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Such as ：int a[3][4]; Its “ Line subscript ” The range of phi is zero 0-2.“ Column subscript ” The range of phi is zero 0-3, An access beyond any subscript is a cross-border access .

3. Initialization of 2D array

（1） Because the two-dimensional array is stored linearly in memory , So you can write all the data in a curly bracket ：

int a[3][4]={1,2,3,4,5,6,7,8,9,10,11,12};

The code for ：

#include <stdio.h>

int main()
{
	int a[3][4] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12};
	int i,j;
	
	for(i = 0;i < 3; i++)
	{
		for(j = 0;j < 4;j++)
		{
			printf("%d ",a[i][j]);
		}
		printf("\n");
	}
	
	return 0;
	
}

Running results ：

（2） In order to more intuitively represent the distribution of elements , You can enclose the elements of each line with braces ：

int a[3][4]={
   {1,2,3,4},{5,6,7,8},{9,10,11,12}};

perhaps ：

int a[3][4]={
    {1,2,3,4},
    {5,6,7,8},
    {9,10,11,12}
};

（3） A two-dimensional array can also assign initial values to only some elements ：

int a[3][4]={
   {1},{5},{9}};

The code for ：

#include <stdio.h>

int main()
{
	int a[3][4] = {
   {1},{5},{9}};
	int i,j;
	
	for(i = 0;i < 3; i++)
	{
		for(j = 0;j < 4;j++)
		{
			printf("%d ",a[i][j]);
		}
		printf("\n");
	}
	
	return 0;
	
}

Running results ：

（4） If you want the entire two-dimensional array to be initialized to 0, Then write a... Directly in braces 0 that will do ：

int a[3][4]={0];

If it is not initialized , Then the read value is random , as follows ：

#include <stdio.h>

int main()
{
	int a[3][4];
	int i,j;
	
	for(i = 0;i < 3; i++)
	{
		for(j = 0;j < 4;j++)
		{
			printf("%d ",a[i][j]);
		}
		printf("\n");
	}
	
	return 0;
	
}

Running results ：

（5）C99 Also added a new feature ： Specifies the initialized element . In this way, only some specified elements in the array can be initialized and assigned , Elements that are not assigned values are automatically initialized to 0：

int a[3][4] = {[0][0] = 1,[1][1] = 2,[2][2] = 3};

The code for ：

#include <stdio.h>

int main()
{
	int a[3][4] = {[0][0] = 1,[1][1] = 2,[2][2] = 3};
	int i,j;
	
	for(i = 0;i < 3; i++)
	{
		for(j = 0;j < 4;j++)
		{
			printf("%d ",a[i][j]);
		}
		printf("\n");
	}
	
	return 0;
	
}

Running results ：

（6） Initialization of two-dimensional arrays can also be lazy , Let the compiler calculate the length of the array according to the number of elements . But only the second 1 The number of elements of a dimension can be omitted , Other dimensions must be written ：

int a[][4]={
   {1,2,3,4},{5,6,7,8},{9,10,11,12}};

4. Code combat ： Enter the grades in the table below , And realize matrix transpose

The code is as follows ：

#include <stdio.h>

int main()
{
	int a[4][5] = {
	
	{80,92,85,86,99},
	{78,65,89,70,99},
	{67,78,76,89,99},
	{88,68,98,90,99}
	};
	
	// Enter the score  
	int i,j;
	
	for(i = 0;i < 4; i++)
	{
		for(j = 0;j < 5;j++)
		{
			printf("%d ",a[i][j]);
		}
		printf("\n");
	}
	printf("\n");

	// Matrix transposition 
	for(i = 0;i < 5; i++)
	{
		for(j = 0;j < 4;j++)
		{
			printf("%d ",a[j][i]);
		}
		printf("\n");
	}
	
	return 0;
	
}

Running results ：

Two 、 The pointer （*）

1. Pointers and pointer variables

Popular speaking , Pointer means address .C There are special pointer variables in the language to store pointers , It is different from ordinary variables , The pointer variable holds the address , Ordinary variables store data .
Pointer variables have types , Its type is the data type pointed to by the stored address

2. Defining pointer variables ： Type name * Pointer variable name

char *pa;// Define a pointer variable that points to a character type
int *pb;// Define a pointer variable that points to an integer

3. Take address operator and value operator

If you need to get the address of the base variable , You can use the address operator （&）：

char *pa=&a;
int *pb=&f;

If you need to access the data pointed to by the pointer variable , You can use the value operator （*）：

printf（"%c,%d\n", *pa, *pb）;

The code for ：

#include <stdio.h>

int main()
{
	char a = 'F';
	int f = 123;
	
	char *pa = &a;
	int *pb = &f;
	
	printf("a = %c\n", *pa);
	printf("f = %d\n", *pb);
	
	*pa = 'C';
	*pb += 1;
	
	printf("now,a = %c\n", *pa);
	printf("now,f = %d\n", *pb);
	
	printf("the addr of a is : %p\n", pa);
	printf("the addr of b is : %p\n", pb);
	
	return 0;
}

Running results ：