[C language] detailed explanation of pointer and array written test questions

author ：@ Ordinary person 1

special column ：《C Language from 0 To 1》

In a word ： In the past , All is prologue

explain : The past is irreparable , The future can change

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Preface

We have learned all the relevant knowledge points of pointer before , Start with the concept of pointer , Learn the meaning of pointer types , Operate the pointer , Then there is the pointer array , And the meaning of array names , And array pointers , A function pointer , Function pointer array, etc . We have learned the knowledge of pointer on the whole . In this blog, we will practice pointer and array topics , The whole process is full of content , Don't be distracted ！

One dimensional array

#include <stdio.h>
int main()
{
    
	// One dimensional array 
	int a[] = {
     1,2,3,4 };
	printf("%d\n", sizeof(a));
	printf("%d\n", sizeof(a + 0));
	printf("%d\n", sizeof(*a));
	printf("%d\n", sizeof(a + 1));
	printf("%d\n", sizeof(a[1]));
	printf("%d\n", sizeof(&a));
	printf("%d\n", sizeof(*&a));
	printf("%d\n", sizeof(&a + 1));
	printf("%d\n", sizeof(&a[0]));
	printf("%d\n", sizeof(&a[0] + 1));
	return 0;
}

Let's analyze ：

Before the start , We need to know sizeof() The size of bytes occupied by the calculated space , The array name is the first element address ( Two exceptions ：1.& The array name represents the address of the entire array ,sizeof( Array name ) Represents the entire array )

#include <stdio.h>
int main()
{
    
	// One dimensional array 
	int a[] = {
     1,2,3,4 };
	printf("%d\n", sizeof(a));//16
	//sizeof（ Array name ）, The array name represents the entire array , It calculates the size of the entire array , Unit is byte 

	printf("%d\n", sizeof(a + 0));//4/8
	//32 The platform is 4,64 The platform is 8
	//a+0 Not a separate array name a, So at this time a Represents the address of the first element of the array , First element address +0 Or the first element address 
	// The address value is 4/8, The current platform is x86 platform , The answer for 4

	printf("%d\n", sizeof(*a));//4
	//*a Medium a Is the address of the first element of the array ,*a Is to dereference the address of the first element 
	// The size of the first element is 4 Bytes 

	printf("%d\n", sizeof(a + 1));//4/8
	// there a Is the address of the first element of the array ,a+1 Is the address of the second element 
	// The size of the address is 4/8

	printf("%d\n", sizeof(a[1]));//4
	// The size of the second element is 4 Bytes 

	printf("%d\n", sizeof(&a));//4/8
	//&a The address of the extracted array , Address of array , It's just an address 
	// The size of the address is 4/8

	printf("%d\n", sizeof(*&a));//16
	//&a What you get is the address of the entire array , That is to say ---int(*)[4], The dereference access to it is an array 
	// So the size is 16
	// It can be simply understood as * and & Offset each other 

	printf("%d\n", sizeof(&a + 1));//4/8
	//&a What you get is the address of the array 
	//&a+1 From an array a The address of skipped backwards (4 Of an integer element ) Size of array 
	//&a+1 Or the address , The address size is 4/8

	printf("%d\n", sizeof(&a[0]));//4/8
	// The size of the address of the first element 
	// The size of the address is 4/8


	printf("%d\n", sizeof(&a[0] + 1));//4/8
	//&a[0]+1 Is the address of the second element 
	// The address size is 4/8 Bytes 
	return 0;
}

A character array

1

#include <stdio.h>
#include <string.h>
int main()
{
    
	char arr[] = {
     'a','b','c','d','e','f' };
	printf("%d\n", sizeof(arr));
	printf("%d\n", sizeof(arr + 0));
	printf("%d\n", sizeof(*arr));
	printf("%d\n", sizeof(arr[1]));
	printf("%d\n", sizeof(&arr));
	printf("%d\n", sizeof(&arr + 1));
	printf("%d\n", sizeof(&arr[0] + 1));

	printf("%d\n", strlen(arr));
	printf("%d\n", strlen(arr + 0));
	printf("%d\n", strlen(*arr));
	printf("%d\n", strlen(arr[1]));
	printf("%d\n", strlen(&arr));
    printf("%d\n", strlen(&arr + 1));
	printf("%d\n", strlen(&arr[0] + 1));
	return 0;
}

Code parsing ：

#include <stdio.h>
#include <string.h>
int main()
{
    
	char arr[] = {
     'a','b','c','d','e','f' };
	printf("%d\n", sizeof(arr));//6
	//sizeof( Array name ), Size of array , by 6

	printf("%d\n", sizeof(arr + 0));//4/8
	//arr+0 Is the address of the first element of the array 

	printf("%d\n", sizeof(*arr));//1
	// First element address dereference ,*arr Is the first element of the array , Size is 1 byte 


	printf("%d\n", sizeof(arr[1]));//1
	// The size of the second element is 1 Bytes 

	printf("%d\n", sizeof(&arr));//4/8
	//&arr Is the address of the array , Yes, the address is 4/8 Bytes 

	printf("%d\n", sizeof(&arr + 1));//4/8
	//&arr+1 Is the address after the array 
	// The address is 4/8

	printf("%d\n", sizeof(&arr[0] + 1));//4/8
	//&arr[0]+1 Is the address of the second element 
	// The address size is 4/8

	/**************************************************************/

	printf("%d\n", strlen(arr));// Random value >=6
	//'\0' The location is uncertain , So the output value is a random value 

	printf("%d\n", strlen(arr + 0));// Random value 
	//arr It's the first element address ,arr+0 Still the same , Still random 

	printf("%d\n", strlen(*arr));//errorr error 
	//strlen() The incoming address is , Pass in *arr when ,*arr It's the first element , Into ’a'
	//97 It's not the address , So there's an error , Similar to the wild pointer problem 

	printf("%d\n", strlen(arr[1]));//error
	// Pass in ‘b’ namely 98 The same result , There is a problem 

	printf("%d\n", strlen(&arr));// Random value 
	//&arr Get the address of the array ,'\0’ Not sure where , Still random 

    printf("%d\n", strlen(&arr + 1));// Random value 
	//&arr+1 Skip this array , Back '\0‘ The location of is still uncertain 
	// It can be understood as the random value above -6

	printf("%d\n", strlen(&arr[0] + 1));// Random value 
	//&arr[0]+1 Represents the address of the second element , Back '\0' The location is still uncertain 
	return 0;
}

2

#include <stdio.h>
#include <string.h>
int main()
{
    
	char arr[] = "abcdef";
	printf("%d\n", sizeof(arr));
	printf("%d\n", sizeof(arr + 0));
	printf("%d\n", sizeof(*arr));
	printf("%d\n", sizeof(arr[1]));
	printf("%d\n", sizeof(&arr));
	printf("%d\n", sizeof(&arr + 1));
	printf("%d\n", sizeof(&arr[0] + 1));


	printf("%d\n", strlen(arr));
	printf("%d\n", strlen(arr + 0));
	printf("%d\n", strlen(*arr));
	printf("%d\n", strlen(arr[1]));
	printf("%d\n", strlen(&arr));
	printf("%d\n", strlen(&arr + 1));
	printf("%d\n", strlen(&arr[0] + 1));
	return 0;
}

Code parsing ：strlen() Find the string length , Be careful ’\0’ The number of characters that appear before

#include <stdio.h>
#include <string.h>
int main()
{
    
	char arr[] = "abcdef";
	//a b c d e f \0
	printf("%d\n", sizeof(arr));//7
	// Array 7 Bytes 

	printf("%d\n", sizeof(arr + 0));//4/8
	// here arr Represents the address of the first element of the array ,arr+0 unchanged 
	// The address size is 4/8

	printf("%d\n", sizeof(*arr));//1
	// The first element address is dereferenced as the first element , The size is 1 Bytes 

	printf("%d\n", sizeof(arr[1]));//1
	// The byte size of the second element is 1

	printf("%d\n", sizeof(&arr));//4/8
	// The address size is 4/8

	printf("%d\n", sizeof(&arr + 1));//4/8
	// The address size is 4/8

	printf("%d\n", sizeof(&arr[0] + 1));//4/8
	// The address size is 4/8

	/**********************************************************/

	printf("%d\n", strlen(arr));//6
	// The array length is 6

	printf("%d\n", strlen(arr + 0));//6
	// There is no change 

	//printf("%d\n", strlen(*arr));//error
	// There is a problem 
	//printf("%d\n", strlen(arr[1]));//error
	// There is a problem 

	printf("%d\n", strlen(&arr));//6
	// Entire array , The length is 6

	printf("%d\n", strlen(&arr + 1));// Random value 
	//&arr+1 After '\0‘ The location is uncertain 

	printf("%d\n", strlen(&arr[0] + 1));//5
	// Start with the address of the second element 
	return 0;
}

3

#include <stdio.h>
#include <string.h>
int main()
{
    
	char* p = "abcdef";
	printf("%d\n", sizeof(p));
	printf("%d\n", sizeof(p + 1));
	printf("%d\n", sizeof(*p));
	printf("%d\n", sizeof(p[0]));
	printf("%d\n", sizeof(&p));
	printf("%d\n", sizeof(&p + 1));
	printf("%d\n", sizeof(&p[0] + 1));


	printf("%d\n", strlen(p));
	printf("%d\n", strlen(p + 1));
	printf("%d\n", strlen(*p));
	printf("%d\n", strlen(p[0]));
	printf("%d\n", strlen(&p));
	printf("%d\n", strlen(&p + 1));
	printf("%d\n", strlen(&p[0] + 1));
	return 0;
}

Code parsing ：

#include <stdio.h>
#include <string.h>
int main()
{
    
	char* p = "abcdef";
	printf("%d\n", sizeof(p));//4/8
	//p Is a pointer variable , The size is 4/8

	printf("%d\n", sizeof(p + 1));//4/8
	// The address size is 4/8

	printf("%d\n", sizeof(*p));//1
	// It's the first element 

	printf("%d\n", sizeof(p[0]));//1
	// First element 

	printf("%d\n", sizeof(&p));//4/8
	// The address is 4/8

	printf("%d\n", sizeof(&p + 1));//4/8
	// The address is 4/8

	printf("%d\n", sizeof(&p[0] + 1));//4/8
	// The address is 4/8

	/*********************************************************/
	printf("%d\n", strlen(p));//6
	// From the first element address to the end 6 Elements 

	printf("%d\n", strlen(p + 1));//5

	printf("%d\n", strlen(*p));//error

	printf("%d\n", strlen(p[0]));//error

	printf("%d\n", strlen(&p));// Random value 
	// stay p From the angle of , hinder '\0' Not sure 

	printf("%d\n", strlen(&p + 1));// Random value 
	// Or random values 

	printf("%d\n", strlen(&p[0] + 1));//5
	// Start with the address of the second element 
	return 0;
}

Two dimensional array

#include <stdio.h>
int main()
{
    
	int a[3][4] = {
     0 };
	printf("%d\n", sizeof(a));
	printf("%d\n", sizeof(a[0][0]));
	printf("%d\n", sizeof(a[0]));
	printf("%d\n", sizeof(a[0] + 1));
	printf("%d\n", sizeof(*(a[0] + 1)));
	printf("%d\n", sizeof(a + 1));
	printf("%d\n", sizeof(*(a + 1)));
	printf("%d\n", sizeof(&a[0] + 1));
	printf("%d\n", sizeof(*(&a[0] + 1)));
	printf("%d\n", sizeof(*a));
	printf("%d\n", sizeof(a[3]));
	return 0;
}

Code parsing ：

#include <stdio.h>
int main()
{
    
	int a[3][4] = {
     0 };
	printf("%d\n", sizeof(a));//48
	//3 That's ok 4 Column ,12*4=48

	printf("%d\n", sizeof(a[0][0]));//4
	// First row, first column element 

	printf("%d\n", sizeof(a[0]));//16
	// first line ： That is, the size of one-dimensional array ：4*4=16

	printf("%d\n", sizeof(a[0] + 1));//4/8
	//a[0] It represents the address of the first element of the entire one-dimensional array in the first row , That is, the address in the first row and the first column 
	//a[0]+1 It's the address in the first row and the second column , Yes, the address is 4/8

	printf("%d\n", sizeof(*(a[0] + 1)));//4
	// It's the elements in the first row and the second column , Size is 4 Bytes 

	printf("%d\n", sizeof(a + 1));//4/8
	//a Although the address of the two-dimensional array , But it is not placed alone sizeof Inside , I didn't take the address 
	//a Represents the address of the first element , The first element of a two-dimensional array is its first row ,a It's the address on the first line 
	//a+1 Just skip the first line , Indicates the address of the second line 
	// The address is 4/8 Bytes 

	printf("%d\n", sizeof(*(a + 1)));//16
	// Equivalent to the size of the second line 

	printf("%d\n", sizeof(&a[0] + 1));//4/8
	//&a[0] What you get is the address on the first line 
	//&a[0]+1 What you get is the address on the second line 
	// The address size is 4/8

	printf("%d\n", sizeof(*(&a[0] + 1)));//16
	// What you get is the size of the second line 

	printf("%d\n", sizeof(*a));//16
	//a Represents the address of the first element , It's the address on the first line 
	//*a It is the dereference of the address in the first line , What you get is the first line 
	// The size of the first line is 16

	printf("%d\n", sizeof(a[3]));//16
	// Some people here may say that the cross-border visit , There will be problems. 
	// But it doesn't ：
	// We did not operate on it , Just visit 
	// For example ：int a = 10;sizeof(int);sizeof(a);
	// We calculated a When , Just need to know a Only a few bytes are known for the type of 
	// about a[3] It's the same thing , The type is certain , So for 16

	return 0;
}

summary

• We need to understand what array names mean

• know sizeof() and strlen() The role of

• Understand and grasp the pointer , What is involved

Practice with these pointers and arrays , We have a new understanding of pointer , Achieved the purpose of this blog . That's all for now