C语言编程：英文第二版精华解读

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"《C程序设计语言》是第二版的英文版，由Prentice Hall出版，是一本经典的C语言教程。这本书旨在引导初学者入门C语言编程，同时对已经熟悉C语言的开发者也有深入的指导价值。书中涵盖了一系列基础到高级的C语言概念和技术，包括变量、表达式、控制流、函数以及程序结构等。” 《C程序设计语言》第一版序言介绍了该书的初衷和目标，而第二版在此基础上可能进行了更新和改进，以适应编程语言的发展和新的教学需求。第一章“教程介绍”是学习C语言的起点，内容包括： 1.1 开始编程：如何设置开发环境，编译和运行你的第一个C程序。 1.2 变量与算术表达式：讲解了C语言中的基本数据类型和算术运算。 1.3 for语句：C语言中最常用的循环结构之一。 1.4 符号常量：用#define定义常量及其在代码中的作用。 1.5 字符输入与输出：介绍标准输入输出函数如printf和scanf，以及简单的文件操作，如文件复制。 1.5.1 文件复制示例：演示如何读取一个文件并将其内容写入另一个文件。 1.5.2 字符计数：计算文件中字符的数量。 1.5.3 行计数：统计文件中的行数。 1.5.4 单词计数：识别并计数文件中的单词数量。 1.6 数组：用于存储一组相同类型的数据。 1.7 函数：将代码组织成可重复使用的模块。 1.8 参数传递：讨论函数调用时参数值的传递方式——按值传递。 1.9 字符数组：特别关注字符串的处理，因为C语言中字符串本质上是字符数组。 1.10 外部变量与作用域：了解变量在不同范围内的可见性。第二章“类型、运算符和表达式”进一步深入到C语言的核心语法： 2.1 变量命名规则：如何为变量选择合法的标识符。 2.2 数据类型和大小：了解C语言中的基本数据类型，如int、char、float等，以及它们的内存占用。 2.3 常量：包括数值常量、字符常量和字符串常量。 2.4 声明：声明变量、常量以及函数。 2.5 算术运算符：加减乘除、取模等操作。 2.6 关系和逻辑运算符：比较（==, !=, <, >, <=, >=）和逻辑（&&, ||, !）运算。 2.7 类型转换：自动和强制类型转换的概念。 2.8 自增自减运算符：++和--在变量前后的不同用法。 2.9 位运算符：对二进制位进行操作的运算符，如按位与、或、非、异或。 2.10 赋值运算符和表达式：赋值操作以及复杂的赋值表达式。 2.11 条件表达式：三目运算符（? :）的使用。 2.12 运算符优先级和结合性：理解哪些运算符先执行，以及如何解析复杂的表达式。第三章“控制流”探讨了决定程序执行路径的结构： 3.1 语句和块：简单语句和复合语句（用花括号包围的一组语句）。 3.2 if-else：条件判断的基本形式。 3.3 else-if：多个条件分支的处理。 3.4 switch：多路选择结构。 3.5 循环：while和for循环的使用。 3.6 do-while：至少执行一次的循环结构。 3.7 break和continue：跳出循环或跳过剩余循环体的控制语句。 3.8 goto和标签：不推荐但有时必要的无条件跳转。第四章“函数和程序结构”则专注于如何组织代码和重用功能： 4.x 这一部分可能涵盖函数的更深入话题，如递归、函数指针、局部和全局变量的管理，以及如何通过头文件和库来组织大型程序。这本书通过实例和清晰的解释，帮助读者逐步掌握C语言，为后续的系统编程、嵌入式开发、算法实现等高级主题打下坚实的基础。无论是初学者还是有经验的程序员，都能从中受益。

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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