C language series - Section 4 - arrays

1. One dimensional array creation and initialization

1.1 Array creation

An array is a set of Elements of the same type Set .
How to create an array ：

type_t   arr_name   [const_n];
//type_t  Is the element type of the exponential group 
//const_n  Is a constant expression , Used to specify the size of an array 

An instance created by an array ：

// Code 1
int arr1[10];/* Uninitialized array */
 Be careful ：
/* If the array is defined in a function ,（ It is defined in the stack area ）（ understand ）, Will be randomly defined   value . But if defined in a global variable , Or use statistic Definitions are not randomly defined   Initial value , A character array char The default is \0, integer array int The default is 0*/

// Code 2
int count = 10;
int arr2[count];

// Arrays can be created normally ？
 Can not be , Variables cannot be used to define array sizes 
 Unless with const Define constants 
const int count = 10;

// Code 3
char arr3[10];
float arr4[1];
double arr5[20];
 Be careful ：
 Such as code 1

// Code 4
char a = 87;
printf("%c",a);  The output is  ascll In code 87 The corresponding   character 

Add : Uninitialized array
as follows

char a[3] = {
    'a','b','c'};// There is no value in the back  
char b[5] = {
    'e','d','f'};// Wherever defined , The first 4 The default is... In the future \0
char c[3] = "abc"; // This is not allowed , Because the string is followed by '\0'

1.2 Initialization of an array

The initialization of an array refers to , While creating an array, give the contents of the array some reasonable initial values （ initialization ）.
Look at the code

int arr1[10] = {
    1,2,3};  // Not fully initialized 
int arr2[] = {
    1,2,3,4};	 /* Initialization of unknown size , But after initialization , Convenient and accurate   Fixed size .  After this definition ,arr2 The array is 4 individual , It can no longer be expanded */
int arr3[5] = {
    1,2,3,4,5};
char arr4[3] = {
    'a',98, 'c'};
char arr5[] = {
    'a','b','c'};
char arr6[] = "abcdef"; // Note that  7  individual 

If you want to create an array without specifying the size of the array, you have to initialize it . The number of elements of the array depends on the initialized Content To make sure .
But for the following code to distinguish , How to allocate in memory .

char arr1[] = "abc";  // Note that this definition is  4 Characters （ At the end of the string is \0）
char arr2[3] = {
    'a','b','c'};

1.3 The use of one-dimensional arrays

For the use of arrays, we introduced an operator ： [] , Subscript reference operator . It's actually an array access operator .
So let's look at the code ：

#include <stdio.h>
int main()
{
    
	int arr[10] = {
     0 };// Incomplete initialization of arrays 

	   // Count the number of elements in an array 
	int sz = sizeof(arr) / sizeof(arr[0]);
	// Assign values to the contents of the array , Arrays are accessed using subscripts , Subscript from 0 Start . therefore ：
	
	int i = 0;// Do subscripts 
	for (i = 0; i < 10; i++)// Write here 10, Ok or not ？
	{
    
		arr[i] = i;
	}
	// Output the contents of the array   Output an array with a loop （ String direct output is available %s）
	for (i = 0; i < 10; ++i)
	{
    
		printf("%d ", arr[i]);
	}
	return 0;
}

summary :

1. Arrays are accessed using subscripts , The subscript is from 0 Start .
2. The size of the array can be calculated .

1.4 One dimensional array storage in memory

Next, we discuss the storage of arrays in memory .
Look at the code ：

#include <stdio.h>
int main()
{
    
	int arr[10] = {
     0 };
	int i = 0;
	int sz = sizeof(arr) / sizeof(arr[0]);

	for (i = 0; i < sz; ++i)
	{
    
		printf("&arr[%d] = %p\n", i, &arr[i]);
	}
	return 0;
}
/* Be careful ：%p Is used to output the address .  Output found , Each address differs by 4 Bytes */

The output is as follows

Watch the output carefully , We know , As the array subscript grows, the address of the element , It's also increasing regularly .
So we can draw a conclusion ： Arrays are continuously stored in memory .

And the bytes occupied by each value , Determined by the array type

2. The creation and initialization of two-dimensional array

2.1 The creation of two-dimensional array

// Array creation 
int arr[3][4];
char arr[3][5];
double arr[2][4];

2.2 Initialization of 2D array

// Array initialization 
int arr[3][4] = {
    1,2,3,4};
int arr[3][4] = {
    {
    1,2},{
    4,5}};
int arr[][4] = {
    {
    2,3},{
    4,5}};

If the two-dimensional array is initialized , Lines can be omitted , Columns cannot be omitted

2.3 The use of two-dimensional arrays

Two dimensional arrays are also used by subscripts .
Look at the code ：

#include <stdio.h>
int main()
{
    
	int arr[3][4] = {
     0 };
	int i = 0;
	for (i = 0; i < 3; i++)
	{
    
		int j = 0;
		for (j = 0; j < 4; j++)
		{
    
			arr[i][j] = i * 4 + j;
		}
	}
	for (i = 0; i < 3; i++)
	{
    
		int j = 0;
		for (j = 0; j < 4; j++)
		{
    
			printf("%d ", arr[i][j]);
		}
	}
	return 0;
}

2.4 Two dimensional array storage in memory

Like a one-dimensional array , Here we try to print each element of a two-dimensional array .

#include <stdio.h>
int main()
{
    
	int arr[3][4];
	int i = 0;
	for (i = 0; i < 3; i++)
	{
    
		int j = 0;
		for (j = 0; j < 4; j++)
		{
    
printf("&arr[%d][%d] = %p\n", i, j, &arr[i][j]);
		}
	}
	return 0;
}

The output is as follows ：
Through the results, we can analyze , In fact, two-dimensional arrays are also stored continuously in memory .
And the bytes occupied by each value , Determined by the array type

3. An array

The subscript of an array is range limited .
The next stipulation of the array is from 0 At the beginning , If the array has n Elements , The subscript of the last element is n-1.
So if the subscript of the array is less than 0, Or greater than n-1, Namely Array out of bounds access , Access beyond the legal space of the array .
C The language itself does not check the bounds of array subscripts , The compiler does not necessarily report an error , But the compiler does not report an error , That doesn't mean the program is right ,
So when programmers write code , You'd better do cross-border inspection yourself .

#include <stdio.h>
int main()
{
    
 int arr[10] = {
    1,2,3,4,5,6,7,8,9,10};
    int i = 0;
    for(i=0; i<=10; i++)
   {
    
        printf("%d\n", arr[i]);
        // When i be equal to 10 When , Cross border visit 
   }
 return 0; }

Rows and columns of two-dimensional arrays may also be out of bounds .

4. Arrays as function arguments

Often when we write code , Will pass the array as an argument to a function , such as ： I want to implement a bubble sort （ Here is the idea of algorithm ） function

Bubble sort ：
Sort an integer array .
Bubble sort algorithm （ Super detailed ）

4.1 Wrong design of bubble sort function

// Method 1：
#include <stdio.h>
void bubble_sort(int arr[])
{
    
	int sz = sizeof(arr) / sizeof(arr[0]);
	// Is that right ？
	/* incorrect , because arr【】 The address is accepted , thereafter arr It means   The address of the first element */
	int i = 0;
	for (i = 0; i < sz - 1; i++)
	{
    
		int j = 0;
		for (j = 0; j < sz - i - 1; j++)
		{
    
			if (arr[j] > arr[j + 1])
			{
    
				int tmp = arr[j];
				arr[j] = arr[j + 1];
				arr[j + 1] = tmp;
			}
		}
	}
}
int main()
{
    
	int arr[] = {
     3,1,7,5,8,9,0,2,4,6 };
	bubble_sort(arr);// Whether it can be sorted normally ？
	for (i = 0; i < sizeof(arr) / sizeof(arr[0]); i++)
	{
    
		printf("%d ", arr[i]);// The output is not ordered 
	}
	return 0;
}

therefore , Arrays as function arguments , It is accepted that Array first address

4.2 Correct design of bubble sorting function

#include <stdio.h>
void bubble_sort(int arr[],int sz)
{
    
	int i = 0;
	for (i = 0; i < sz - 1; i++)
	{
    
		int j = 0;
		for (j = 0; j < sz - i - 1; j++)
		{
    
			if (arr[j] > arr[j + 1])
			{
    
				int tmp = arr[j];
				arr[j] = arr[j + 1];
				arr[j + 1] = tmp;
			}
		}
	}
}
int main()
{
    
	int arr[] = {
     3,1,7,5,8,9,0,2,4,6 };
	int i;
bubble_sort(arr, sizeof(arr) / sizeof(arr[0]));
for (i = 0; i < sizeof(arr) / sizeof(arr[0]) ;i++)
	{
    
		printf("%d ", arr[i]);
	}
	return 0;
}

4.3 What is the array name ？

#include <stdio.h>
int main()
{
    
	int arr[10] = {
     1,2,3,4,5 };
	printf("%p\n", arr);
	printf("%p\n", &arr[0]);
	printf("%d\n", *arr);
	// Output results 
	return 0;
}

Conclusion ：

The array name is the address of the first element of the array .（ There are two exceptions ）

4.3 The array name is a special case of the address of the first element of the array

 special case 
int arr[10] = {
    0};
printf("%d\n", sizeof(arr));
 The output is 40

special case ：

1. sizeof( Array name ), Calculate the size of the entire array ,sizeof Put a separate array name inside , The array name represents the entire array .
2. & Array name , What we get is the address of the array .& Array name , The array name represents the entire array

Such as ：

int a[10];
&a ->  Represents the address of the entire array 
&a+1  Will cross the border 

In addition to this 1,2 Except for two cases , All array names represent arrays First element The address of .

5. Data instance

5.1 Application examples of arrays 1： Sanzi

Sanzi （ stand-alone ） Reflect human-computer interaction , Engineering system thinking code . show VS The charm of

5.2 Application examples of arrays 2： The Minesweeper game

C Language minesweeps （vs Engineering function reflects ）（ Logic is important ）