C编程语言教程：从入门到精通

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"《C程序设计语言》是一本关于C语言的经典教程，涵盖了C语言的基础概念和编程技术。书中的内容包括变量、算术表达式、控制流、函数以及类型和运算符等核心主题。" 《C程序设计语言》是C语言学习者的必备参考书，由Brian W. Kernighan和Dennis M. Ritchie合著，通常被称为K&R（KnR）C书。书中以简洁明了的方式介绍了C语言的基本元素和编程技巧。第一章“教程介绍”引导读者逐步进入C语言的世界，从开始编写简单的程序入手，讲解了变量和算术表达式的概念。其中，`for`语句用于循环控制，使程序员能够重复执行一段代码。`symbolic constants`（符号常量）提供了一种定义不可修改的值的方法，增强了代码的可读性。接着，章节详细介绍了字符输入和输出，包括文件复制、字符计数、行计数和单词计数的实现，这些都是基本的文本处理任务。数组是C语言中的重要数据结构，允许存储一组相同类型的元素。此外，还讨论了函数的使用，以及参数传递的机制——值传递。字符数组是处理字符串的关键，而外部变量和作用域的概念则帮助理解变量在程序不同部分的行为。第二章“类型、运算符和表达式”深入探讨了变量命名规则、数据类型及其大小、常量的表示，以及声明语法。各种算术、关系和逻辑运算符被逐一介绍，包括自动类型转换，这在不同数据类型之间的操作中非常重要。增量和减量运算符（`++` 和 `--`）提供了便捷的值增减方式，位运算符则提供了对二进制位的操作。赋值运算符和表达式让程序的计算变得更加灵活。条件表达式（三元运算符）和运算符优先级与结合性是编写高效且清晰代码的关键。第三章“控制流”阐述了如何根据条件改变程序的执行路径，如`if-else`、`else-if`和`switch`语句。`while`、`for`和`do-while`循环提供了不同程度的灵活性，用于重复执行代码块。`break`和`continue`语句用于跳出循环或跳过循环的剩余部分，`goto`和标签则提供了非结构化的控制转移，虽然使用时需谨慎，以免导致复杂的程序结构。第四章“函数和程序结构”继续扩展对函数的理解，包括函数的定义、调用以及它们如何构成程序的结构。本章还可能涉及其他高级话题，如指针、结构体和联合体，以及如何使用这些工具来构建更复杂的数据结构和程序。《C程序设计语言》是一本全面覆盖C语言基础的教科书，无论对于初学者还是有经验的程序员，都能从中获益匪浅，掌握C语言的核心概念和技术。通过这本书，读者将能够编写出高效、可靠的C程序，并为后续学习更高级的计算机科学概念打下坚实基础。

/* 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 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)

++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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