C语言(第2版)：从入门到控制结构

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《C程序设计语言(英文第2版)》是一本经典的计算机编程教材，由Bjarne Stroustrup等人编著，深入浅出地介绍了C语言的核心概念和技术。本书分为多个章节，旨在引导读者逐步掌握这门广泛应用于系统开发、嵌入式编程等领域的语言。在第一部分的“ Preface”中，作者回顾了上一版的内容，并阐述了第二版更新的目的，可能包括对C语言新标准的支持、教学方法的改进以及针对现代程序员需求的补充。接下来的“Chapter 1 - A Tutorial Introduction”是学习C语言的入门章节，通过一系列实践示例，让初学者了解如何编写简单的程序。这一章涵盖了： 1.1 部分介绍了如何开始编程，如创建一个简单的程序、设置环境变量和编译器。 1.2 讲解了变量和算术表达式的使用，强调数据类型的概念，以及如何进行基本的数学运算。 1.3 介绍了循环结构，特别是for循环，这是控制程序流程的关键构造。 1.4 符号常量的使用，帮助定义不变的值，提高代码可读性。 1.5 重点讲述了字符输入和输出，包括文件复制操作，以及计数字符、行和单词的基本函数，这些都是处理文本数据的基础。 1.6 探讨了数组，这是存储一组相同类型数据的重要数据结构。 1.7 函数的引入，包括参数传递方式（如按值传递），以及基本函数的定义和调用。 1.8 特别关注了字符数组的使用，它们在字符串处理中起着关键作用。 1.9 外部变量和作用域的概念，确保了变量在程序中的正确生命周期管理。第二章进一步深入探讨了类型、运算符和表达式，涉及变量命名规则、数据类型及其大小、不同类型的常量、声明和初始化，以及各种算术、比较、逻辑、位操作和赋值表达式的使用。这部分内容强调了C语言的严谨性和灵活性。第三章专门讲解控制流，包括基本的语句和块结构，if-else和嵌套的else-if语句，switch语句用于多路分支，以及while、for循环的使用。还讨论了break和continue语句控制循环流程，以及 goto和标签的高级控制技术。最后，第四章集中于函数和程序结构，深入解析函数的定义、调用、参数传递，以及如何组织代码以形成模块化的程序结构。这部分内容对于理解C语言的模块化编程思想至关重要。总体而言，《C程序设计语言(英文第2版)》为学习者提供了一个坚实的基础，使他们能够熟练掌握C语言的基本语法和核心编程技巧，进而进入更高级的主题，如指针、内存管理和错误处理。对于想要在这个领域深入发展的程序员来说，这是一本不可或缺的参考书籍。

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.

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 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.

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 c 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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