First acquaintance with C language (Part 2)

keyword

typedef

typedef unsigned int uint;
typedef struct Node
{
    
	struct Node* next;
	int data;
}Node;
int main()
{
    
	unsigned int a = 0;
	// When we write an unsigned integer  unsigned int  For a long 
	// We can't write well   This is what we can use typedef  simplify 
	uint num = 0;
	struct Node n;
	Node N;
	return 0;
}

static（ Static ）

1. Modify local variables
2. Modify global variable
3. Modify function

//  Modify local variables 
void test()
{
    
	static int a = 1;
	a++;
	printf("%d ", a);
	// Print 2~11
}
int main()
{
    
	int a = 0;
	while (a < 10)
	{
    
		test();
		a++;
	}
	return 0;
}

Think about it without static What will be printed
I don't need code to demonstrate after modifying global variables and functions , Because you need to create another source file , Is that when static Modifying global variables and functions will turn their external link attributes into internal link attributes , That is, in another source file, we extern Less than their value , It can be understood that we have reduced their scope .

define（ Not a keyword, but a preprocessing instruction ）

//define  Defining macro 
//ADD  Macro name  (a,,b) Macro parameters   Parameter has no type  ((a)+(b))  Macrobody 
#define ADD(a,b) ((a)+(b));
int main()
{
    
	int a = 10;
	int b = 20;
	int c = ADD(a, b);// It's actually replacement , hold ADD(a,b) Replace with  int c = ((a)+(b));
	printf("%d\n", c); 
	return 0;
}

Another function is to define constants as mentioned above

#define M 100
int main()
{
    
	int a = M;
	int M = 0;// This is wrong 
	printf("%d \n", a);
	printf("%d \n", M);
}

register（ register ）

int main()
{
    
	// because cpu Now the speed of processing data is faster and faster   So we need memory with high speed to read and write files 
	// Otherwise cpu No matter how fast it is , Registers will soon meet cpu
	// Where computers store data   From high to low - Slower and slower - More and more memory - The price is getting more and more expensive 
	//  register 
	//  Cache (cache)
	//  Memory 
	//  Hard disk 
	//register  It is recommended that the compiler put num = 3 Put the value of in the register , It is applicable to a large number of num Value 
	register num = 3;
	return 0;
}

The pointer

When talking about pointers, we need to talk about memory first

Memory (Memory) Is an important part of a computer , Also called memory and main memory , It's for temporary storage CPU Operation data in , And data exchanged with external memory such as hard disk . It is external storage and CPU Bridge for communication , All programs in the computer run in memory , Memory performance affects the overall level of computer play . As soon as the computer starts running , The operating system will transfer the data to be calculated from memory CPU In the process of operation , When the operation is complete ,CPU Send the results out .

So memory is divided into memory units , Then number these memory units , The number is their address , And the pointer is the address

int main()
{
    
	// Memory unit  
	// Number  -  Address  -  A pointer is an address 
	int num = 10;// Memory is 4 Bytes , Inside the store 10
	// 0000 0000 0000 0000 0000 0000 0000 1010
	// 0 0 0 0 0 0 0 10
	// 0x0000000a
	int* p = #// p Represents a pointer variable  int Indicates that the address type of pointer variable storage is int
	//*  Represents the definition of a pointer variable  & Is to put num Take out the number of this memory unit and give it to p
	printf("%p\n", p);
	printf("%d\n", *p);
	//* Dereference operator   It is through the stored address to find the value in this address 
	return 0;
}

The size of the pointer variable

int main()
{
    
	printf("%d\n", sizeof(char*));
	printf("%d\n", sizeof(int*));
	printf("%d\n", sizeof(short*));
	printf("%d\n", sizeof(long*));
	printf("%d\n", sizeof(float*));
	printf("%d\n", sizeof(double*));
	// All output 4
	return 0;
}

32 The computer of bit , The pointer variable size is 4 Bytes
64 The computer of bit , Pointer variable size bits 8 Bytes

Selection structure

int main()
{
    
	int n = 0;
	printf(" choice 1/0:");
	scanf("%d",&n);
	if (n == 0)
		printf(" You gave up studying , I can only sell sweet potatoes \n");
	else
		printf(" You keep learning , You can find a good job \n");
	return 0;
}

Loop structure

int main()
{
    
	// The number of valid codes exceeds 2w That's ok   You are Daniel 
	int line = 0;
	while (line < 20000)
	{
    
		printf(" Still need to work hard to continue refuelling %d\n",line);
		line++;
	}
	printf(" You are Daniel ！\n");
	return 0;
}

Structure

int ,float,char,long,short,double These are all C Language built-in data types , But when we want to express a complex object , These types don't seem to express well , This is our custom type struct Type of structure , Structure type is the method of combining these single types .

struct Stu
{
    
	char name[20];
	int age;
	char sex[10];
	char tele[12];
};
void test(struct Stu* ps)
{
    
	// Structure pointer variable -> Member name 
	printf("%s %d %s %s\n", ps->name, ps->age, ps->sex, ps->tele);
}
int main()
{
    
	struct Stu  s = {
     "zhangsan", 18, "man", "15833299302" };
	printf("%s %d %s %s\n", s.name, s.age,s.sex,s.tele);
	// Structural variable . Member name 
	struct Stu* p = &s;
	test(&s);
	printf("%s %d %s %s\n", (*p).name, (*p).age, (*p).sex, (*p).tele);
	return 0;
}

Add ：define It is a preprocessing instruction, not a keyword , We will explain in detail later when we talk about preprocessing instructions
extern Is to declare external symbols