C language learning notes - P23 (< C language high level > + advanced level of pointer < 1 > + problem example + illustration)

Posted by dgs on Tue, 18 Jan 2022 00:59:52 +0100

We have already touched the topic of pointer in the chapter of pointer at the primary stage. We know the concept of pointer:
1. A pointer is a variable used to store an address, which uniquely identifies a piece of memory space.
2. The size of the pointer is fixed 4/8 Bytes( 32 Bit platform /64 Bit platform).
3. Pointers are typed, and the type of pointer determines the type of pointer +- The step size of an integer and the permission of pointer dereference operation.
4. Pointer operation.

In this chapter, we continue to explore the advanced topic of pointers.

1. Character pointer

Among the pointer types, we know that one pointer type is character pointer char* ;

General use:

int main()
{
    char ch = 'w';
    char *pc = &ch;
    *pc = 'w';
    return 0;
}

There is another way to use it as follows:
int main ()
{
    const char* pstr = "hello bit" ;  // Here is to put a string into pstr Is it in the pointer variable?
    printf ( "%s\n" , pstr );
    return 0 ;
}

Code const char* pstr = "hello bit."; It's very easy for students to think that they put the string hello bit into the character pointer pstr, but / in essence, they put the string hello bit The address of the first character is placed in pstr.

The above code means that the first character of a constant string h The address of is stored in the pointer variable pstr Yes. 

int main()
{
	//int a = 10;
	//printf("%p\n",  &a);
	//char ch = 'w';
	//char* p = &ch;

	const char* p = "abcdef";
	//*p = 'w';

	return 0;
}
Then there are such interview questions: 
#include <stdio.h>
int main()
{
	char str1[] = "hello bit.";
	char str2[] = "hello bit.";
	const char* str3 = "hello bit.";
	const char* str4 = "hello bit.";
	if (str1 == str2)
		printf("str1 and str2 are same\n");
	else
		printf("str1 and str2 are not same\n");

	if (str3 == str4)
		printf("str3 and str4 are same\n");
	else
		printf("str3 and str4 are not same\n");

	return 0;
}

int main()
{
	char arr1[] = "abcdef";
	char arr2[] = "abcdef";

	const char* str1 = "abcdef";
	const char* str2 = "abcdef";

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

	if (str1 == str2)
		printf("str1==str2\n");
	else
		printf("str1!=str2\n");

	return 0;
}

The final output here is:

Here, str3 and str4 point to the same constant string. C/C + + will store the constant string in a separate memory area when several pointers. When pointing to the same string, they actually point to the same block of memory. However, when initializing different arrays with the same constant string, different memory blocks will be opened up. So str1 and str2 are different, and str3 and str4 are different.

 2. Pointer array

In the "pointer" chapter, we also learned the pointer array, which is an array of pointers.
Let's review here. What does the following pointer array mean?
int* arr1 [ 10 ];             // Array of integer pointers
char * arr2 [ 4 ];           // Array of first level character pointers
char ** arr3 [ 5 ];         // Array of secondary character pointers

 

 

ypedef int* pint;
#define PINT int*
int main()
{
	//int a, b;//a int b int
	//int *pa, pb;
	pa -> int*
	pb -> int

	//int * pa, * pb;
	//
	//pint pa, pb;
	//pa int*
	//pb int*

	PINT pa, pb;//int * pa,pb;
	//pa -> int*
	//pb -> int

	return 0;
}
//Integer array - an array that holds integers
//Character array - an array of characters
//
//Pointer array - an array of pointers


int main()
{
	char* arr[] = { "abcdef", "qwer", "zhangsan" };
	int i = 0;
	int sz = sizeof(arr) / sizeof(arr[0]);

	for (i = 0; i < sz; i++)
	{
		printf("%s\n", arr[i]);
	}
	return 0;
}
int main()
{
	int arr1[] = { 1,2,3,4,5 };
	int arr2[] = { 2,3,4,5,6 };
	int arr3[] = { 3,4,5,6,7 };

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

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

	return 0;
}

Postscript:
● due to the limited level of the author, it is inevitable that there are fallacies in the article. Please correct the slang, and sincerely hope to give advice!

-- By author: Xinxiao · old acquaintance

Topics: C

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