What is C language

A basic computer programming language.

Chapter 1: Introduction to the C Language

The Development of the C Language

The C language was introduced in the early 1970s. The C language was officially published in 1978 by American Telephone and Telegraph Company (AT&T) Bell Labs. At the same time by B.W. Kernighan and D.M. Ritchit co-authored the famous "THE C PROGRAMMING LANGUAGE" book. Usually referred to as "K&R", also known as "K&R" standard. However, in the "K& R" does not define a complete standard C language, later by the American National Standards Institute on this basis to develop a C language standard, published in 1983. It is commonly referred to as ANSI C.

The best programming language of our time

Early C was mainly used on UNIX systems. Due to the powerful functions and various advantages of C language is gradually recognized by people, to the eighties, C began to enter other operating systems, and soon in all kinds of large, medium, small and micro-computers on the use of a wide range. It has become one of the best programming languages of our time.

Features of C

C is a structured language. It has a clear hierarchy, easy to organize the program in a modular way, easy to debug and maintain.C language is extremely expressive and processing power. It not only has a wealth of operators and data types, easy to realize all kinds of complex data structures. It can also directly access the physical address of the memory, bit (bit) level operations. Because C language realizes the programming operation of hardware, C language combines the functions of high-level language and low-level language in one. It is suitable for both system software development and application software development. In addition, C language is characterized by high efficiency and high portability. Therefore, it has been widely transplanted to various types of computers, thus forming a variety of versions of the C language.

C Language Versions

The most popular C languages are as follows:

-Microsoft C or MS C

-Borland Turbo C or Turbo C

-AT&T C

These C versions not only realize the ANSI C standard. These C versions not only implement the ANSI C standard, but also make some extensions to each of them to make them more convenient and perfect.

Object-Oriented Programming Language

On the basis of C, in 1983, Bjarne Strou-strup of Bell Labs introduced C++. C++ extends and refines C to become an object-oriented programming language. the latest popular versions of C++ are Borland C++ 4.5, Symantec C++ 6.1, and Microsoft VisualC++ 2.0. C++ proposes more in-depth concepts, and the object-oriented concepts it supports make it easy to map problem space directly to program space. The object-oriented concepts supported by C++ make it easy to map problem space directly to program space, providing programmers with a different way of thinking and programming than traditional structured programming. As a result, it also increases the complexity of the language as a whole, making it somewhat difficult to master.

C and C++

However, C is the foundation of C++, and C++ is compatible with C in many ways. Therefore, after mastering C, further study of C++ will enable you to learn an object-oriented language with a familiar syntax, thus achieving twice the result with half the effort.

The structural characteristics of C source programs

To illustrate the structural characteristics of C source programs, let's look at the following programs. These programs, from the simple to the difficult, show the characteristics of the C source program in the composition of the structure. Although the content has not yet been introduced, these examples can be understood from the basic parts of the composition of a C source program and the writing format. main()

{

printf("Hello c language world www.vcok.com! \n");

}

main is the function name of the main function, indicating that it is a main function. Every C source program must have, and can only have, one main function (main function). The function call statement, printf function's function is to send the content to be output to the monitor to display. printf function is a standard function defined by the system, can be called directly in the program.

#include

#include

main()

{

double x,s;

printf("input number:\n");

scanf("%lf",&x);

s=sin(x);

printf("sine of %lf is %lf\n",x,s);

}

Per-line comments

include is called a file include command extension .h is also known as a header file or a header file

Defining two real variables to be be used by later programs

Display prompt

Get a real number x from the keyboard

Find the sine of x and assign it to the variable s

Display the result of the program's operations

End of the main function

The function of the program is to take a number x from the keyboard, find the sine of x, and then output the result. The two lines before main() are called preprocessor commands (see later). There are several other preprocessing commands, here include is called the file include command, the meaning of the pointed brackets "" or quotation marks & lt; & gt; specified in the file included in the program to become part of the program. The included file is usually provided by the system and has the extension .h. It is therefore also known as a header file or a first file.The C header file contains the function prototypes of each standard library function. Therefore, whenever a library function is called in a program, the header file in which the function prototype is located must be included. In this example, three library functions are used: the input function scanf, the sine function sin, and the output function printf. sin is a mathematical function whose header file is math.h, so the main function of the program uses the include command to include math.h before the main function. scanf and printf are standard input and output functions whose header file is stdio.h, and are included in the main function. .h, stdio.h is also included with the include command before the main function.

It should be noted that the C language specifies that the two functions scanf and printf can be omitted from their header file include command. So in this example, you can also delete the include command #include in the second line. similarly, the printf function is used in Example 1.1, and the include command is also omitted.

The main function body in the example is divided into two parts, a description part and an execution part. The description is the type description of the variable. The example does not use any variables, so there is no description section.C states that all variables used in a source program must be described before they are used, or an error will occur. This is a feature of compiled high-level programming languages, which is different from the interpreted BASIC language. The description section is a very important part of the C source program structure. Two variables, x, s, are used in this example to represent the input independent variable and the value of the sin function. Since the sin function requires that these two quantities be double-precision floating-point, the type specifier double is used to describe these two variables. The four lines following the description section are the execution section or called the executing statement section, which is used to complete the function of the program. The first line of the execution section is an output statement that calls the printf function to output a prompt string on the display asking the operator to enter the value of the independent variable x. The second line is an input statement that calls the printf function to output the value of the independent variable x on the display. The second line is an input statement that calls the scanf function, which accepts the number entered on the keyboard and stores it in the variable x. The third line is a call to the sin function. The third line is a call to the sin function and sends the value of the function to the variable s. The fourth line is the printf function that outputs the value of the variable s, which is the sine of x. The program ends.

printf("input number:\n");

scanf("%lf",'C10F10&x);

s=sin(x);

printf("sine of %lf is %lf\n",'C10F10x,s);

When running this program, the prompt string input number is first given on the monitor screen, which is done by the first line of the executable. The user types a number from the keyboard at the prompt, such as 5, presses the Enter key, and then the result of the calculation is given on the screen.

Input and Output Functions

The input and output functions scanf and printf were used in the first two examples, and we're going to cover them in detail in Chapter Three. Here we first briefly introduce their format for the following use. scanf and printf These two functions are called format input function and format output function respectively. Their meaning is to input and output values in the specified format. Therefore, the parameter list of both functions in parentheses consists of the following two parts: "Format Control String", Parameter List The format control string is a string, which must be enclosed in double quotes, and which indicates the data type of the input and output quantities. The format representations of the various types can be found in Chapter III. It is also possible to have non-format control characters within the format control string in the printf function, when the original text is printed as it is on the display screen. The input or output quantities are given in the parameter list. When there are multiple quantities, they are spaced by commas. For example:

printf("sine of %lf is %lf\n",x,s);

where %lf is a format character indicating that it is treated as a double-precision floating-point number. It is present twice in the format string and corresponds to the x and s variables. The rest of the characters are non-format characters are output on the screen as is

int max(int a,int b);

main(){

int x,y,z;

printf("input two numbers:\n");scanf("%d%d",&x ,&y);

z=max(x,y);

printf("maxmum=%d",z);

}

int max(int a,int b){

if(a>b)return a;else return b;

}

This function takes two integers as input and outputs the larger of them.

/*Description of the function*/

/*Main function*/

/*Description of the variables*/

/*Input x,y values*/

/*Call max function*/

/*Output*/

/*Define max function*/

/*Return the result back to the main function**/

/// Return the result to the main function.

The function of the program in the above example is to input two integers by the user and output the larger of them after the program is executed. The program consists of two functions, the main function and the max function. The functions are related to each other. Other functions can be called from the main function. max function compares two numbers and returns the larger number to the main function. max function is a user-defined function. The max function is a user-defined function, so a description is given in the main function (line 3 of the program). As you can see, in the description part of the program, there can be not only variable descriptions but also function descriptions. Details about functions will be presented in Chapter Five. After each line of the program with / * and * / bracketed by the contents of the comment section, the program does not execute the comment section.

The execution of the program in the above example is, first of all, display a prompt string on the screen, ask the user to enter two numbers, enter the scanf function statement to receive the two numbers into the variable x, y, and then call the max function, and x, y value transmitted to the max function parameters a, b. In the max function to compare the size of a, b, the larger returned to the main function of the variable z, and finally output z on the screen. Finally, the value of z is output on the screen.

Structural features of C source programs

1. A C source program can consist of one or more source files.

2. Each source file can consist of one or more functions.

3. A source program, no matter how many files it consists of, has one and only one main function, the main function.

4. The source program can have preprocessing commands (include command is only one of them), preprocessing commands should usually be placed in the source file or source program at the top.

5. Every description, every statement must end with a semicolon. However, preprocessing commands, function headers, and parentheses "}" cannot be followed by a semicolon.

6. Identifiers, keywords must be spaced by at least one space. If there is already a clear spacer, you can also not add a space to the spacing.

Rules to follow when writing programs

From the point of view of clear writing, easy to read, understand and maintain, the following rules should be followed when writing programs:

1. A description or a statement on one line.

2. The part of the program enclosed in {} usually indicates a hierarchical structure of the program. {} is usually aligned with the first letter of the statement of that structure and occupies a separate line.

3. A lower level of statements or descriptions can be indented than a higher level of statements or descriptions written after a number of frames. In order to look more clear, increase the readability of the program. In programming should strive to follow these rules to develop a good programming style.

The C character set

Characters are the most basic elements of a language, and the C character set consists of letters, numbers, spaces, punctuation, and special characters. You can also use Chinese characters or other representable graphical symbols in character constants, string constants, and comments.

1. Alphabets There are 26 lowercase letters a to z***, and 26 uppercase letters A to Z***

2. Numbers There are 10 numbers from 0 to 9***

3. Whitespace Space, tabs, line breaks, etc. are collectively known as whitespace characters. Blanks only work in character constants and string constants. When they appear elsewhere, they only act as spacers and are ignored by the compiler. Therefore, the use of whitespace in a program does not affect the compilation of the program, but the use of whitespace in appropriate places in the program will increase the clarity and readability of the program.

4. Punctuation and Special Characters

C Vocabulary

Vocabulary used in C is divided into six categories: identifiers, keywords, operators, separators, constants, comment characters etc.

1. Identifiers

Variable names, function names, and symbols used in a program are collectively called identifiers. Except for the function names of library functions, which are defined by the system, the rest are customized by the user.C specifies that identifiers can only be strings of letters (A to Z, a to z), numbers (0 to 9), and underscores (), and that the first character of the string must be a letter or an underscore.

The following identifiers are legal:

a,x, 3x,BOOK 1,sum5

The following identifiers are illegal:

3s begins with a number

s*T appears as the illegal character *

-3x begins with a minus sign

bowy-1 appears as the illegal character -(minus sign)

In the use of identifiers must also pay attention to the following points:

(1) Standard C does not limit the length of identifiers, but it is subject to the limitations of the various versions of the C language compilation system, as well as machine-specific limitations. For example, in one version of C, the first eight bits of an identifier are valid, and when two identifiers have the same first eight bits, they are considered the same identifier.

(2) In identifiers, there is a case difference. For example, BOOK and book are two different identifiers.

(3) Although identifiers can be defined at the discretion of the programmer, an identifier is a symbol used to identify a quantity. Therefore, the name should try to have the appropriate meaning, so that readers can understand, so that "the name means what it says".

2. Keywords

Keywords are strings of characters with specific meanings defined by the C language, often called reserved words. User-defined identifiers should not be the same as keywords. keywords in C are divided into the following categories:

(1) type specifiers

These are used to define and specify the type of a variable, function, or other data structure. Such as int, double, etc. used in the previous example

(2) statement definer

Used to indicate the function of a statement. For example, if else used in Example 1.3 is the statement definer of a conditional statement.

(3) Preprocessing command word

It is used to indicate a preprocessing command.

3. Operators

The C language is quite rich in operators. Operators, together with variables and functions, form expressions that represent a variety of arithmetic functions. Operators consist of one or more characters.

4. Separators

The two types of separators used in C are commas and spaces. Commas are mainly used in type descriptions and function parameter lists to separate variables. Spaces are used as spacers between words in a statement. In the keyword, there must be more than one space between identifiers for spacing, otherwise there will be a syntax error, for example, int a; written inta; C compiler will be inta as an identifier, the result will be an error.

5. Constants

Constants used in C can be divided into numeric constants, character constants, string constants, symbolic constants, escape characters and so on. In the second chapter will be given a special introduction.

6. Commentaries

The commentaries in C are strings that begin with "/*" and end with "*/". Between "/*" and "*/" are comments. Comments are not processed when the program is compiled. Comments can appear anywhere in the program. Comments are used to indicate or explain the meaning of the program to the user. In the debugging program for the temporary use of the statement can also be used to enclose the comments, so that the translation skipped without processing, to be debugged at the end of the comments and then removed.