C语言教程：从入门到精通

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"C语言编程教程" 这篇文档似乎是一本介绍C语言编程的教程，包含多个章节，旨在引导初学者入门。以下是对每个章节主要内容的详细概述： **第1章：教程入门** 这一章作为教程的开端，介绍了如何开始编写C程序。它涉及以下几个关键点： 1. **开始阶段**：解释了如何设置开发环境和编译器，让读者能够运行第一个C程序。 2. **变量与算术表达式**：讨论了变量的概念，以及如何使用基本的算术运算符进行计算。 3. **for循环**：详细介绍了for循环的结构和用法，这是C语言中最常见的循环结构之一。 4. **符号常量**：讲解了如何使用#define定义常量，以避免在代码中硬编码值。 5. **字符输入与输出**：这部分涵盖了基本的输入输出操作，如printf和scanf函数，还通过示例展示了如何处理字符数据。 - **文件复制**：展示了如何使用C语言读取一个文件并将其内容写入另一个文件。 - **字符计数**：如何计算文件中的字符总数。 - **行计数**：如何统计文件中的行数。 - **单词计数**：演示了如何分析文本并计算其中的单词数量。 6. **数组**：解释了如何声明和使用数组，以及一维数组的基本操作。 7. **函数**：介绍了函数的定义、调用和作用，包括函数的参数传递（按值传递）。 8. **字符数组**：专门讨论了用于存储字符串的字符数组。 9. **外部变量与作用域**：讲解了全局变量和局部变量的区别，以及作用域的概念。 **第2章：类型、运算符与表达式** 此章深入讨论了C语言的数据类型、运算符和表达式： 1. **变量名**：讨论了变量命名的规则和约定。 2. **数据类型与大小**：列出了C语言中的基本数据类型（如int, float, char等）及其内存占用。 3. **常量**：涵盖了各种类型的常量，如整型、浮点型、字符型和字符串字面量。 4. **声明**：解释了如何声明变量，包括类型和初始值的指定。 5. **算术运算符**：详述了加、减、乘、除、取余等运算符的用法。 6. **关系与逻辑运算符**：包括等于、不等于、小于、大于等比较运算符，以及逻辑与、或、非运算符。 7. **类型转换**：讨论了自动类型转换和强制类型转换的规则。 8. **增量与减量运算符**：如++和--，它们可以用来增加或减少变量的值。 9. **位运算符**：介绍了一位操作，如按位与、或、异或和位移。 10. **赋值运算符与表达式**：涵盖了简单的赋值以及复合赋值运算符。 11. **条件表达式**：讲解了三元运算符的使用。 12. **优先级与求值顺序**：解释了运算符的优先级和结合性，以确保正确理解表达式的计算过程。 **第3章：控制流** 这一章关注程序的流程控制： 1. **语句与块**：介绍了一系列语句类型，如简单语句、复合语句（用花括号包围的一组语句）。 2. **if-else**：解释了条件语句，以及如何根据条件执行不同的代码路径。 3. **else-if**：说明了如何在if语句中添加更多的条件分支。 4. **switch**：详细介绍了switch语句，它提供了多路选择的另一种方式。 5. **循环**：覆盖了while和for循环，以及它们在迭代中的应用。 6. **do-while**：介绍了do-while循环，它的特点是至少会执行一次循环体。 7. **break与continue**：讲解了如何中断循环或跳过当前循环迭代的语句。 8. **goto与标签**：尽管不推荐，但仍然提到了goto语句及其在程序中的使用。 **第4章：函数**（内容未给出完整）这部分很可能是关于更深入的函数使用，可能包括函数指针、递归函数、函数原型声明、函数的内存管理等方面。这个教程全面地介绍了C语言的基础知识，从变量、运算符到控制流，为学习者提供了一个系统的学习路径。

}

The two lines

/* print Fahrenheit-Celsius table

for fahr = 0, 20, ..., 300 */

are a comment, which in this case explains briefly what the program does. Any characters between /* and */

are ignored by the compiler; they may be used freely to make a program easier to understand. Comments may

appear anywhere where a blank, tab or newline can.

In C, all variables must be declared before they are used, usually at the beginning of the function before any

executable statements. A declaration announces the properties of variables; it consists of a name and a list of

variables, such as

int fahr, celsius;

int lower, upper, step;

The type int means that the variables listed are integers; by contrast with float, which means floating

point, i.e., numbers that may have a fractional part. The range of both int and float depends on the

machine you are using; 16-bits ints, which lie between -32768 and +32767, are common, as are 32-bit ints.

A float number is typically a 32-bit quantity, with at least six significant digits and magnitude generally

between about 10

-38

and 10

C provides several other data types besides int and float, including:

char character - a single byte

short short integer

long long integer

double double-precision floating point

The size of these objects is also machine-dependent. There are also arrays, structures and unions of these

basic types, pointers to them, and functions that return them, all of which we will meet in due course.

Computation in the temperature conversion program begins with the assignment statements

lower = 0;

upper = 300;

step = 20;

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.

4 Chapter 1 - A Tutorial Introduction

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);

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

Chapter 1 - A Tutorial Introduction 5

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 */

fahr = lower;

while (fahr <= upper) {

celsius = (5.0/9.0) * (fahr-32.0);

printf("%3.0f %6.1f\n", fahr, celsius);

fahr = fahr + step;

}

This is much the same as before, except that fahr and celsius are declared to be float and the formula

for conversion is written in a more natural way. We were unable to use 5/9 in the previous version because

integer division would truncate it to zero. A decimal point in a constant indicates that it is floating point,

however, so 5.0/9.0 is not truncated because it is the ratio of two floating-point values.

If an arithmetic operator has integer operands, an integer operation is performed. If an arithmetic operator has

one floating-point operand and one integer operand, however, the integer will be converted to floating point

before the operation is done. If we had written (fahr-32), the 32 would be automatically converted to

floating point. Nevertheless, writing floating-point constants with explicit decimal points even when they have

integral values emphasizes their floating-point nature for human readers.

The detailed rules for when integers are converted to floating point are in Chapter 2. For now, notice that the

assignment

fahr = lower;

and the test

while (fahr <= upper)

also work in the natural way - the int is converted to float before the operation is done.

The printf conversion specification %3.0f says that a floating-point number (here fahr) is to be printed

at least three characters wide, with no decimal point and no fraction digits. %6.1f describes another number

(celsius) that is to be printed at least six characters wide, with 1 digit after the decimal point. The output

looks like this:

6 Chapter 1 - A Tutorial Introduction

0 -17.8

20 -6.7

40 4.4

...

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.

%d print as decimal integer

%6d print as decimal integer, at least 6 characters wide

%f 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.

"%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

Chapter 1 - A Tutorial Introduction 7

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.

8 Chapter 1 - A Tutorial Introduction

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C语言教程：从入门到精通

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