C语言编程教程（第二版-英文版）

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"《C程序设计语言（第二版-英文版）》是C语言的经典教材，英文版被认为能提供更好的学习体验。本书旨在教授C语言的基础知识，包括变量、算术表达式、控制结构、数组、函数、类型、运算符、表达式以及流程控制等核心概念。" 本书分为多个章节，每章都深入浅出地介绍了一个关键主题： 1. 序言：在第一版的序言中，作者可能会讨论C语言的发展历程和编写本书的目的，为读者提供学习的背景和动力。 2. 第1章 - 实用教程介绍：这一章引导初学者入门，讲解如何开始编写C程序。内容涵盖变量、算术表达式、for循环、符号常量、字符输入输出。还通过几个实例，如文件复制、字符计数、行计数和单词计数，来演示基本的I/O操作和数组应用。 3. 第2章 - 类型、运算符和表达式：详细介绍变量命名规则、数据类型和大小、常量、声明、算术、关系和逻辑运算符、类型转换、增量和减量运算符、位运算符、赋值运算符和表达式、条件表达式，以及运算符的优先级和求值顺序。 4. 第3章 - 控制流：讲解了各种控制语句，如if-else、else-if、switch语句，以及不同类型的循环（while、for和do-while），还包括break、continue语句的使用，以及goto语句和标签的作用，这些都是程序流程控制的关键。 5. 第4章 - 函数（未完全显示）：这部分可能涵盖了函数的详细使用，包括函数定义、参数传递（调用时传值）、函数返回值等。 6. 其他章节（未列出）：可能涉及到更高级的主题，如指针、内存管理、结构体、位字段、预处理器、错误处理和输入/输出系统调用等。通过这本书，读者不仅可以学习到C语言的基本语法和编程技巧，还能了解到如何组织和优化代码，为今后的软件开发打下坚实的基础。由于是英文版，读者在学习过程中还可以提升英语阅读能力，更好地理解计算机科学的专业术语。

/* 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

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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.

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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.

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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 of

!= 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 c 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)

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++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 nc. 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 %f 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 or 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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