First acquaintance with C language (1)

                                          Hello everyone , I first met Yu Dong .

【 Statement 】： Because the author's record level is limited , Mistakes will inevitably appear in future blog posts , Readers are expected to criticize and put forward your valuable opinions , Looking forward to your comments .

List of articles

• Preface
• One 、 What is? c Language ？
• Two 、 first C Language program
  • How to use vs2019
  • Programming
• 3、 ... and 、 data type
• Four 、 Variable . Constant
• summary

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Preface

Basic understanding C Basic knowledge of language , Yes C Have a general understanding of language .

One 、C What is language ？

C Language is a general computer programming language , Widely used in the underlying development .C The design goal of the language is to provide a way to compile 、 Processing low-level memory 、 Generate a small amount of machine code and a programming language that can run without any support of the running environment .

Application field ： Application software （ Baidu SkyDrive /QQ）、 operating system （win/linux）、 Driving layer, etc

C The origin of language ： With the development of machine language , Binary cannot meet the increasingly complex work requirements . There is a mnemonic derived from binary . This gave birth to assembly language , And then it happened B Language , Finally, it forms what we use today C Language .

C Common compilers for languages are ：Clang、GCC、WIN-TC、SUBLIME、MSVC、Turbo C etc. .

Two 、 first C Language program

1. How to use vs2019

Tips ： As a beginner , Bloggers want to recommend you to use vs2019 This software ！ Really super friendly to Xiaobai ！

Recommended reasons ： You can prompt your errors in time when writing , And support many practical shortcuts ！

                                                      I don't say much nonsense , On dry ！

① open vs2019, Click to create a new project .

② Create a new project

 

 ③ Configure new projects

 ④ Create new item

 ⑤ establish C Language source file

2. Programming

#include<stdio.h>

int main()
{
    printf("hello word!\n");
    return 0;
}

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  explain ： This code means printing on the screen ：hello word!

a. #include<stdio.h> It refers to the inclusion of header files .

b. <stdio>  ------std（ standard ）   intput( Input ） output（ Input ） It refers to standard input and output .

c.  <.h>------header( The header file ）

d.main It means main function （ It's the entrance to the program , There is and only one ）

e. int For integer （ The return type of the function ： integer ——> Corresponding （return 0;））

f. "   " Enclosed is called string .

g. printf It's a library function , The use of library functions is to include header files ——>  <stdio.h>

h.  \n Means line break .

Be careful ：C The habit of language is that the return value is 0 Yes means normal return , The return value is not 0 Indicates abnormal return .

Share vs2019 Use tips ：

Click the line of code you want to copy （ctrl+c）, And then use （ctrl+v） Paste directly .

（ctrl+k+c） For comment   （ctrl+k+u） uncomment   （Fn+F10） debug  

（home） Jump to the beginning of the line   （end） Jump to the end of the line

3、 ... and 、 data type

data type	name	length （ byte ）
Character	char	1
Short	short	2
integer	int	4
Long integer	long	4
Longer integers	long long	8
Single-precision floating-point	float	4
Double precision floating point	double	8

Common storage unit of computer

8bit=1byte

1KB=1024byte

1MB=1024KB

1GB=1024MB

1TB=1024GB

1PB=1024TB

notes ： Type is to create variables

example ：int age=20;

float weight=55.6;（ here 55.6 The default is double type ）

float weight=55.6f;（ This is the case float type ）

Four 、 Variable . Constant

Some values in life are constant （ such as ： PI , Gender , Id card number , Blood type and so on ）

And some values are variable （ such as ： Age 、 weight 、 Salary ）

Constant value ,C The concept of constant is used in language to express , Variable value C In language, variables are used to represent .

① How to define variables

int age=22;
float weight=45.5f;
char ch='f';

② Variable name

• Only letters （ Including uppercase and lowercase ）、 Numbers and underscores （_） form
• Cannot start with a number
• The length cannot exceed 63 character
• Variable names are case sensitive
• Variable names cannot use keywords

③ Classification of variables

• local variable
• Global variables

#include<stdio.h>
int global=2022;// Global variables 
int main()
{
    int local=2020;// local variable 
    int global=2021;// local variable 
    printf("global=%d\n",global);
    printf("local=%d\n",local);
    return 0;
}

summary ：

Outside the braces are global variables , Internal are local variables

  expand ：

Local variables outside braces global There is no problem with variables ！ But when the local variable and the global variable have the same name , Local variables take precedence

  Use of variables

#include<stdio.h>
int main()
{
   int num1=0;
   int num2=0;
   int num3=0;
   printf(" Please enter two numbers ：");
   scanf("%d %d" &sum1,&sum2);
   sum3=sum1+sum2;
   printf("sum3=%d\n,sum3");
   return 0;
}

  notes ：printf Enter a statement for ,scanf For the output statement .

④ The scope and life cycle of variables

Scope

Scope （scope） It's a programming concept , Generally speaking , The names used in a piece of program code 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 （ Other projects in the project can also be accessed ）

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 .

⑤ Constant

C The definition forms of constants and variables of language are different

C Language constants are divided into the following

• 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 
      500;// 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 Demonstration of identifier constants 
      #define MAX  100
      printf("max=%d\n",MAX);

     // Enumeration constants demonstrate 
     printf("%d\n,"MALE);
     printf("%d\n,"FEMALE);
     printf("%d\n,"SECRET);
     // notes ： The default of enumerating variables is from 0 Start , Descending in turn 
     return 0;
 }

  notes ：

In the above example pai go by the name of const Modified constant variable ,const The modified constant is C In language, it is only the grammatical level that restricts the variable beat from being directly changed , however pai The essence is a variable , So it's called a constant variable .

summary

The above is the initial stage I summarized for you C Language dry goods , If there are mistakes, welcome baozi to criticize and correct in the comment area ！ I also hope the knowledge I summarized can bring you harvest ！