[C Advanced] pointer array vs array pointer

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1. Preface

The happiest thing in life is to fight , Not success , The most painful thing in life is laziness , Not failure . Hello everyone , Here is ecember. Today we are going to discuss the concept and application of pointer array and array pointer in pointer （ The following results are in VS2022 Chinese compiler ）.

2. Character pointer

We know that there is a type of pointer called character pointer, that is char*

int main()
{
    
	char ch = '0';
	char* pc = &ch;
	*pc = 'a';
	printf("%c\n", ch);

	return 0;
}

We have to use character pointer to store the address of characters .

Pen test 1

int main()
{
    
	const char* p1 = "abcdef";// Constant string _ And const irrelevant 
	const char* p2 = "abcdef";// Will not open up another space 

	char arr1[] = "abcdef";
	char arr2[] = "abcdef";// The space opened up by the array is two spaces 
	
	if (p1 == p2)
	{
    
		printf("p1 == p2\n");
	}
	else
	{
    
		printf("p1 != p2\n");
	}

	if (arr1 == arr2)
	{
    
		printf("arr1 == arr2\n");
	}
	else
	{
    
		printf("arr1 != arr2\n");
	}

	return 0;
}

What does the above code output ？ Here we must know that when we store strings with pointers , It is actually the address of the first character of the stored string , here p1, p2 Point to the same string , Therefore, the contents stored are the same . And we specify Different arrays when , Even if Same content , Opened up in memory The address is different .

3. Pointer array

seeing the name of a thing one thinks of its function , An array of pointers is an array , An array of pointers .

int main()
{
    
	// Shape array 
	//int arr[10];// Store integer 

	 A character array 
	//char arr2[5];// Store characters 

	//int* arr[10];// An array of integer pointers 
	//char* ch[5];// An array of character pointers 

	int a = 10;
	int b = 20;
	int c = 30;

	/*int* p1 = &a; int* p2 = &b; int* p3 = &c;*/

	int* arr[3] = {
     &a,&b,&c };
	int i = 0;
	for (i = 0; i < 3; i++)
	{
    
		printf("%d ", *arr[i]);// Dereference the contents of the array to get the data 
	}

	return 0;
}

application

int main()
{
    
	int arr1[5] = {
     1,2,3,4,5 };
	int arr2[5] = {
     2,3,4,5,6 };
	int arr3[5] = {
     3,4,5,6,7 };

	int* parr[3] = {
     arr1, arr2, arr3 };

	int i = 0;
	for (i = 0; i < 3; i++)
	{
    
		int j = 0;
		for (j = 0; j < 5; j++)
		{
    
			//printf("%d ", parr[i][j]);
			printf("%d ", *(parr[i] + j));
		}
		printf("\n");
	}

	return 0;
}

there parr Array is pointer array , The address of the first element of the above three arrays is stored ,parr[ i ] It is equivalent to the array name of each array , I.e. on behalf of The address of the element at the beginning of each line , You can add j To access each element of the array .

4. Array pointer

Array pointers are pointers ？ Or arrays ？ The answer is ： The pointer .
We are already familiar with ：
Shaping the pointer ： int * pint; A pointer that can point to shaped data . Floating point pointer ： float * pf; A pointer that can point to floating-point data . The array pointer should be ： can Pointer to array .
What about the basic structure of pointer array ？

int *p1[10];
int (*p2)[10];
//p1, p2 What are the differences ？

explain ：p2 The first and * combination , explain p Is a pointer variable , Then point to a size of 10 An array of integers . therefore p It's a pointer , Point to an array , It's called Array pointer .
Pay attention here ：[ ] Priority is higher than * The no. , So we have to add （） To guarantee p The first and * combination .

For the priority of operators , You can see my blog ： Operator details .

int main()
{
    
	int arr[10] = {
     0 };

	int(*p)[10] = &arr;// Array pointer 

	//p That is, the array pointer type name ——int(*)[10] Adding one indicates skipping an array 

	return 0;
}

practice

int main()
{
    
	char* arr[5] = {
     0 };

	char*(*p)[5] = &arr;

	return 0;
}

among p What is it? ？ First p The first and * Combine to form a pointer , Then we looked at another one behind [ ] Indicates that it points to a 5 Array of elements , What type of array elements ？——char * , So p Is an array element pointing to char* The pointer to the array of .

Little science popularization

&arr VS arr

What's the difference between the two ？

Usually , The array name we are talking about is the address of the first element of the array
But there are two exceptions ：
1.sizeof( Array name ), This represents the entire array ,sizeof It calculates the size of the entire array
2.& Array name , Here is the address of the whole array

Application of array pointer

void print2(int(*p)[5], int c, int r)
{
    
	int i = 0;
	for (i = 0; i < c; i++)
	{
    
		int j = 0;
		for (j = 0; j < r; j++)
		{
    
			//p + i Is pointing to the i Yes 
			//*(p + i) It's equivalent to getting the third i That's ok , Also equivalent to the second i Row array name 
			printf("%d ", *(*(p + i) + j));//p[i][j]
		}
		printf("\n"); 
	}
}
int main()
{
    
	int arr[3][5] = {
     {
    1,2,3,4,5},{
    2,3,4,5,6},{
    3,4,5,6,7} };
	// Function print arr Array 
	print2(arr, 3, 5);// What passed here is equivalent to the line pointer of the first line , That's the address on the first line 

	return 0;
}

We know that the address of the first line is actually passed by the parameters of the two-bit array , that p + i It is equivalent to getting the No i The address of the line , And then I'll explain the quotation , It's equivalent to getting the array name , namely The address of the first element of each line , Re pass +j Then dereference is to access every element of the two-dimensional array .

5. Conclusion

Here we are , our 《 Pointer array VS Array pointer 》 It's near the end , I will continue to update later C Language related content , Endless learning , It will always be left to those who are prepared . I hope my blog can help you , If you like it, you can give the thumbs-up + Collection Oh , You are also welcome to point out my mistakes in the comment area at any time .