C language from entry to soil (I)

1、 first C Language program

Study C Language is the first thing to know C The basic framework of language

This is the standard C Language The standard The main function is written （ need Keep in mind ）

#include <stdio.h>

int main()
{
    // sentence 
    return 0;
}

In some books, you can see the following two ways

#include <stdio.h>

void main()
{

    // sentence 
}

#include <stdio.h>

main()
{

    // sentence 
}

These are relatively early ways of writing , You can meet

understand ：c Language is based on a house ,main Function is quite a gate （ The main door ）,PS： There will be a side door ( This is equivalent to a custom function ), But use these side doors , Always step into the main door first

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2、 data type

type	Express	Size of bytes accessed Company (  The bit  - bit - B)	Output format
char	Character	1	%c  /  %s ( character string )
short	Short	2	%hd
int	integer	4	%d
long	Long integer	4	%ld
long long	long long	8	%lld
float	Single precision floating point	4	%f
double	Double precision floating point	8	%lf

Learn early C It's enough to know these when speaking

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3、 Constants and variables

3.1、 What is? Constant ？ Variable ？

（1） Constant

stay C In language , Frequently used Constant value To represent a constant

 for example :
      3.14;
      2022;
      666;

（2） Variable  

  Empathy , stay C In language , The value may change at any time It's a variable

  stay C In language , To use variables , First of all, know what you are going to use The type of variable , And then for this variable Take a name

for example ：

int i; // It defines a   The type is   integer  , The name is  i  The variable of 
char ch;// It defines a   The type is   Character  , The name is  ch  The variable of 

3.2、 Define the representation of variables

int i = 1;// by   Variable i  Initialize to  1

char ch = 'a';// by   Variable ch  Initialize to character  a

 3.3、 Classification of variables

（1） Global variables

understand ： stay   Curly braces   {  }    outside The variable is the global variable

（2） local variable

understand ： stay   Curly braces   {  }   Inside Variables are local variables

（3） Be careful ：

1、 Variable must be in front （ Whether global or local ）
2、 The same variable name cannot be defined repeatedly
3、 When the name of the global variable is the same as that of the local variable , Priority local variable

Examples are as follows ： 

Conclusion

All right. , That's all for this issue , I hope everyone can achieve their goals , Come on , Strive to achieve a higher level , See you next time , Bye-bye