Overview of basic knowledge of C language

ps: Basic understanding C Basic knowledge of language , Yes C There is a general understanding of language .
Every knowledge point is a simple understanding , Don't explain it in detail , Subsequent complement .

1. What is? C Language

C Language is a general computer programming language , Widely used in the underlying development .C The goal of language design is to provide a way to make it easy
The way to compile 、 Processing low-level memory 、 Generate a small amount of machine code and programming language that can run without any running environment support
said .
Even though C Language provides many low-level processing functions , But it still has good cross platform characteristics , Written in a standard specification
C Language programs can be compiled on many computer platforms , It even includes some embedded processors （ Single chip computer MCU） And super
Level computer and other operating platforms .
The 1980s , In order to avoid the use of C There are differences in language grammar , By the National Bureau of standards C Language system
A complete set of American national standard grammar , be called ANSI C, As C The original standard of language . [1] at present 2011 year 12 month 8
Japan , International Organization for Standardization （ISO） And the International Electrotechnical Commission （IEC） released C11 The standard is C The third official mark of language
accurate , It's also C The latest standard of language , The standard better supports Chinese character function name and Chinese character identifier , To a certain extent, it has realized Han
Word programming .
C Language is a process oriented computer programming language , And C++,Java Object oriented programming languages such as .
Its compiler mainly includes Clang、GCC、WIN-TC、SUBLIME、MSVC、Turbo C etc. .

2. first C Language program

#include <stdio.h>
int main()
{
    
    printf("hello bit\n");
    printf("he he\n");
    return 0; }
// explain ：
//main Function is the entry of a program 
// In a project main There is and only one function 

3. data type

char        // Character data type 
short       // Short 
int         // plastic 
long        // Long integer 
long long   // Longer plastic surgery 
float       // Single-precision floating-point 
double      // Double precision floating point 

ps: The size of the data type is bytes （byte）

ps: There are so many types , In fact, it is to express various values in life more abundantly .

4. Variable , Constant

Some values in life are constant （ such as ： PI , Gender , Blood type and so on ）
Some values are variable （ such as ： Age , weight , Salary ）.
Constant value ,C Used in language Constant The concept of , Become value C Used in language Variable To express .

4.1 How to define variables

char name[] = "eleven";
int height = 180;

4.2 Classification of variables

• local variable
• Global variables

#include <stdio.h>
int year = 2021;// Global variables 
int main()
{
    
    // As defined below year Will there be a problem ？
    int year = 2022;// local variable 
    printf("year = %d\n", year);
    return 0;
}

ps: The local variable above year There is no problem with the definition of variables ！
When a local variable has the same name as a global variable , Local variables take precedence .

4.3 Use of variables

#include <stdio.h>
int main()
{
    
    int num1 = 0;
   int num2 = 0;
    int sum = 0;
    printf(" Enter two operands :>");
    scanf("%d %d", &num1, &num2);
    sum = num1 + num2;
    printf("sum = %d\n", sum);
    return 0; }
    //scanf  Input function 
    //printf  Output function 

4.4 The scope and life cycle of variables

Scope
Scope （scope） It's a programming concept , Generally speaking , The names used in a program are not always valid / Usable , The scope of the code that defines the usability of the name is the scope of the name .

1. The scope of a local variable is the local scope of the variable .
2. The scope of global variables is the whole project .

Life cycle
The life cycle of a variable is the period between the creation of a variable and its destruction

1. The life cycle of a local variable is ： Enter the scope lifecycle begins , Out of scope life cycle ends .
2. The life cycle of global variables is ： The whole life cycle of the program .

4.5 Constant

C There are differences in the form of definitions between constants and variables in languages .
C Constants in a language can be divided into the following categories ：

• Literal constants
• const Modified constant variable
• #define Defined identifier constant
• Enumeration constants

#include <stdio.h>
// give an example 
enum Sex
{
    
 MALE,
 FEMALE,
 SECRET
};
// In brackets MALE,FEMALE,SECRET It's an enumeration constant 
int main()
{
    
    // Literal constants demonstrate 
    3.14;// Literal constants 
    1000;// Literal constants 
    
    //const  Modified constant variable 
    const float pai = 3.14f;   // there pai yes const Modified constant variable 
    pai = 5.14;// It can't be modified directly ！
    
    //#define The identifier constant for   demonstration 
 #define MAX 100
    printf("max = %d\n", MAX);
    
    // Enumeration constants demonstrate 
    printf("%d\n", MALE);//0
    printf("%d\n", FEMALE);//1
    printf("%d\n", SECRET);//2
    // notes ： The default of enumeration constants is from 0 Start , Increase in descending order 1 Of 
    return 0;

ps: In the above example pai go by the name of const Modified constant variable , const The modified constant is C In language, variables are limited only at the grammatical level pai Can't be changed directly , however pai Is essentially a variable , So it's called a constant variable .

5. character string + Escape character + notes

5.1 character string

"hello world"

This is made up of double quotation marks （Double Quote） A string of characters is called a string literal （String Literal）, Or string for short .

ps: The end of a string is a \0 The escape character of . When calculating the length of a string \0 It's the end sign , It doesn't count as string content .

#include <stdio.h>
// The following code , What is the result of printing ？ Why? ？（ prominent '\0' Importance ）
int main()
{
    
    char arr1[] = "hello";
    char arr2[] = {
    'h', 'e', 'l', 'l', 'o'};
    char arr3[] = {
    'h', 'e', 'l', 'l', 'o', '\0'};
    printf("%s\n", arr1);//5
    printf("%s\n", arr2);// Random value 
    printf("%s\n", arr3);//5
    return 0;
}

5.2 Escape character

If we want to print a directory on the screen ： c:\code\test.c
How do we write code ？

#include <stdio.h>
int main()
{
    
    printf("c:\code\test.c\n");
    return 0;
}

In fact, the result of the program is ：

I have to mention the escape character here . Escape characters, as the name suggests, change meaning .
Let's look at some escape characters ：

Escape character	paraphrase
\?	Use... When writing multiple consecutive question marks , Prevent them from being parsed into three letter words
\’	Used to represent character constants ’
\"	Double quotation marks used to represent the inside of a string
\\	Used to indicate a backslash , Prevent it from being interpreted as an escape sequence character
\a	Warning characters , Beep
\b	Back space
\f	Change the page
\n	Line break
\r	enter
\t	Horizontal tabs
\v	Vertical tabs
\ddd	ddd Express 1~3 Eight octal numbers . Such as ： \130 X
\xdd	dd Express 2 Hexadecimal numbers . Such as ： \x30 0

Small test
1

#include <stdio.h>
int main()
{
    
    // problem 1： Print a single quote on the screen ', How do you do it? ？
    // problem 2： Print a string on the screen , The content of the string is a double quote “, How do you do it? ？
    return 0;
}

2

// What does the program output ？
#include <stdio.h>
int main()
{
    
    printf("%d\n", strlen("abcdef"));
    printf("%d\n", strlen("c:\test\628\test.c"));
    return 0;
}

Refer to the answer

1.
printf("%c\n", '\'');
printf("%s\n", "\"");
2.
6
14// /t,\62 Parsed into an escape character 
 Hee hee , Did you do it right 

5.3 notes

1. There are unnecessary codes in the code that can be deleted directly , You can also comment out
2. Some of the code is hard to understand , You can add the annotation text

such as ：

#include <stdio.h>
int Add(int x, int y)
 {
    
    return x+y;
 }
/*C Language style notes  int Sub(int x, int y) { return x-y; } */
int main()
{
    
    //C++ Annotation style 
    //int a = 10;
    // call Add function , Complete the addition 
    printf("%d\n", Add(1, 2));
    return 0; 
 }

There are two styles of annotation ：

• C Notes on language style /* xxxxxxx*/
  defects ： You can't nest comments
• C++ Style notes //xxxxxxxx
  You can comment on one line or multiple lines

6. Select statement

#include <stdio.h>
int main()
{
    
    int coding = 0;
    printf(" Would you type the code ？（ choice 1 or 0）:>");
    scanf("%d", &coding);
    if(coding == 1)
   {
    
       printf(" come on. , Bai Fumi is waving to you \n");
   }
    else
   {
    
       printf(" Go home and farm \n");
   }
    return 0; }

Click on this blog , That's the choice ！
give the thumbs-up , Comment on , Collection , Focus on , It's also a choice ！

7. Loop statement

C How to implement a loop in a language ？

• while sentence
• for（ Subsequent complement ）
• do…while( Subsequent complement ）
  Code up

//while Examples of cycles 
#include <stdio.h>
int main()
{
    
    int row = 0;
    while(row<=30000)
   {
    
        printf(" I'm going to keep typing \n");
        row++;
   }
    if(line>30000)
    {
    
        printf(" good offer\n");
    }
    return 0;
}

8. function

The feature of the function is to simplify the code , Code reuse .
Go straight to the code ：

#include <stdio.h>
int main()
{
    
    int num1 = 0;
   int num2 = 0;
    int sum = 0;
    printf(" Enter two operands :>");
    scanf("%d %d", &num1, &num2);
    sum = num1 + num2;
    printf("sum = %d\n", sum);
    return 0;
}
 The above code , Write the function as follows ：
#include <stdio.h>
int Add(int x, int y) 
{
    
   int z = x+y;
   return z;
}
int main()
{
    
    int num1 = 0;
   int num2 = 0;
    int sum = 0;
    printf(" Enter two operands :>");
    scanf("%d %d", &num1, &num2);
    sum = Add(num1, num2);
    printf("sum = %d\n", sum);
    return 0;
}

9. Array

To store 1-10 The number of , How to store ？
C The definition of array is given in the language ： A collection of elements of the same type

9.1 To define an array

int arr[10] = {1,2,3,4,5,6,7,8,9,10};// Define an integer array , Put at most 10 Elements

9.2 Index of the array

C Language policy ： Each element of an array has a subscript , The subscript is from 0 At the beginning .
Array can be accessed by subscript .

int arr[10] = {0};
// If the array 10 Elements , The scope of the subscript is 0-9

9.3 Use of arrays

#include <stdio.h>
int main()
{
    
 int i = 0;
 int arr[10] = {
    1,2,3,4,5,6,7,8,9,10};
 for(i=0; i<10; i++)
 {
    
       printf("%d ", arr[i]);
 }
 printf("\n");
    return 0;
 }

11. The operator

First, a brief introduction , Make up later

• arithmetic operator

+ - * / %

• Shift operator

>> <<

• Bit operators

& ^ |

• Assignment operator

= += -= *= /= &= ^=  |=    >>=   <<=

• Monocular operators

!            Logical anti operation 
-            negative 
+            Comes at a time 
&            Address fetch 
sizeof       The type length of the operands （ In bytes ）
~            To reverse the binary of a number 
--           In front of 、 After --
++           In front of 、 After ++
*            Indirect access operators ( Dereference operator )
( type )        Cast 

• Relational operator

>
>=
<
<=
!=         It's not equal 
==         equal 

• Logical operators

&&        Logic and 
||        Logic or 

• Conditional operators

exp1 ? exp2 : exp3

• Comma expression

- exp1, exp2, exp3, ...expN 

• Subscript reference 、 Function calls and structure members

[] () . ->

12. Common keywords

C The language provides a wealth of keywords , These keywords are preset by the language itself , Users cannot create keywords themselves .

13.define Define constants and macros

Code up. , Easy to understand

//define Define identifier constants 
#define MAX 1000
//define Defining macro 
#define ADD(x, y) ((x)+(y))
#include <stdio.h>
int main()
{
    
    int sum = ADD(2, 3);
    printf("sum = %d\n", sum);
    
    sum = 10*ADD(2, 3);
    printf("sum = %d\n", sum);
    
    return 0; }

14. The pointer

14.1 Memory

Memory is a very important memory on a computer , All programs in a computer run in memory .
So in order to use memory efficiently , Divide the memory into small memory units , The size of each memory unit is 1 Bytes .
In order to effectively access every unit of memory , The memory units are numbered , These numbers are called the The address of the memory unit .

Memory	Address
A byte	0x00000002
A byte	0x00000001
A byte	0x00000000

Variables are created in memory （ To allocate space in memory ）, Each memory cell has an address , So variables also have addresses .
Take out the variable address as follows ：

#include <stdio.h>
int main()
{
    
 int num = 10;
 &num;// Take out num The address of 
    // notes ： here num Of 4 Bytes , Every byte has an address , What is taken out is the address of the first byte （ Smaller address ）
 printf("%p\n", &num);// Print address ,%p Is printed as an address 
 return 0;
}

How to store the address , Need to define pointer variables .

int num = 10;
int *p;//p For an integer pointer variable 
p = #

The use of pointers ：

#include <stdio.h>
int main()
{
    
 int num = 10;
 int *p = &num;
 *p = 20;
 return 0;
}

Take the example of a reshaped pointer , It can be extended to other types , Such as :

#include <stdio.h>
int main()
{
    
 char ch = 'w';
 char* pc = &ch;
 *pc = 'q';
 printf("%c\n", ch);
 return 0;
}

14.2 The size of the pointer variable

#include <stdio.h>
// The size of the pointer variable depends on the size of the address 
//32 The address under the bit platform is 32 individual bit position （ namely 4 Bytes ）
//64 The address under the bit platform is 64 individual bit position （ namely 8 Bytes ）
int main()
{
    
    printf("%d\n", sizeof(char *));
    printf("%d\n", sizeof(short *));
    printf("%d\n", sizeof(int *));
    printf("%d\n", sizeof(double *));
    return 0; }

ps: The size of the pointer is 32 Bit platform is 4 Bytes ,64 Bit platform is 8 Bytes .

15. Structure

The structure is C Especially important knowledge points in language , The structure makes C Language has the ability to describe complex types .
For example, describe students , Students include ： name + Age + Gender + Student number These information .
Only structures can be used to describe .
for example ：

struct Stu
{
    
    char name[20];// name 
    int age;      // Age 
    char sex[5];  // Gender 
    char id[15]; // Student number 
};

Initialization of structure ：

// Print structure information 
struct Stu s = {
    " Zhang San ", 20, " male ", "20180101"};
//. Access operators for structure members 
printf("name = %s age = %d sex = %s id = %s\n", s.name, s.age, s.sex, s.id);
//-> The operator 
struct Stu *ps = &s;
printf("name = %s age = %d sex = %s id = %s\n", ps->name, ps->age, ps->sex, ps- >id);

Summary ：
Come here C I have probably passed the elementary knowledge of language , Thank you for watching , Later, bloggers will send out detailed versions after learning new knowledge , The first time I write a blog, there may be some things I haven't done well , Please forgive me , If there are mistakes, you are welcome to point them out .
Cough , You see this , I'll go if I like it ~