C language pointer classic interview question - the first bullet

Catalog

 sizeof() and char Type one-dimensional array

 strlen and char Type one-dimensional array

 sizeof() And string arrays

 sizeof And string arrays

strlen And string

Character pointer and sizeof()

  Character pointer and strlen（）

Two dimensional arrays and sizeof（）

summary

---

#include<stdio.h>
int main()
{
	int a[] = { 1,2,3,4 };
	printf("%d\n", sizeof(a));
	// The result is 16, Here is the whole array 
	printf("%d\n", sizeof(a + 0));
	// The result is 4 or 8, Here is the size of the first address 
	printf("%d\n", sizeof(*a));
	// The result is 4, Here is the size of the first element 
	printf("%d\n", sizeof(a + 1));
	// The result is 4 or 8, Here is the size of the second address 
	printf("%d\n", sizeof(a[1]));
	// The result is 4, Here is the size of the second element 
	printf("%d\n", sizeof(&a));
	// The result is 4 or 8, Here is the address size of the entire array 
	printf("%d\n", sizeof(*&a));
	// The result is 16, Here we first take out the entire array address , Then dereference the entire array , That is, the size of the entire array is calculated 
	printf("%d\n", sizeof(&a + 1));
	// The result is 4 or 8, The entire array is skipped here , Calculate the address size after the array , So it is 4 or 8,&a The type is int(*)[4]
	printf("%d\n", sizeof(&a[0]));
	// The result is 4 or 8, Here is the address size of the first element 
	printf("%d\n", sizeof(&a[0] + 1));
	// The result is 4 or 8, Here is the address size of the second element 
	return 0;
}

Be careful ： here &a The type is int(*)[4], Array pointer ,

&a+1 Is to skip the entire array  

 sizeof() and char Type one-dimensional array

	char arr[] = { 'a','b','c','d','e','f' };
	printf("%d\n", sizeof(arr));
	//6, Here we calculate the whole array 
	printf("%d\n", sizeof(arr + 0));
	//4 or 8, Here we calculate the address of the first element 
	printf("%d\n", sizeof(*arr));
	//1, Here we calculate the size of the first element 
	printf("%d\n", sizeof(arr[1]));
	//1, Philosophy calculates the size of the second element 
	printf("%d\n", sizeof(&arr));
	//4 or 8, Here we calculate the address size of the whole array 
	printf("%d\n", sizeof(&arr + 1));
	// still 4 or 8, The calculation here is after skipping the entire array , The size of the entire address 
	printf("%d\n", sizeof(&arr[0] + 1));
	// still 4 or 8, Here we calculate the address size of the second element 

 strlen and char Type one-dimensional array

	char arr[] = { 'a','b','c','d','e','f' };
	printf("%d\n", strlen(arr));
	// Random value , Here we calculate the whole array , That is, calculate from the first element to find \0 Location , The random value is because there is no \0
	printf("%d\n", strlen(arr + 0));
	// Random value , here strlen Is counting back from the first element , The value of random value is the same as the above value 
	printf("%d\n", strlen(*arr));
	// Report errors , Here is the first element , The first element is a value ,strlen（） What is in brackets must be char * Pointer variable of type , And the elements in it are char The type is not char * type , If the 'a' Transfer the past , It is equivalent to 97 As a pointer through 
	printf("%d\n", strlen(arr[1]));
	// Report errors , The second element is passed , Same as the above value 
	printf("%d\n", strlen(&arr));
	// Random value , It's the same as before , It's the whole address , The entire address starts with the address of the first element , Random values will be generated at random , Because it starts from the first element , So the result will be the same as the first 
	printf("%d\n", strlen(&arr + 1));
	// Random value , Here is skipping the entire array , Then start calculating , encounter \0 Then stop , It is different from the first random value 6
	printf("%d\n", strlen(&arr[0] + 1));
	// Random value , Calculate from the address of the second element , encounter \0 stop it , It is different from the first random value 1

 sizeof() And string arrays

	char arr[] = "abcdef";
	printf("%d\n", sizeof(arr));
	//7, Yes \0
	printf("%d\n", sizeof(arr + 0));
	//4 or 8, The address size is calculated , Start with the address of the first element 
	printf("%d\n", sizeof(*arr));
	//1, What we calculate is the size of the first element 
	printf("%d\n", sizeof(arr[1]));
	//1, It calculates the size of the second element 
	printf("%d\n", sizeof(&arr));
	//4 or 8, The whole address is calculated 
	printf("%d\n", sizeof(&arr + 1));
	//4 or 8, After skipping this array , Calculate the size of the following address 
	printf("%d\n", sizeof(&arr[0] + 1));
	//4 or 8, After skipping the first element , Calculate the size of the following address , Instead of the size of the contents in the array 

 sizeof And string arrays

	char arr[] = "abcdef";
	printf("%d\n", sizeof(arr));
	//7, Yes \0
	printf("%d\n", sizeof(arr + 0));
	//4 or 8, The address size is calculated , Start with the address of the first element 
	printf("%d\n", sizeof(*arr));
	//1, What we calculate is the size of the first element 
	printf("%d\n", sizeof(arr[1]));
	//1, It calculates the size of the second element 
	printf("%d\n", sizeof(&arr));
	//4 or 8, The whole address is calculated 
	printf("%d\n", sizeof(&arr + 1));
	//4 or 8, After skipping this array , Calculate the size of the following address 
	printf("%d\n", sizeof(&arr[0] + 1));
	//4 or 8, After skipping the first element , Calculate the size of the following address , Instead of the size of the contents in the array 

strlen And string

	char arr[] = "abcdef";
	printf("%d\n", strlen(arr));
	//6, encounter \0 Stop printing 
	printf("%d\n", strlen(arr + 0));
	//6, encounter \0 Stop printing 
	printf("%d\n", strlen(*arr));
	// Report errors , It's not char * type 
	printf("%d\n", strlen(arr[1]));
	// Report errors , Not wearing char* type 
	printf("%d\n", strlen(&arr));
	//6, The address of the whole array is passed , Start with the address of the first element 
	printf("%d\n", strlen(&arr + 1));
	// Random value , After spanning the entire array , Starting calculation encountered \0 stop it 
	printf("%d\n", strlen(&arr[0] + 1));
	//5, Start with the second element , encounter \0 stop it 

Character pointer and sizeof()

	char* p = "abcdef";
	printf("%d\n", sizeof(p));
	//4 or 8, there p Is a pointer ,p Inside the village is the address of the first element ,
	printf("%d\n", sizeof(p + 1));
	//4 or 8, Point to the address of the second element 
	printf("%d\n", sizeof(*p));
	//1, The size of the first element 
	printf("%d\n", sizeof(p[0]));
	//1, The size of the first element 
	printf("%d\n", sizeof(&p));
	//4 or 8,&p It's a secondary pointer 
	printf("%d\n", sizeof(&p + 1));
	//4 or 8, Across the entire array 
	printf("%d\n", sizeof(&p[0] + 1));
	//4 or 8, Calculate the size of the address from the second element 

  Character pointer and strlen（）

	char* p = "abcdef";
	printf("%d\n", strlen(p));
	//6,p It's the first element address 
	printf("%d\n", strlen(p + 1));
	//5, Start with the second element 
	printf("%d\n", strlen(*p));
	// Report errors , Because what you wear is a value , No char * Type of 
	printf("%d\n", strlen(p[0]));
	// Report errors , It passes a value , No char * Type of 
	printf("%d\n", strlen(&p));
	// Random value ,p Is a first level pointer ,&p It's just a secondary pointer , That is to say, here strlen It's from p The address of starts counting backwards \0 stop it 
	printf("%d\n", strlen(&p + 1));
	// Random value , Across the entire array , Then start to calculate the encounter \0 Location 
	printf("%d\n", strlen(&p[0] + 1));
	// Random value , Calculate from the first element encountered \0 until , The number that differs from the above is also a random value , Not a definite number , Because they met \0 The situation is different 
	// The value of the difference between the penultimate and the penultimate , It's a random value , because &p It's from p Starting number of addresses , When not &p[0]+1 when , There may be \0 It may not happen \0, and &p[0]+1, Start counting from the address of the second element , When did you meet \0 It's impossible 

 

 

 &p+1 and &p[0]+1 The reason why the difference of results is random , Are likely to p[0]+1 Move to the right and don't reach &p+1 When I met \0, It may also be with &p+1 Meet the same \0, Because of the \0 With uncertainty , So the difference between them is a random value .

Two dimensional arrays and sizeof（）

int a[3][4] = { 0 };
printf("%d\n", sizeof(a));
//48, Here is the size of the entire array 
printf("%d\n", sizeof(a[0][0]));
//4, Here we calculate the size of the first element 
printf("%d\n", sizeof(a[0]));
//16,a[0] The size of the first line of elements 
printf("%d\n", sizeof(a[0] + 1));
//4/8, there a[0] No longer put it alone () Inside , Yes +1 The operation of , So here a[0] Represents the address of the first element ,+1 It means the size of the address of the second element , So here is the address 
printf("%d\n", sizeof(*(a[0] + 1)));
//4, This is the first line , The size of the second element 
printf("%d\n", sizeof(a + 1));
//4/8,a Yes +1 The operation of , therefore a Represents the address of the first element , The first line ,a+1 It's the address on the second line , Because it's an address, it's 4/8
printf("%d\n", sizeof(*(a + 1)));
//16, The size of the element in the second line 
printf("%d\n", sizeof(&a[0] + 1));
//4/8, The address on the second line 
printf("%d\n", sizeof(*(&a[0] + 1)));
//16, The array size of the second row 
printf("%d\n", sizeof(*a));
//16,a Represents the address of the first element ,*a Is to dereference , So it is 16
printf("%d\n", sizeof(a[3]));

summary

The meaning of array names ：

1. sizeof( Array name ), The array name here represents the entire array , It calculates the size of the entire array .

2. & Array name , The array name here represents the entire array , It takes out the address of the entire array .

3. In addition, all array names represent the address of the first element .