Fundamentals of C language -- similarities and differences between arrays and pointers

1. The difference

• The array name cannot be changed , The pointer to the array can be changed .
• Pointer to string , The string cannot be changed ; The string defined by the character array , Characters can be changed .
• When finding the length of an array , The array length can be obtained by borrowing the array name , You can't get the array length by borrowing the pointer .

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1. The array name cannot be changed , The pointer to the array can be changed

Traversal mode 1 ：

#include <stdio.h>

int main(void)
{
    
   int a[5] = {
     0, 1, 2, 3, 4 }, * p = a;
   char i;

   //  Array traversal mode I 
   for (i = 0; i < 5; i++)
   {
    
   	printf("a[%d] = %d\n", i, *p++);
   }

   return 0;
}

a[0] = 0
a[1] = 1
a[2] = 2
a[3] = 3
a[4] = 4

Array traversal mode I ： Use the pointer to traverse the array elements ,*p++ Equivalent to *(p++), That is, the address pointed to by the pointer moves back one unit at a time , Then take the value on the address . One unit here is sizeof(int) Bytes .

Traversal mode 2 ：( error )

#include <stdio.h>

int main(void)
{
    
	int a[5] = {
     0, 1, 2, 3, 4 }, * p = a;
	char i;

	//  Array traversal mode 2 
	for (i = 0; i < 5; i++)
	{
    
		printf("a[%d] = %d\n", i, *a++);
	}

	return 0;
}

• error: value required as increment operand
• Because the array name cannot be changed , Using the autoincrement operator autoincrement will change its direction , It's not right , Array names can only point to the beginning of an array .

Traversal mode 2 ：( correct )

#include <stdio.h>

int main(void)
{
    
	int a[5] = {
     0, 1, 2, 3, 4 }, * p = a;
	char i;

	//  Array traversal mode 2 
	for (i = 0; i < 5; i++)
	{
    
		printf("a[%d] = %d\n", i, *(a+i));
	}

	return 0;
}

• *(a+i) So the pointer passes through the offset , Find the specific value , Instead of changing the pointer itself
• *(a+i) And a[i] It is equivalent.

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2. A string of 2 Initialization methods

// String definition method 1 
char str[] = "happy";

// String definition method 2 
char *str = "happy";

• Mode one ： The characters in a string can be changed . If you can use something like str[3]=’q’ Such a statement to change the characters . The reason is that ： The strings defined in this way are stored in the global data area or stack area , It's readable and writable .
• Mode two ： The characters in a string cannot be changed . The reason is that ： The string defined in this way is saved in the constant area , It's not modifiable .

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3. The group name can find the length of the array , The pointer cannot get the length of the array

#include <stdio.h>


int main(void)
{
    
	int a[] = {
     0, 1, 2, 3, 4 }, * p = a;
	char len = 0;

	//  How to find the length of an array 
	printf(" Mode one ：len=%d\n", sizeof(a) / sizeof(int));

	//  How to find the length of an array 
	printf(" Mode two ：len=%d\n", sizeof(p) / sizeof(int));

	return 0;
}

 Mode one ：len=5
 Mode two ：len=1

• Mode one ： Borrow the array name to find the length of the array , The evaluable array has 5 Elements , correct .
• Mode two ： Using a pointer to find the length of an array , The obtained length is 1,

error . as a result of ：

• p It's just a point to int Pointer to type , The compiler does not know whether it points to an integer or an array .
• sizeof(p) What you get is p The number of bytes occupied by the pointer variable itself , Instead of the number of bytes occupied by the entire array .

• For arrays a, Its type is int [6], It means that this is a possession 6 individual int A collection of data ,1 individual int The length of is 4,6 individual int The length of is 4×6 = 24,sizeof Easy to find .
• For pointer variables p, Its type is int *, stay 32 Length in bit environment 4, stay 64 Length in bit environment 8.
• in the final analysis ,a and p The types of these two symbols are different , The data referred to are also different , They are not the same thing ,sizeof The length is calculated according to the symbol type ,a and p Different types of , The length obtained is naturally different .

The compiler will create a special table during compilation , It is used to save the name and the data type corresponding to the name 、 Address 、 Scope, etc .sizeof Is an operator , It's not a function , Use sizeof The length of the corresponding symbol can be queried from this table .

Above contents , Source blog ：《【C Language notes 】 Arrays are not equivalent to pointers 》

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2. Similarities

1. When using indexes

1	2	3
a[i] = p[i]	*(p+i) = *(a+i)	*p + 1 = *a + 1

#include <stdio.h>


int main(void)
{
    
	int a[] = {
     1, 2, 3, 4, 5 }, * p, i = 2;

	p = a;

	printf("p[i]---------%d \n", p[i]);
	printf("a[i]---------%d \n", a[i]);
	printf("\n");

	printf("*(p + i)---------%d \n", *(p+i));
	printf("*(a + i)---------%d \n", *(a+i));
	printf("\n");

	printf("*p + i---------%d \n", *p + i);
	printf("*a + i---------%d \n", *a + i);

}

p[i]---------3
a[i]---------3

*(p + i)---------3
*(a + i)---------3

*p + i---------3
*a + i---------3

• Remove the subscript operator [ ] Is based on the pointer , Its function is to add a pointer to an integer , A new pointer is generated , Then from this new pointer （ New address ） Get data on ;
• The two operands of the subscript operator are interchangeable , It doesn't care about the order of the operands . If you wish to visit page 3 Elements , Then you can write a[3], You can also write 3[a]
• a[3] Equivalent to *(a + 3),3[a] Equivalent to *(3 + a), Just transpose the two operands of addition .
• When using Subscripts , The compiler will automatically adjust the subscript step size to the size of the array elements . Array a Every element in is int type , The length is 4 Bytes , that a[i+1] and a[i] The distance in memory is 4（ instead of 1）.

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2. When an array is used as a function parameter

• Equating arrays as formal parameters with pointers is for efficiency reasons .
• An array is a collection of data of the same type , There is no limit to the number of data , Maybe only a few , There could be thousands of . If you want to pass the entire array , The overhead in both time and memory space can be very large . And in most cases , We don't really need a copy of the entire array , We just want to tell the function which particular array is of interest at that moment .

#include <stdio.h>

void func(char arr[]) {
    
	arr[2] = 'd';
	printf("arr[2]--------%c \n", arr[2]);
}


int main()
{
    
	char arr[] = "abcd";
	func(arr);				//  here , Pass the array name to the function , Is the pointer representing the array 
}

arr[2]--------d

The above blog posts , source ：《 When exactly will the array be converted to a pointer 》