C语言经典教程：Brian Kernighan与Dennis Ritchie合著

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《C编程语言》是由Brian W. Kernighan 和 Dennis M. Ritchie 联合编著的经典计算机科学教材，于1988年由Prentice-Hall出版社发行。这本书的ISBN分别是0-13-110362-8（平装版）和0-13-110370-9，旨在向读者介绍C语言的基础知识和核心概念。在本书的第一部分，"A Tutorial Introduction"，作者以一种循序渐进的方式引导读者入门。首先，"Getting Started"介绍了C语言的基本环境设置和命令行交互。接下来，章节详细讲解了变量和算术表达式，包括整型、浮点型等数据类型以及基本运算。对于循环结构，"The for statement"展示了如何使用for循环控制程序流程。 "Symbolic Constants"部分介绍了常量的概念，使得程序中的数值可以有更易读的标识。字符输入与输出是后续章节的重点，通过示例如文件复制、字符计数、行计数和单词计数来演示操作。数组作为数据存储的重要方式，在这里得到了深入剖析，包括一维数组和函数的使用。第二章深入探讨了数据类型、运算符和表达式。变量名的命名规则、各种数据类型的特性和大小、以及不同类型的常量都是讨论内容。此外，还有算术、关系和逻辑运算符，以及类型转换、自增自减和位操作符的讲解。这部分强调了表达式的优先级和求值顺序，帮助读者理解代码的执行逻辑。第三章"Control Flow"涵盖了控制结构，如基本语句和条件语句，如if-else结构。通过递归、选择结构和循环结构（如while和do-while），读者可以学习如何根据条件改变程序的执行路径。流程控制的理解对编写复杂的算法至关重要。除了以上内容，全书还包括函数的定义和调用，其中重点介绍了按值传递参数的方式，以及字符数组和外部变量的作用域管理。每章都配有丰富的示例和练习，旨在帮助读者通过实践巩固理论知识。《C Programming Language》不仅是C语言的入门指南，也是理解编程基础和结构的良好资源。它不仅适合初学者，也对希望深入理解C语言和编程思想的专业人员具有很高的参考价值。通过阅读和实践，读者能够掌握一门强大且广泛应用的编程语言。

Chapter 1 - A Tutorial Introduction

to print this sequence of characters

} \n represents the newline character

The first C program

The statements of a function are enclosed in braces { }. The function main contains only one statement,

printf("hello, world\n");

A function is called by naming it, followed by a parenthesized list of arguments, so this calls the function printf

with the argument "hello, world\n". printf is a library function that prints output, in this case the string

of characters between the quotes.

A sequence of characters in double quotes, like "hello, world\n", is called a character string or string

constant. For the moment our only use of character strings will be as arguments for printf and other functions.

The sequence \n in the string is C notation for the newline character, which when printed advances the output to

the left margin on the next line. If you leave out the \n (a worthwhile experiment), you will find that there is no

line advance after the output is printed. You must use \n to include a newline character in the printf argument;

if you try something like

printf("hello, world

");

the C compiler will produce an error message.

printf never supplies a newline character automatically, so several calls may be used to build up an output line

in stages. Our first program could just as well have been written

#include <stdio.h>

main()

{

printf("hello, ");

printf("world");

printf("\n");

}

to produce identical output.

Notice that \n represents only a single character. An escape sequence like \n provides a general and extensible

mechanism for representing hard-to-type or invisible characters. Among the others that C provides are \t for tab,

\b for backspace, \" for the double quote and \\ for the backslash itself. There is a complete list in

Section 2.3.

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Chapter 1 - A Tutorial Introduction

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.

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);

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Chapter 1 - A Tutorial Introduction

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

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 */

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