Analysis of classical pointer and array written test questions in C language

We have learned about arrays before , Pointer initial and advanced . After learning the above knowledge, I haven't practiced many topics , Let's take a look c Analysis of language classic pointer and array written test questions

One dimensional array

#include<stdio.h>
int main()
{
    
	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 review the basic knowledge before the topic exercise
sizeof() The calculation is the size of the space occupied by the data, and the unit is bytes .
The array name represents the address of the first element of the array , But there are two exceptions ：

• sizeof( Array name ) It calculates the size of the entire array , Note that only the array name , Other operations are not the size of the entire array .
• & Array name , It takes out the address of the entire array ,+1,-1 Skip the entire array .

The size of the address is 4/8, Computer related .

title

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

	printf("%d\n", sizeof(a + 0));// 4/8
	//sizeof() Inside a It doesn't exist alone , either &
	// So the array name represents the address of the first element of the array 
	printf("%d\n", sizeof(*a));//4
	//*a in a Is the address of the first element of the array ,*a What you find is the first element of the array 
	// The first element is shaping , Size is 4 byte 

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

	printf("%d\n", sizeof(&a));
	//&a Is the address of the entire array , As long as the address size is 4/8

	printf("%d\n", sizeof(*&a));//16
	// There are two ways of understanding 
	//1.&a<-->int (*)[4]
	// &a What you get is the address of the array name , The type is int (*)[4], It's an array pointer 
	//  The array pointer points to an array ,*&a-->a
	//
	//2.& and * Offset ,*&a-->a
	
	printf("%d\n", sizeof(&a + 1));// 4/8
	//&a What we get is the address of the array ,&a<-->int (*)[4]
	//&a+1  Is from an array a The address of skipped a backward (4 A plastic element ) Size of array .
	//&a+1 Or the address , Yes, the address is 4/8 byte 

	printf("%d\n", sizeof(&a[0]));//4/8
	//a[0] Array head element ,&a[0] Address of the first element of the array 
	
	printf("%d\n", sizeof(&a[0] + 1));// 4/8
	//&a[0]+1, The address of the first element of the array +1--> The address of the second element of the array .

	return 0;
}

A character array

Additional knowledge is needed ,strlen Is the function , The header file <string.h>, Is a special function to obtain the length of a string , The end flag of the string is ’\0’, When does the character array appear ‘\0’ Namely strlen Function to get the length of the string .

The first group –> A character array

#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 ), The space occupied by the array ,1*6=6byte
	printf("%d\n", sizeof(arr + 0));// 4/8
	//arr+0, The array name does not appear separately in sizeof Inside , The array name represents the address of the first element of the array 
	//arr+0 The address of the first element of the array 
	printf("%d\n", sizeof(*arr));// 1
	//*arr, Dereference the address of the first element of the array , Array head element .
	// The first element of the array is of character type , The size is 1byte
	printf("%d\n", sizeof(arr[1]));// 1
	// The size of the first element of the array 1byte
	printf("%d\n", sizeof(&arr));// 4/8
	//&arr, Fetch the address of the entire array , It's an address 
	// Yes, the address size is 4/8
	printf("%d\n", sizeof(&arr + 1));//4/8
	//&arr + 1 Is the address after the array .
	printf("%d\n", sizeof(&arr[0] + 1));//4/8
	//&arr[0] + 1 Is the address of the second element of the array 

	printf("%d\n", strlen(arr));// Random value 1
	// In the character array 'a','b','c','d','e','f'.
	// After the character array '\0' I don't know where it is .
	// So the size is random 
	printf("%d\n", strlen(arr + 0));/// Random value 
	//arr+0 Address of the first element of the array , and arr The effect is the same 
	
	//printf("%d\n", strlen(*arr));// Report errors 
	//*arr Is the first element of the array 'a',ASCII Code for 97
	//strlen('a')-->strlen(97)
	// The address is 97 The memory of belongs to the system , Users are not allowed to use .
	// The address visited cannot predict the effect ,*arr It can be regarded as a wild pointer 
	// Changing the code will report an error 
	
	//printf("%d\n", strlen(arr[1]));// Report errors 
	//a[1]-->'b',
	//strlen('b')-->strlen(98)
	// The reason for the error is *a identical 
	printf("%d\n", strlen(&arr));// Random value 1
	//&a For the address of the entire array , But address access still starts from the address of the first element 
	// So the result is also a random value 
	printf("%d\n", strlen(&arr + 1)); // Random value 1 - 6
	//&arr+1, Skip the entire array , The address is the address after the array 
	// It is also a random value , But the random value is a random value 1-6
	printf("%d\n", strlen(&arr[0] + 1)); // Random value 1 - 1
	//&arr[0]+1 Is the address of the second element of the array , The result is also a random value 
	// But the random value is   Random value 1-1

	return 0;
}

character string

character string "abcdef", Stored in character array ,f Post tacit view ’\0’,
strlen Is to find the length of the string , Focus on... In the string \0, The calculation is \0 The number of characters that appear before
strlen It's a library function , For strings only
sizeof Only pay attention to the size of memory space , Don't care what's in the memory
sizeof It's the operator

#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;
}

title

#include<stdio.h>
#include<string.h>
int main()
{
    
	char arr[] = "abcdef";
	printf("%d\n", sizeof(arr));//7, Array size 
	printf("%d\n", sizeof(arr + 0));//4/8  The address size of the first element of the array 
	printf("%d\n", sizeof(*arr));//1, The size of the first element of the array 
	printf("%d\n", sizeof(arr[1]));//1, The size of the second element of the array 
	printf("%d\n", sizeof(&arr));//4/8, The size of the entire array address 
	printf("%d\n", sizeof(&arr + 1));//4/8, The size of the address after the array 
	printf("%d\n", sizeof(&arr[0] + 1));//4/8, Size of the address of the second element of the array 

	printf("%d\n", strlen(arr));//6, The size of the string 
	printf("%d\n", strlen(arr + 0));//6
	printf("%d\n", strlen(*arr));// error ,*arr=‘a’; Illegal access address 
	printf("%d\n", strlen(arr[1]));// error ,arr[1]='b'; Illegal access address 
	printf("%d\n", strlen(&arr));//6, The address of the entire array , What you get is also the address at the lowest address 
	printf("%d\n", strlen(&arr + 1));// Random value , Skipping the entire array, I don't know the address behind the array 
	//‘/0’ The location of the is uncertain 
	printf("%d\n", strlen(&arr[0] + 1));//5, The pointer jumps over 1 Bytes of character size 
	// Is the address of the second element , Calculate string length 6-1

	return 0;
}

Constant string

What should be noted is that ,char* p = “abcdef”;p It's a pointer variable . p Stored "abcdef" Is the first address of the string , Instead of stored strings .p+1 What is skipped is the space occupied by a character

Exercises ：

#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;
}

title :

#include<stdio.h>
#include<string.h>
int main()
{
    
	char* p = "abcdef";
	printf("%d\n", sizeof(p));//4/8, The size of the pointer variable 
	printf("%d\n", sizeof(p + 1));//4/8, The address of the second character of the string constant 
	printf("%d\n", sizeof(*p));//1, The size of the first character of a string constant 
	printf("%d\n", sizeof(p[0]));//1, The size of the first character of a string constant 
	printf("%d\n", sizeof(&p));//4/8, Character pointer p The address of 
	printf("%d\n", sizeof(&p + 1));//4/8,+1 Skip the size of a character pointer ,
	// But the space points to who is uncertain .
	printf("%d\n", sizeof(&p[0] + 1));//4/8,p[0]-->*(p+0)-->*p
	//&p[0]+1-->p+1, The address of the second character of the string constant 

	printf("%d\n", strlen(p));//6
	printf("%d\n", strlen(p + 1));//5
	printf("%d\n", strlen(*p));//err
	printf("%d\n", strlen(p[0]));//err
	printf("%d\n", strlen(&p));// Random value ,&p Is to take out the pointer variable p The address of ,
	// Pointer to the variable p The address of the constant string is stored ,strlen The function takes the address of the string constant 
	// Think of it as a string , This is an uncertain value , The storage of the address of the constant string in the pointer variable 
	// Whether it is big end storage or small end storage is still uncertain , So the result is a random value 
	printf("%d\n", strlen(&p + 1));// Random value , But with the &p There is no relationship between two random values 
	//&p+1 Is to skip the size of a pointer , The address pointed to is the same as p It is not certain that '\0'.
	// Or who didn't '\0', Still none '\0'.
	printf("%d\n", strlen(&p[0] + 1));//5,&p[0]+1-->p+1

	return 0;
}

Two dimensional array

Some knowledge that needs attention
A two-dimensional array can be regarded as an array of a one-dimensional array
The array name represents the address of the first element , The first element of a two-dimensional array is a one-dimensional array , So the array name of the two-dimensional array represents the address of the first row of one bit array
int a[3][4] = { 0 };

• a[0] It can be regarded as the address of the first row of a two-dimensional array , And so on a[1],a[2]
• arr Alone in sizeof() Inside , perhaps &arr Is the address of the entire two-dimensional array , The rest is arr[0] The address of , data type int(*)[4]

sizeof() Internal expressions do not operate , Just get the data type of the expression

Example :

#include<stdio.h>
#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;
}

analysis ：

#include<stdio.h>
#include<stdio.h>
int main()
{
    
	int a[3][4] = {
     0 };
	printf("%d\n", sizeof(a));//48
	//3*4*4
	printf("%d\n", sizeof(a[0][0]));//4
	// The size of the first integer element of a two-dimensional array 
	printf("%d\n", sizeof(a[0]));//16
	//a[0], Equivalent to the first row of one-dimensional array name ,sizeof(a[0])
	// Is the size of the first row of the two-dimensional array 
	printf("%d\n", sizeof(a[0] + 1));//4/8
	//a[0],  Equivalent to the first row of one-dimensional array name 
	//a[0]+1, Equivalent to the address of the second element in the first row of a two-dimensional array 
	printf("%d\n", sizeof(*(a[0] + 1)));//4
	//*(a[0] + 1)-->a[0][1]
	// The size of the second element in the first row is calculated 
	printf("%d\n", sizeof(a + 1));//4/8
	//a+1 The address of the element in the second row of the two-dimensional array 
	printf("%d\n", sizeof(*(a + 1)));//16
	//*(a + 1)-->a[1], Equivalent to the second row of one-dimensional array name ,sizeof(a[1])
	// Is the size of the second row of the two-dimensional array 
	printf("%d\n", sizeof(&a[0] + 1));//4/8
	//&a[0]-->&*(a+0)-->a
	//&a[0] + 1-->a+1 
	printf("%d\n", sizeof(*(&a[0] + 1)));//16
	//&a[0] + 1-->a+1
	//*(&a[0] + 1)-->*(a+1)-->a[1]
	//a[1],  Equivalent to the second row of one-dimensional array name ,sizeof(a[1])
	// Is the size of the second row of the two-dimensional array 
	printf("%d\n", sizeof(*a));//16
	//*a-->a[0] Is the address of the first row of the two-dimensional array 
	printf("%d\n", sizeof(a[3]));//16
	//a[3] Is a two-dimensional array 4 The address of the line 
	// Although the two-dimensional array has no fourth row , But the code is not out of bounds 
	//sizeof() Internal expressions do not operate , Just get the data type of the expression 

	return 0;
}