C语言编程教程：从入门到精通

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"《C语言程序设计》(K&R)是C编程的经典教材，涵盖了C语言的基础知识，包括类型、运算符和表达式、控制流程、函数和程序结构、指针和数组等内容。这本书适用于初学者，也适合已经有一定经验的程序员作为参考。" 在本书的第一章“教程入门”中，作者引导读者开始C语言的学习旅程。从最基本的启动程序、声明变量和使用算术表达式开始，帮助读者理解C语言的基础语法。介绍的`for`语句是C语言中常用的循环结构，用于执行一系列重复操作。此外，书中还提到了符号常量的概念，它们是不可修改的值，常用于代码中的固定参数。在字符输入和输出部分，作者通过几个简单的示例，如文件复制、字符计数、行计数和词数统计，展示了如何处理字符输入和输出。这些示例涵盖了基本的文件操作和流处理，是C语言程序设计中的核心技能。第一章还介绍了数组，这是C语言中存储一组相同类型数据的重要结构。数组使得我们可以一次性处理多个数据。接下来，函数是C语言中复用代码和组织程序结构的关键，书中详细讲解了函数的定义、调用以及参数传递（按值调用）。字符数组，即字符串，在C语言中占有特殊地位，书中有专门的讨论。最后，外部变量和作用域的概念帮助读者理解变量在不同上下文中的可见性和生命周期。第二章“类型、运算符和表达式”深入到C语言的语法细节。包括变量名的命名规则、各种数据类型的大小和用途、常量的定义、声明的语法、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、条件运算符以及运算符的优先级和求值顺序。第三章“控制流程”涉及程序的执行路径。`if-else`语句、`else-if`结构、`switch`语句提供了条件分支的实现方式。不同类型的循环（`while`、`for`、`do-while`）让程序可以重复执行某段代码，而`break`和`continue`语句则提供了对循环内部流程的控制。`goto`和标签是跳转语句，虽然在某些情况下有用，但通常鼓励避免过度使用，以保持代码的清晰性和可维护性。第四章“函数和程序结构”则探讨了如何构建大型的、模块化的程序。通过函数来组织代码，可以使程序更易于理解和维护。书中还可能涵盖了函数的返回值、递归、函数指针等高级主题。这个资源标签为“教程 ANSI C”，意味着它遵循的是ANSI C标准，即C89/C90，这是C语言的一个重要版本，定义了许多至今仍被广泛使用的特性。通过学习这本书，读者将能够掌握C语言的基础，为进一步的编程学习打下坚实基础。

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

merely says to print the number as floating point.

print as decimal integer

%6d

print as decimal integer, at least 6 characters wide

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.

#include <stdio.h>

/* 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

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

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

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)

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C语言编程教程：从入门到精通

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