C语言程序设计第二版：入门与实践

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"The C Programming Language Second Edition - 英文版C语言程序设计正版电子书" 本书是《C语言程序设计》第二版，由Brian W. Kernighan和Dennis M. Ritchie共同编写，是一本经典的C语言学习教程。书中详细介绍了C语言的基本概念和编程技巧，适合初学者和有一定经验的程序员阅读。在第一章"一个教程介绍"中，作者首先引导读者如何开始编程，介绍了变量和算术表达式。这部分内容涵盖基础的变量定义、数据类型以及简单的计算。接着，讲解了for循环的使用，它在C语言中是非常常见且重要的控制结构。此外，还提到了符号常量的概念，它们在代码中提供了可读性。在字符输入与输出部分，通过几个简单的示例（如文件复制、字符计数、行计数和单词计数）帮助读者理解基本的输入/输出操作。数组的介绍紧随其后，包括一维数组的声明和使用。函数的引入让读者明白如何将代码模块化，而参数传递（按值调用）则解释了函数参数的工作原理。接着讨论了字符数组，特别是字符串的处理。最后，涉及了外部变量和作用域的概念，这是理解程序内存管理和数据共享的关键。第二章"类型、运算符和表达式"深入探讨了变量命名规则、数据类型和大小、常量的表示。声明语句的使用和不同类型的运算符，如算术、关系、逻辑、位运算和赋值运算符，都在这一章中详尽解释。递增和递减运算符、类型转换、位操作符和赋值表达式的组合使用，以及条件表达式和运算符优先级和求值顺序，都是程序员必备的知识点。第三章"控制流"关注的是程序执行流程的控制。陈述语句和块的概念使得代码组织有序，if-else和else-if用于条件判断，switch语句提供了一种更简洁的多分支选择。循环结构是程序中的核心，while、for和do-while循环各有其应用场景。break和continue语句用于中断循环，而goto和标签则提供了非结构化的跳转，尽管在某些情况下可能增加代码复杂性。第四章"函数"进一步深入函数的使用，包括函数的定义、调用、返回值以及递归函数的应用。第五章"指针"揭示了C语言的强大之处，通过指针可以直接操作内存，实现高效的数据操作和动态内存管理。第六章"结构体和联合"讨论了如何组织复杂的数据结构。第七章"输入/输出"涵盖了标准I/O库的使用，包括格式化输入/输出和文件操作。第八章"预处理"介绍了宏定义和条件编译，这些都是在编译阶段进行代码修改的重要工具。最后，第九章"附录"提供了语言的参考，包括关键字、运算符和约定。通过这本书，读者不仅可以掌握C语言的基础，还能了解到高级特性，为成为一名熟练的C程序员打下坚实基础。

}

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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