Read the basic grammar of C language in one article

Preface —— sketch C History of language development

C The development of language can be traced back to 1972 year . His creation is due to writing Unix Operating system ,Unix The operating system was originally written in assembly language , The biggest advantage of assembly language is that the code runs fast . But its shortcomings are also very obvious , That is, if the code goes wrong, debugging is difficult , The biggest problem is due to poor code portability , Cannot cross platform , Because some instructions of assembly language are strongly related to the platform . This obviously reduces efficiency , And it gets in the way of Unix The development of . And then there was B Language , however B Language also has some limitations , And with Unix The great development of , Make in B Developed on the basis of language C Language . Let's remember such a great man ,C The father of language —— Dennis · Ricci (Dennis Ritchie) .
after one's words C After a brief history of language, we will proceed C The basic grammar of language introduces ！

One 、 Start with a program ——Hello World

We go straight through a Hello World Program to introduce our grammar .hello world Program can be said to be a convention of programming language, commonly known as .hello world Which translates as Hello world , In fact, it is a greeting to the world , Tell the world , I'm coming . Actually, write this hello world Another meaning of the program is that as long as the program can run normally, it means that there is no problem with its local running environment , So when you learn any programming language, you can try hello world This program ！

Source code ：

#include <stdio.h>

int main()
{
    
    printf("Hello World!\n");
    return 0;
}

Running results ：

Hello World!

The code above is very simple , Is to print a sentence to the screen terminal " hello world! " .
Now let's analyze it ：
（1）#include<stdio.h> : This is actually a standard library file , There are many standard functions in this standard library file , By introducing this library file, we can use the functions inside .
（2）int main(){} : Here is the definition of a return value of int Type variable main function , It is worth noting that all of us C The programs are all from main Function starts to run , And to main After running . In other words, as long as it is C The program must be defined main function .
（3）printf(“Hello World!\n”); ： The function of this sentence is to output a hello world.printf() Namely stdio.h A function inside , We will explain his function in detail in the following article .
（4）return 0; : Function return value , The type of the return value should be consistent with the first type of the function .

Two 、C Basic grammar of language

1. Constants and variables

Constant , As the name suggests, it is a constant quantity , image 1, 2, 3, 1.1, -100 Wait, such numbers are constants , And we cannot assign values to constants .
Variable , As the name suggests, it is the amount of change , To define variables, we usually assign data types to variables , Such as char a; int b; ( Define character variables a, Define shaping variables b). And variables can be assigned .

2. data type

When defining variables , We usually assign data types to them , Next we will introduce C The data type of the language .
Basic data type ：
　　char : Character . yes signed char Shorthand for type , That is, signed character type , Abbreviation character type . Take up a byte in memory , Range of values -128~127.
　　unsigned char : No sign character type . In memory 1 Bytes , Range of values 0~255. A character is a character enclosed in single quotation marks . It should be noted that , There can be at most one character in a single quotation mark .
　　short : Short . yes signed short Shorthand for type , That is, signed short integer , Short integer . In memory 2 Bytes , Range of values -32768~32767.
　　unsigned short : Unsigned short . In memory 2 Bytes , Range of values 0~65535.
　　int : integer . yes signed int Shorthand for type , That is, signed integer , It's called integer for short . In memory 4 Bytes (16 Bit and below are 2 byte ), Range of values -2147483648~2147483647.
　　unsigned int : Unsigned integer . In memory 4 Bytes (16 Bit and below are 2 byte ), Range of values 0~4294967295.
　　long : Long integer . yes signed long Shorthand for type , That is, signed long integer , Short for long integer . In memory 4 Bytes (64 In a bit operating system, it's 8 byte ).
　　unsigned log: Unsigned long . In memory 4 Bytes (64 In a bit operating system, it's 8 byte ).
　　long long: Long plastic surgery , In memory 8 Bytes .
　　float : Single precision floating point , In memory 4 byte .
　　double : Double precision floating point , In memory 8 byte .
Empty type ：
　　void : Empty type , In memory 1 byte .
Construct data types ：
　　 Array : Such as char buf[10]; It can store multiple data of the same type .
　　 Structure : struct, It can store multiple data of different types .
　　 Shared body : union, It can store multiple data of different types , The data in the community share the same address space .
Pointer types :
　　* : for example char* int* etc. , Pointer type size 32 The bit operating system is always 4 byte , 64 Bit operation is 8 byte .

3. keyword

C The keywords of the language are 32 individual , These keywords are already defined , We can't change it at will , And these keywords have their special significance .
　　void ： Declares that the function returns no value or takes no arguments , Declare no type pointer （ Basically, these three functions ）
　　char ： Declare character type variables or functions
　　short ： Declare short integer variables or functions
　　int ： Declare an integer variable or function
　　long ： Declare long integer variables or functions
　　float ： Declare floating-point variables or functions
　　double ： Declare a double precision variable or function
　　enum ： Declare enumeration type
　　unsigned ： Declare an unsigned type variable or function
　　signed ： Declare a symbolic type variable or function
　　struct ： Declare a structural variable or function
　　union ： Declare federated data types
　　const ： Declare read-only variable
　　auto ： Declare automatic variables Generally not used
　　do： The body of a loop statement
　　while ： The loop condition of a loop statement
　　for ： A circular statement
　　if： Conditional statements must Branch
　　else ： Conditional statement negates Branch （ And if Continuous use ）
　　switch : Used for branch statements
　　case ： Switch statement Branch
　　continue ： End the current cycle , Start next cycle
　　break： Jump out of current loop
　　default ： In the switch statement “ other ” Branch
　　goto ： Jump statements without conditions
　　sizeof ： Calculate the data type length
　　volatile ： Explain that variables can be implicitly changed during program execution
　　static ： Declare static variables
　　register： Declare register variables
　　typedef： Used to alias data types
　　extern： Declare variables are being declared in other files （ It can also be seen as a reference variable ）
　　return ： Program return statement

4. identifier

When we define variable names or function names , We will define the identifier , for example int a; This defines a int Type variables a , This a It's an identifier , Empathy , We can also define char b; float c; These variables are identifiers , The name of the variable is our own ,C Language has requirements for identifiers ：
（1） Identifiers can only be made of letters 、 Numbers 、 Underline composition .
（2） Identifiers can only start with letters or underscores .
（3） Identifiers cannot be with C Language keyword conflict .
although C The identifier of a language can be named by ourselves , But in order to develop good coding habits , It is still recommended that you know the meaning of the name when naming . Otherwise, if the code capacity is huge , You may not know what you are doing with this identifier .

5. notes

If a piece of code is annotated , When the program is running code , Will not run the code in the comments , The contents in the comments will be automatically ignored when the program is running . The content in the comments is usually to annotate or explain the code , Make it easy for yourself or others to read the code . Can be interpreted as , The code is written by people and run by computers , Notes are written for people to see .
C There are three ways to add comments to languages .
（1）// Double slash notes , Is the content after the double slash in the comment line .
（2）/* */ This annotation method is to annotate all the contents between the two symbols , If the annotation is multi line, this method can be used faster .
（3）#if 0
　　　　 The content in the middle will be annotated
　　＃else
　　　　 The content in the middle will work normally
　　#endif
In fact, the third method cannot be regarded as annotation in a strict sense , But this practice can also achieve the purpose of annotation .

6. Hexadecimal number

stay C There are octal numbers that can be recognized in the language 、 Decimal system 、 Hexadecimal .
Octal numbers require numbers 0 start , Such as 012 It's an octal number , His decimal system is 10
We are very familiar with decimal numbers , Such as 10, 12 etc.
Hexadecimal numbers need 0x start , Such as 0xa It's a hexadecimal number , His decimal system is 10
namely 012, 10, 0xa Their values are equal , Just different base .

7. Operator

C There are many language operators , Depending on the operands , Operators can be divided into monocular operators 、 Binocular operator 、 Ternary operator . If you use an operator, you only need one operand to complete , We call this operator a monocular operator , And so on . We know that four operations in mathematics have priority , Empathy . We C There are also operational priorities in languages .
c The relationship between all operators and priorities in the language can be seen in the following figure .

8. Escape character

because C Some characters in language should be given special meanings , In order to ensure that it does not conflict with its original intention , Therefore, the concept of importing escape characters . Escape character is to use \ Change the original meaning of characters , Give it a new meaning .
Common escape characters are ：
　　\n ： Line break
　　\t ： Horizontal tabs , Equivalent to one Tab key
　　\v ： Vertical tabs
　　\f ： Change the page
　　\\ ： Representing one \ character
　　\ddd ： 1 To 3 Any character represented by an octal number ,d Represents any octal number
　　\xhh ： 1 To 2 Any character represented by hexadecimal digits ,h Represents any hexadecimal number

9. C Language type conversion

C The type conversion of language can be divided into forced type conversion and implicit type conversion .
Cast :
　　 Casting is to cast one type into another . It's our forced conversion .
Implicit type conversion .
　　 Implicit type conversion is also converting one type to another . This conversion is done automatically by the compiler .
　　
When doing forced type conversion , If a large type is converted to a small type, precision will be lost , This is worth noting .

3、 ... and 、 Sample code

So much has been said about C Grammar of language , Here we will pass a complete example to let you further understand it .
Source code ：

#include <stdio.h>

int main()
{
    
    /*  Defining variables  */
    char a = 'x';
    short b = 23;
    int c = 10;
    long d = 67;
    float e = 3.14;
    double f = 12.13;

    printf("a = %c, b = %d, c = %d, d = %d, e = %f, f = %lf\n", a, b, c, d, e, f);
    printf("%d, %d\n", a, 10);   // Variables can be assigned at will , Constants cannot be .
    printf("%d, %d, %d\n", 012, 10, 0xa);   // Put octal 、 Decimal system 、 Hexadecimal numbers are printed in decimal .

    /*C The language division operator is the acquisition quotient , The remainder operator is to get the remainder */
    printf("%d %d %d\n", b+c, b/10, b%10); 

    /* octal 61 It's the decimal system 49,49 Corresponding ASCII The character is 1*/
    printf("\61, \x32\n");

    printf("c = %d\n", (int)e);   // take float Type cast to int Type output 

    return 0;
}

Running results ：

a = x, b = 23, c = 10, d = 67, e = 3.140000, f = 12.130000
120, 10
10, 10, 10
33 2 3
1, 2
c = 3