经典C语言教程：入门到控制流详解

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《C Programming Language》是一本经典的C语言教程，全面介绍了C语言的基础知识和编程技巧。本书共分为四个主要章节，涵盖了从入门到进阶的内容。在第1章“一个教程介绍”中，作者引导读者快速上手C语言。首先，通过“Getting Started”，介绍了如何设置开发环境并编写简单的程序。接着，章节详细解释了变量和算术表达式（包括基本数据类型和运算符），为后续计算操作打下基础。对于控制结构，1.3节讲解了for循环，让学习者理解如何控制程序流程。此外，该章还涉及符号常量、字符输入输出以及一些基本操作，如文件复制、字符计数、行计数和单词计数，展示了C语言处理文本的能力。数组的概念也在这一章引入，为数据存储和操作提供关键工具。第二章“类型、运算符和表达式”深入探讨了变量命名规则、数据类型及其大小、常量的使用，以及如何进行数据声明和不同类型的操作。这里涵盖了算术、关系和逻辑运算符，以及类型转换、自增自减和位操作符。条件表达式和运算符的优先级及执行顺序也是本章重点。第三章“控制流”详细解析了程序的基本结构，如语句和代码块。if-else语句用于基于条件执行不同的代码路径，else-if用于扩展条件判断，switch则用于多选项选择。接下来，while和for循环展示了不同类型的循环控制，do-while确保至少执行一次，而break和continue则用来改变程序流程。Goto语句和标签则是对复杂控制流程的一种高级处理方式。第四章“函数与程序结构”是C语言的核心内容，讲解了如何定义和调用函数，以及参数传递机制（call by value）。章节讨论了字符数组的应用，以及外部变量的作用域问题，帮助读者理解如何组织和模块化代码。此外，函数的嵌套和递归也被涵盖，这些都是构建大型软件项目的基石。《The C Programming Language》以清晰易懂的方式传授C语言基础知识，从语法到高级概念，为初学者提供了扎实的编程基础，并对进阶开发者提升技能同样有价值。通过阅读这本书，学习者将掌握C语言的关键要素，为后续深入学习和实践打下坚实基础。

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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