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  This issue focuses on ： Two dimensional array related understanding chain

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Catalog

Two dimensional array

Memory layout of two-dimensional array

Three dimensional array memory  

Two dimensional array name

Two dimensional array example

Dereference related

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Two dimensional array

Memory layout of two-dimensional array

First of all, in our daily study , The schematic diagram of two-dimensional array is mostly in the form of matrix , Well, that's right ？

Take an example to analyze :

	char a[4][5] = { 0 };

	for (int i = 0; i < 4; i++)
	{
		for (int j = 0; j < 5; j++)
		{
			printf("%p\n", &a[i][j]);
		}
	}
    return 0;

Watch carefully , It is found that the memory in the two-dimensional array is continuous , This is normal , After all, when we talk about arrays, we say that arrays should open up space as a whole , So it will continue , Well, it seems different from the usual drawing , It should look like the following .

  Only in this way can it meet the memory layout of such a two-dimensional array , In this way, we can think of a two-dimensional array as 4 It consists of a one-dimensional array , Each array is int [5] type .

Three dimensional array memory  

What about multidimensional arrays ？（ Three dimensional array ）

  We can think of three-dimensional arrays as 4 One by one int[4][5] A one-dimensional array of , and int[4][5] It can also be seen as 4 It consists of a one-dimensional array , Each array is int [5] type . This is a bit like a successive downgrade , therefore No matter how many dimensional arrays are, they can be regarded as composed of some one-dimensional arrays .

Two dimensional array name

There is also some use of two-dimensional array names, which is a little confusing , Let's have a deep understanding of array names .

Look at a code ：

    char c[3][4] = { 0 };
 	printf("%p\n", &c);
	printf("%p\n", c);
	printf("%p\n", &c[0][0]);

  Here we can see that the expressions of different array names are the same value , Now let's draw and analyze ：

 &c Is the overall address of the array ,c Is the address of the first element of the array （ Consider a two-dimensional array as the first element type after a one-dimensional array ）,&c[0][0] Is the first... In the array char Type of address , Because for any target that can get an address , The address it takes out is the lowest address , Just the lowest address is &c[0][0] The address of , So in numerical terms, the number of addresses is the same , But the step size is the same , The root cause is because of different types .

Two dimensional array example

Let's take a classic example , To further understand two-dimensional arrays , Pointer array

	int a[5][5] = { 0 };
	int(*p)[4] = a;
	printf("%p\n%d\n", &p[4][2] - &a[4][2], &p[4][2] - &a[4][2]);

Now draw a picture to explain ：

First of all, the starting address is the same ,p It's an array pointer , The type of array is int [4], Every time p+1 Is to move back four shaping size spaces , That is, the color part on the figure , And two-dimensional arrays int [5][5] Can be seen as 5 One dimensional array , The type is int [5], Every time +1 Is backward access 5 The size of a plastic .

therefore p[4][2] It's light brown ,&p[4][2] It's the place shown in the picture ,a[4][2] It's dark brown ,&a[4][2] As shown in the figure , Subtract the address value , Represents the number of types of the first element in an array , then %p No match -4,%d It's symbolic -4, That's easy .

Dereference related

About two-dimensional array dereference related .

	int a = 10;
	int *p = &a;
	int **pp = &p;

	p = 100;
	*p = 100;
	pp = 100;
	*pp = 100;
	**pp = 100;

  This is the general direction diagram , First p Yes a The lowest address ,pp Yes p The lowest address .p discharge a Get address

pp discharge p The address of .

First p = 100 Refer to p The direction of has changed .

*p = 100, because *p yes a Value , Is to put a Change the value in to 100.

pp = 100, Is actually the pp The direction of has been changed .

*pp = 100, It's actually *pp Namely p The content of , Namely &a, So the p The direction of has changed , and p = 100 The effect is the same .

**pp = 100 In fact, it is to dereference and access p The content of , In dereference , Become right p Quoting , visit a Value , hold a The value of the into 100, Actually sum *p = 100 The effect is the same .

  Next up

In the next issue, we'll talk about function pointers

The next issue is more exciting ~！~！