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I would like to share with you an example analysis of the basic grammar of the C language. I believe most people don't know much about it, so share this article for your reference. I hope you will learn a lot after reading this article. Let's learn about it!

1. Identifiers and keywords

The name given to the variable is called the variable name, and defining the name of the variable needs to follow the naming rules of the identifier.

Identifiers are valid character sequences used to identify variables, symbolic constants, arrays, functions, files, and so on.

Naming rules for identifiers:

1. Can only consist of letters, numbers, and underscores (for example, Score1,Stu_name)

two。 The first character must be a letter or an underscore

3. Case-sensitive letters

4. Identifiers (that is, keywords) specified within C language cannot be used as user identifiers.

Keywords of C language:

Keywords in C language are reserved words with special functions, with a total of 32 keywords.

Keyword description auto declaration automatic variable break jumps out of the current loop case switch statement branch char declares character variable or function return type const definition constant, if a variable is modified by const, then its value can no longer be changed continue ends the current loop Start the next round of the "other" branch of the default switch statement the loop body of the do loop statement double declaration double precision floating point variable or function return value type enum declaration enumerated type extern declaration variable or function is defined in another file or elsewhere in this file float declaration floating point variable or function return value type for A loop statement goto unconditional jump statement if conditional statement int declaration whole Type variable or function long declaration long integer variable or function return value type register declaration register variable return subroutine return statement (you can take parameters Unsigned declaration unsigned type variable or function union declaration common body type void declaration function no return value or no parameter, declaration untyped pointer volatile indicates that the variable can implicitly change the loop condition of the while loop statement during program execution

C99 New keyword:

_ Bool 、 _ Complex 、 _ Imaginary 、 inline 、 restrict

C11 New keyword:

_ Alignas 、 _ Alignof 、 _ Atomic 、 _ Generic 、 _ Noreturn 、 _ Static_assert 、 _ Thread_local

2. Constant and symbolic constant

Constant: the amount by which the value of the program does not change while it is running.

Variable: the amount by which the value of the program can be sent while the program is running

(1) constant and constant symbol

The constant types provided by C language are integer constant, real constant, character constant, string constant and symbolic constant.

An identifier is used to represent a constant, which is called a symbolic constant and is generally in the form of:

# define identifier constant

For example:

# define PI 3.1415926

About the usage and difference between typede and defin:

1.#defien is a preprocessing instruction, which is simply replaced during compilation preprocessing without correctness checking, regardless of whether the meaning is correct or not. Possible errors will be found and reported only when the compiler has been expanded by the source program.

For example: # define PI 3.1415926

Program: area = PIrr replace 3.1415926r

2.rtypedef is handled at compile time and is usually used to alias existing data types, while # define is simply replaced.

For example: typedef unsigned int uint

This introduces a new mnemonic uint for the unsigned int type, and later uses uint to directly represent the unsigned int type.

For example: typedef PI 3.1515926

The compiler will report an error directly because PI is not an existing type, and secondly, "3.1515926" does not conform to the naming convention.

(2) variable

1. The variable occupies a certain storage unit in memory, and the variable value is stored in the storage unit.

two。 All variables used in the program must have a variable name

3. Variable name and variable value are two different concepts.

4. Different types of variables occupy different numbers of storage units in memory and have different storage formats, for example: char ch = '1storage int I = 1

5. Variables must be "defined before you use". The purpose of this is:

1. It is convenient for the compiler to check whether the operation of the variable is legal. For example, integer variables can be calculated by modulus (remainder), while real variables can not be calculated by modulus.

2. The correct use of variable names in the program

3. C language data type

(1) Integer constant

Integer constants are integers and have three representations.

1. Decimal integer constant: the usual way to write an integer with a numeric value of 0-9. For example: 25,-623, 0, etc.

two。 Octal integers: prefixed with 0, numeric value 0-7

For example:

020 (hexadecimal 16)

016 (hexadecimal 14)

0177777 (hexadecimal 65535)

-010 (decimal number-8)

Several illegal octal numbers:

256 (no prefix 0)

079 (including non-octal number 9)

3. Hexadecimal integer constant: begins with the number and the letter 0x, that is, 0x as the prefix, and the numeric values are 0-9 and Amurf (or the letter Amurf)

Several legal hexadecimal numbers:

0x2A (decimal number 42)

0x1AB0 (decimal number 6832)

Several illegal hexadecimal numbers:

5A (no prefix), 0x2H (including non-hexadecimal digits H)

Integer variable

Integer variables are divided into basic integer, long integer, short integer and unsigned type.

Definition and initialization of integer variables:

The general form of defining variables and initialization is:

Type descriptor variable name 1 = value 1

For example:

Int arecrum bdhoc; / / defines arecrine bpenc as the integer variable long x = 10; / / defines x as the long integer variable, with the initial value 10unsigned int pcent2 and qdepartment 5m; and / / defines pquotient qPerry m as an unsigned integer variable. P with an initial value of 2 and Q with an initial value of 5

Integer variables need to be noted:

1. Allow multiple variables of the same type to be defined after a type descriptor, with at least one space between the type descriptor and the variable name, and a comma between the variable names

2. The last variable name ends with a ";" sign.

3. The definition of the variable must be placed before the variable is used, usually at the beginning of the function body.

4. Just because there is no initial value assigned to the variable does not mean that there is no value in the variable, only there is no definite value in the variable, so the direct use of this variable may produce inexplicable results and may lead to operational errors.

Routine:

# include void main (void) {int a = 3, b = 5, c int d; c = aquib; d = Amurb; printf ("axib% dmai arelb% d\ n", c Magi d);} original code, reverse code and complement code

The number in the computer is divided into signed and unsigned. They are expressed in binary form and can be expressed in three ways, namely, the original code and the complement.

The three representations are composed of symbolic bits and numerical bits.

The symbol bit is in the highest position, with 0 for "positive" and 1 for "negative".

Take the remainder as a numerical bit

Source code: the absolute value of a number is represented by the binary method, and the original code of the number is obtained by this method.

Inverse code: the inverse code of a positive number is the same as its original code, and the inverse code of a negative number is the inverse of its original code except the symbol bits.

Complement: the complement of a positive number is the same as its original code, while the complement of a negative number is to add 1 at the end after inverting the original code except the symbol bit.

The original and negative codes of positive numbers. The complement is the same; the original code, inverse code and complement of negative numbers are different. The values are stored in the form of complements in the computer.

For example: ask for the complement of-127

(2) Real data

Real data, also known as real or floating-point numbers, refers to non-integer values with decimal parts.

Real constant

In C, real constants can only be represented by decimal numbers, and there are two ways to express them:

1. Decimal form: consists of the number 09and the decimal point. When the decimal part is 00:00, the decimal point cannot be omitted. For example, 0.02b 24.4,-56.33, etc. are legal real constants.

two。 Exponential form: consists of a decimal number, an addition code marking the lowercase letter "e" (or an uppercase letter "E") and a step code (which must be an integer).

For example: 2.3E5 (equivalent to 2.3 x 10 ^ 5)

The following are illegal real constants:

123 (no decimal point)

E4 (no number before step code "E")

23.-E3 (the minus sign is not in the right position)

2.7e (no order code)

Real variable

Definition and initialization of real variables

For example:

Float and / or define double as a single-precision real variable double mens3.5 and nforth as a double-precision real variable, and assign 3.5,5.6 (3) character data respectively.

A character constant is a character enclosed in a pair of single quotation marks

Example

Such as:'b','z','=','?','5'

Character constant characteristics:

1. A character constant can only be a single character, not multiple characters.

two。 Character constants are stored as ASCII codes, and a character constant takes up one byte of space.

ASCII code

ASCII code stands for American Standrd Code for Information Interchange. ASCII coding can represent a data range of 000000000000001111111, a total of 2 ^ 8, that is, 256 data.

The standard ASCII code has 128 characters, of which:

1.0-31 is the control character, for example, enter newline, file end flag, string end flag

2. The code values of 10 Arabic '0century 9' are continuous.

3.26 uppercase letters are contiguous, while 26 and uppercase and lowercase letters are contiguous.

4. Character constants can also participate in various operations, such as: the ASCII code value of the character'B' minus the ASCII code value of 66 character'A' is 1.)

Character variable

1. Character-type variables occupy 1 byte in memory and can only store one character in the form of the value of the character's ASCII code.

two。 In C language, a character data can be operated according to its ASCII code.

Character data storage space and value range

The value range of type bytes is char1-128~127unsigned char10~255

Routine: assign an integer value to a character variable

# include int main () {char ch2=65,ch3=66; printf ("% cMagne% c\ n", ch2,ch3); printf ("% djine% d\ n", ch2,ch3);} escape character characters

Escape character: a character with a specific meaning specified by a backslash ""

The common escape characters are as follows

Escape characters are used as follows:

# include int main () {printf ("\" china\ "\ n"); printf ("My\ tCountry.\ n"); printf ("I am hap\ 160\ x79.\ n");} string constant

A string constant is an abbreviated string, which is a string of characters enclosed by a pair of double apostrophes (western double quotation marks), including the number and length of characters.

For example: "Hello World". "a", "C language" are string constants.

The difference between a string constant and a character constant

1. Character constants are characters enclosed in single quotes on the right, while string constants are characters enclosed in double quotes. Although both'a 'and' a 'contain a character, they both have different meanings in C programs.

two。 A character constant can only be one character, and a string constant can be multiple characters.

3. You can assign a string constant to a character variable, but not a character constant to a character variable.

String variable

There is no special string type in C language, so we can only use arrays or pointers to store strings indirectly.

Char str1 [] = "Hello World!" char * str2 = "Hello world!"

Str1 and str2 are the names of strings, followed by [] and preceded by * are fixed. For the time being, beginners can think that these two storage methods are equivalent, and both of them can be output through special puts function and general printf function.

# include int main () {char web_url [] = "hHello World!"; char * web_name = "Hello World"; puts (web_url); puts (web_name); printf ("% s\ n% s\ n", web_url, web_name); return 0;} above is all the content of the article "sample Analysis of basic Grammar of C language". Thank you for reading! I believe we all have a certain understanding, hope to share the content to help you, if you want to learn more knowledge, welcome to follow the industry information channel!

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