C语言入门指南：从基础到实践

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"这是一本全面介绍C语言的入门教程，旨在帮助初学者从零基础逐渐精通C语言。教程覆盖了C语言的基础概念、语法、控制流和函数等多个方面，通过实例教学，使学习者能够逐步掌握编程技能。" 本教程以清晰易懂的方式引导读者进入C语言的世界。首先，第1章“教程介绍”提供了初步的引导，包括如何开始，变量和算术表达式的概念，以及常用的控制结构——`for`语句。这一章还讲解了符号常量的使用，以及字符输入和输出的相关操作，如文件复制、字符计数、行计数和单词计数，这些基本操作是所有编程工作中的基石。此外，章节中介绍了数组、函数的概念，以及参数传递（按值调用）的方式，字符数组的处理，外部变量和作用域，这些都是理解C语言程序结构的关键。第2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、常量的定义。它详细阐述了声明的作用，各种算术、关系和逻辑运算符的使用，以及自动类型转换。本章还涵盖了增量和减量运算符，位运算符，赋值运算符和表达式，以及条件表达式的运用，最后讲解了运算符的优先级和求值顺序，这些都是编写复杂逻辑时不可或缺的知识点。第3章“控制流”则围绕程序执行流程展开，包括语句和块的概念，`if-else`、`else-if`分支结构，以及多分支选择`switch`语句。此外，教程详细介绍了不同类型的循环结构：`while`、`for`和`do-while`，并讨论了`break`和`continue`语句在控制循环中的作用，以及`goto`和标签的使用。这些内容帮助读者理解如何根据条件控制程序的执行路径，实现复杂的逻辑控制。第4章“函数和程序结构”可能涉及函数的更深入探讨，包括函数的定义、调用，以及如何通过函数组织程序结构，这在大型项目中尤为重要。函数的合理设计和使用是提高代码复用性和可维护性的关键。这本C语言入门教程通过逐步引导，从基础知识到高级特性，全面覆盖了C语言的核心概念和实践技巧，是初学者理想的自学资料。通过深入学习和实践，读者可以逐步建立扎实的C语言编程基础，为后续的编程学习或职业发展奠定坚实基础。

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 %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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