C语言编程经典原版：The C Programming Language

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"C Programming Language" 《C Programming Language》是由Brian W. Kernighan和Dennis M. Ritchie共同编写的经典书籍，首次出版于1988年，被广泛认为是学习C语言的权威教材。这本书是C语言编程者的必备参考，其中包含了C语言的核心概念和实践技巧。在书中，作者首先引入了C语言的基础概念，帮助读者快速入门。"Chapter 1: A Tutorial Introduction"部分涵盖了编程的起始步骤，包括如何启动编程环境、变量和算术表达式、for循环、符号常量、字符输入和输出等基础内容。通过一系列简单的示例，如文件复制、字符计数、行计数和单词计数，使初学者能够熟悉C语言的基本操作。接下来，"Chapter 2: Types, Operators, and Expressions"深入讲解了C语言的数据类型、大小、常量、声明、算术运算符、关系和逻辑运算符、类型转换、自增自减运算符、位运算符、赋值运算符和表达式，以及它们的优先级和求值顺序。这部分内容是理解C语言程序逻辑的关键。在"Chapter 3: Control Flow"中，作者探讨了程序控制流，包括语句和块、选择结构（if和switch）、循环（while、do...while和for），以及跳转语句（break和continue）。这些章节帮助读者掌握如何根据条件和循环来组织代码，实现复杂逻辑。除此之外，书中还涵盖了数组、函数（包括参数传递，如值传递）、字符数组、外部变量的作用域、指针、结构体等更高级的主题，这些都是C语言强大的特性和功能。这些章节不仅讲解了基本用法，还提供了实际应用的示例，以加深理解。这本书的特色在于其简洁明了的写作风格，每个概念都配以实例，使得学习过程更为直观。无论是初学者还是有经验的程序员，都可以从中受益。通过阅读这本书，读者可以系统地学习到C语言的核心知识，并具备编写高效、可靠C程序的能力。

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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