。 - 2.7 【TypeConversions】类型转换：自动和强制转换。 - 2.8 【IncrementandDecrementOperators】自增和自减运算符，简化代码并影响变量值。 - 2.9 【BitwiseOperators】位运算符：在二进制级别操作数据。 - 2.10 【AssignmentOperatorsandExpressions】赋值运算符和表达式：组合赋值和表达式的计算。 - 2.11 【ConditionalExpressions】条件表达式（三元运算符），根据条件选择值。 2.12 【PrecedenceandOrderofEvaluation】解释了运算符的优先级和求值顺序，这对于正确计算表达式至关重要。第三章【3-ControlFlow】关注程序的控制流，如何根据条件和循环改变执行路径： 3.1 【StatementsandBlocks】概述了语句和代码块的结构。 3.2 至3.4详细介绍了条件语句： - 3.2 【If-Else】基础的if-else结构。 - 3.3 【Else-If】更复杂的if-else-if结构。 - 3.4 【Switch】switch语句，用于多条件分支。 3.5 至3.8涉及循环控制： - 3.5 【Loops-WhileandFor】while和for循环的用法。 - 3.6 【Loops-Do-While】do-while循环，确保至少执行一次。 - 3.7 【BreakandContinue】break和continue语句，控制循环的退出和跳过。 - 3.8 【Gotoandlabels】goto语句和标签，虽然不推荐，但在某些情况下仍可能使用。第四章【4-FunctionsandProgramStructure】则深入到函数的使用和程序结构，包括函数的声明、定义、调用以及更复杂的程序组织。这本书涵盖了C语言的各个方面，适合初学者和有一定经验的程序员，通过实例和练习帮助读者逐步提升C语言编程技能。无论是对C语言的初探还是深入研究，它都是一个不可或缺的参考。

Width and precision may be omitted from a specification: %6f says that the number is to be at

least six characters wide; %.2f specifies two characters after the decimal point, but the width

is not constrained; and %f merely says to print the number as floating point.

print as decimal integer

%6d

print as decimal integer, at least 6 characters wide

print as floating point

%6f

print as floating point, at least 6 characters wide

%.2f

print as floating point, 2 characters after decimal point

%6.2f

print as floating point, at least 6 wide and 2 after decimal point

Among others, printf also recognizes %o for octal, %x for hexadecimal, %c for character, %s

for character string and %% for itself.

Exercise 1-3. Modify the temperature conversion program to print a heading above the table.

Exercise 1-4. Write a program to print the corresponding Celsius to Fahrenheit table.

1.3 The for statement

There are plenty of different ways to write a program for a particular task. Let's try a variation

on the temperature converter.

#include <stdio.h>

/* print Fahrenheit-Celsius table */

main()

{

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 is

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 c 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 c 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 c to be a type

big enough to hold any value that getchar returns. We can't use char since c 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 c 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 c, 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)

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