C语言入门教程：从零开始掌握编程基础

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“The C programming Language”是一本国外的经典C语言教程，旨在帮助读者掌握C语言的基础知识，并在学习过程中提升英语阅读能力。书中的内容详细且深入，适合初学者逐步理解C语言的核心概念。该书的第一章“1. A Tutorial Introduction”介绍了C语言的基本结构和编程入门。1.1节“Getting Started”引导读者开始编程旅程，讲解如何启动和编写简单的C程序。1.2节“Variables and Arithmetic Expressions”涉及变量的声明和基本算术运算的使用。1.3节“The for statement”讲解了C语言中的循环控制结构for循环，它是编写重复任务的关键。1.4节“Symbolic Constants”介绍常量的定义与使用，使得代码更具可读性。1.5节探讨了字符输入和输出，包括1.5.1节的文件复制，1.5.2节的字符计数，1.5.3节的行计数，以及1.5.4节的单词计数，这些都是实际编程中的常见操作。1.6节“Arrays”和1.7节“Functions”分别讲述了数组的运用和函数的定义，它们是C语言中组织数据和实现模块化的重要工具。1.8节“Arguments - Call by Value”介绍了函数参数传递的机制，即值传递。1.9节“Character Arrays”讨论字符数组，尤其是字符串的处理。1.10节“External Variables and Scope”解释了外部变量和作用域的概念，这对于理解程序的内存管理至关重要。第二章“2. Types, Operators and Expressions”详细阐述了C语言的数据类型、运算符和表达式。2.1节“Variable Names”讲解变量命名规则，2.2节“Data Types and Sizes”介绍了不同类型的变量所占用的内存大小。2.3节“Constants”涵盖了常量的种类。2.4节“Declarations”讲述了变量和类型的声明。接下来的章节详细讨论了各种运算符，如2.5节的算术运算符，2.6节的关系和逻辑运算符，2.7节的类型转换，2.8节的增量和减量运算符，2.9节的位运算符，2.10节的赋值运算符和表达式，以及2.11节的条件表达式和2.12节的运算符优先级与求值顺序。第三章“3. Control Flow”关注程序的流程控制。3.1节“Statements and Blocks”讲解了语句和代码块的组织方式，3.2节到3.4节介绍了if-else，else-if和switch语句用于条件判断。3.5节至3.7节分别讲解了while，for和do-while循环，以及3.8节的break和continue语句，它们提供了控制循环行为的手段。3.9节的“goto and labels”虽然不推荐使用，但仍然是C语言的一个组成部分，用于无条件跳转。第四章“4. Functions and Program Structure”则深入到函数的使用和程序结构，这在大型项目的开发中尤为重要，因为它涉及到代码的组织和重用。通过本书的学习，读者不仅可以掌握C语言的基础语法，还能了解其背后的逻辑和程序设计思想，为后续的编程实践打下坚实的基础。

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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