C语言经典：K&R第二版，完美版式与链接

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"C语言(K&R)带目录带链接,完美排版——这是一本关于C语言编程的经典著作，由Kernighan和Ritchie合著，第二版。书中内容排版精美，包含了详细的目录及内部链接，是学习C语言的必备参考书。" 在C语言的学习中，《The C Programming Language》(通常称为K&R) 是一本不可或缺的经典教材。它由C语言的开发者之一Brian Kernighan和Dennis Ritchie共同撰写，第二版详细介绍了C语言的基础和高级特性，适合初学者和有经验的程序员阅读。 1. **简介** - 本书首先通过一个简短的前言介绍C语言的历史和目的，引导读者进入C的世界。 - 第一章是一个教程式的入门，帮助读者快速上手，包括如何开始编写程序、变量与算术表达式、for循环、符号常量、字符输入输出等基础概念。 2. **变量、类型、运算符和表达式** - 第二章深入讲解变量命名规则、数据类型及其大小、常量、声明、算术运算符、关系和逻辑运算符、类型转换、增量和减量运算符、位运算符、赋值运算符以及条件表达式。这些是构建C程序的基本元素。 3. **控制流** - 第三章讨论了程序执行的控制结构，如语句和块、if-else语句、else-if语句、switch语句、不同类型的循环（while、for、do-while），以及break和continue语句的用法。此外，还涵盖了goto语句和标签，虽然它们在结构化编程中不被推荐，但在某些特定情况下仍然有用。 4. **函数** - 第四章详细介绍了函数的概念，包括函数定义、调用、参数传递（值传递）、局部和全局变量，以及如何处理函数返回值。 5. **数组和指针** - 后续章节会涉及数组，特别是如何使用数组进行数据存储，以及指针的使用，这是C语言的一大特色。指针允许直接操作内存地址，增强了程序的灵活性和效率。 6. **结构体和联合** - 书中还将讨论结构体（structs）和联合（unions），它们允许将不同类型的数据组合成一个单一的结构，是实现复杂数据结构的基础。 7. **预处理器、输入输出和错误处理** - 预处理器（预处理指令如#include和#define）用于在编译前修改源代码。标准输入输出库（stdio.h）和错误处理是程序与用户交互和调试的关键部分。 8. **内存管理** - 动态内存分配（malloc和free）和内存管理对于创建复杂的程序至关重要，书中也会涉及这部分内容。 9. **位字段和枚举** - 位字段（bit fields）允许在结构中高效地存储位，而枚举（enumerations）提供了一种定义整数常量的简洁方式。 10. **文件操作** - 最后，文件I/O部分介绍了如何读写文件，这对于实现持久化数据存储至关重要。这本书以其清晰的解释和简洁的示例代码，深受程序员喜爱，是掌握C语言精髓的重要参考资料。每个章节都包含大量实例，读者可以通过实践来巩固所学知识。通过阅读和理解K&R，你可以系统地学习C语言，并具备编写高效、可靠的C程序的能力。

}

The two lines

/* print Fahrenheit-Celsius table

for fahr = 0, 20, ..., 300 */

are a comment, which in this case explains briefly what the program does. Any characters between /* and */

are ignored by the compiler; they may be used freely to make a program easier to understand. Comments may

appear anywhere where a blank, tab or newline can.

In C, all variables must be declared before they are used, usually at the beginning of the function before any

executable statements. A declaration announces the properties of variables; it consists of a name and a list of

variables, such as

int fahr, celsius;

int lower, upper, step;

The type int means that the variables listed are integers; by contrast with float, which means floating

point, i.e., numbers that may have a fractional part. The range of both int and float depends on the

machine you are using; 16-bits ints, which lie between -32768 and +32767, are common, as are 32-bit ints.

A float number is typically a 32-bit quantity, with at least six significant digits and magnitude generally

between about 10

-38

and 10

C provides several other data types besides int and float, including:

char character - a single byte

short short integer

long long integer

double double-precision floating point

The size of these objects is also machine-dependent. There are also arrays, structures and unions of these

basic types, pointers to them, and functions that return them, all of which we will meet in due course.

Computation in the temperature conversion program begins with the assignment statements

lower = 0;

upper = 300;

step = 20;

which set the variables to their initial values. Individual statements are terminated by semicolons.

Each line of the table is computed the same way, so we use a loop that repeats once per output line; this is the

purpose of the while loop

while (fahr <= upper) {

...

}

The while loop operates as follows: The condition in parentheses is tested. If it is true (fahr is less than or

equal to upper), the body of the loop (the three statements enclosed in braces) is executed. Then the

condition is re-tested, and if true, the body is executed again. When the test becomes false (fahr exceeds

upper) the loop ends, and execution continues at the statement that follows the loop. There are no further

statements in this program, so it terminates.

4 Chapter 1 - A Tutorial Introduction

The body of a while can be one or more statements enclosed in braces, as in the temperature converter, or a

single statement without braces, as in

while (i < j)

i = 2 * i;

In either case, we will always indent the statements controlled by the while by one tab stop (which we have

shown as four spaces) so you can see at a glance which statements are inside the loop. The indentation

emphasizes the logical structure of the program. Although C compilers do not care about how a program

looks, proper indentation and spacing are critical in making programs easy for people to read. We recommend

writing only one statement per line, and using blanks around operators to clarify grouping. The position of

braces is less important, although people hold passionate beliefs. We have chosen one of several popular

styles. Pick a style that suits you, then use it consistently.

Most of the work gets done in the body of the loop. The Celsius temperature is computed and assigned to the

variable celsius by the statement

celsius = 5 * (fahr-32) / 9;

The reason for multiplying by 5 and dividing by 9 instead of just multiplying by 5/9 is that in C, as in many

other languages, integer division truncates: any fractional part is discarded. Since 5 and 9 are integers. 5/9

would be truncated to zero and so all the Celsius temperatures would be reported as zero.

This example also shows a bit more of how printf works. printf is a general-purpose output formatting

function, which we will describe in detail in Chapter 7. Its first argument is a string of characters to be printed,

with each % indicating where one of the other (second, third, ...) arguments is to be substituted, and in what

form it is to be printed. For instance, %d specifies an integer argument, so the statement

printf("%d\t%d\n", fahr, celsius);

causes the values of the two integers fahr and celsius to be printed, with a tab (\t) between them.

Each % construction in the first argument of printf is paired with the corresponding second argument, third

argument, etc.; they must match up properly by number and type, or you will get wrong answers.

By the way, printf is not part of the C language; there is no input or output defined in C itself. printf is

just a useful function from the standard library of functions that are normally accessible to C programs. The

behaviour of printf is defined in the ANSI standard, however, so its properties should be the same with any

compiler and library that conforms to the standard.

In order to concentrate on C itself, we don't talk much about input and output until chapter 7. In particular, we

will defer formatted input until then. If you have to input numbers, read the discussion of the function scanf

in Section 7.4. scanf is like printf, except that it reads input instead of writing output.

There are a couple of problems with the temperature conversion program. The simpler one is that the output

isn't very pretty because the numbers are not right-justified. That's easy to fix; if we augment each %d in the

printf statement with a width, the numbers printed will be right-justified in their fields. For instance, we

might say

printf("%3d %6d\n", fahr, celsius);

to print the first number of each line in a field three digits wide, and the second in a field six digits wide, like

this:

0 -17

20 -6

Chapter 1 - A Tutorial Introduction 5

40 4

60 15

80 26

100 37

...

The more serious problem is that because we have used integer arithmetic, the Celsius temperatures are not

very accurate; for instance, 0

F is actually about -17.8

C, not -17. To get more accurate answers, we should

use floating-point arithmetic instead of integer. This requires some changes in the program. Here is the second

version:

#include <stdio.h>

/* print Fahrenheit-Celsius table

for fahr = 0, 20, ..., 300; floating-point version */

main()

{

float fahr, celsius;

float lower, upper, step;

lower = 0; /* lower limit of temperatuire scale */

upper = 300; /* upper limit */

step = 20; /* step size */

fahr = lower;

while (fahr <= upper) {

celsius = (5.0/9.0) * (fahr-32.0);

printf("%3.0f %6.1f\n", fahr, celsius);

fahr = fahr + step;

}

This is much the same as before, except that fahr and celsius are declared to be float and the formula

for conversion is written in a more natural way. We were unable to use 5/9 in the previous version because

integer division would truncate it to zero. A decimal point in a constant indicates that it is floating point,

however, so 5.0/9.0 is not truncated because it is the ratio of two floating-point values.

If an arithmetic operator has integer operands, an integer operation is performed. If an arithmetic operator has

one floating-point operand and one integer operand, however, the integer will be converted to floating point

before the operation is done. If we had written (fahr-32), the 32 would be automatically converted to

floating point. Nevertheless, writing floating-point constants with explicit decimal points even when they have

integral values emphasizes their floating-point nature for human readers.

The detailed rules for when integers are converted to floating point are in Chapter 2. For now, notice that the

assignment

fahr = lower;

and the test

while (fahr <= upper)

also work in the natural way - the int is converted to float before the operation is done.

The printf conversion specification %3.0f says that a floating-point number (here fahr) is to be printed

at least three characters wide, with no decimal point and no fraction digits. %6.1f describes another number

(celsius) that is to be printed at least six characters wide, with 1 digit after the decimal point. The output

looks like this:

6 Chapter 1 - A Tutorial Introduction

0 -17.8

20 -6.7

40 4.4

...

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.

%d print as decimal integer

%6d print as decimal integer, at least 6 characters wide

%f 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.

"%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

Chapter 1 - A Tutorial Introduction 7

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.

8 Chapter 1 - A Tutorial Introduction

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