C language from entry to Earth - array

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1. One dimensional array creation and initialization

1.1 Array creation

Q： What is? Array ？

A： A group of The same type Elemental aggregate

Q： How arrays are created ？

type arr_name[const_n];
//type  Is the element type of the exponential group 
//const_n  Constant expression , Used to determine the size of the array 

for example ：

// Code 1
int arr1[10];

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

// Code 3
char arr3[10];
float arr4[10];
double arr5[10];

notes ： In the use of Code 2 When creating an array , Pay attention to whether the compiler you use supports C99 standard , No support for C99 Standard compiler , [] Only one Constant , You can't put variables . For arrays like this , We usually call it Variable length array .

1.2 Initialization of an array

The initialization of array refers to , Assign values to the contents of the array while creating the array .

for example ：

int arr1[10] = {
     0 };
int arr2[10] = {
     1,2,3 };
int arr3[] = {
     1,2,3 };

char arr4[3] = {
     'a','b','c' };
char arr5[] = {
     'a',98,'c' };
char arr6[] = "abcdef";

reflection ：

1. Array is not initialized , What is inside the array ？

2. When initializing an array , Initialize only the first digit , What is inside the array ？

3. Press above arr6 Error initializing character array , How many characters are there ？

problem 1 :

Array is not initialized , What is inside the array ？

obviously , If the array is not initialized , The contents of the array are all indefinite

problem 2 :

When initializing an array , Initialize only the first digit , What is inside the array ？

ordinary F10 debugged , You can see , The first digit is the initialized value , The rest defaults to 0, For such initialization , We also call it —— Incomplete initialization .

problem 3 :

Press above arr6 Error initializing character array , How many characters are there ?

You can see , Use " " To create a string , There will be one more at the end '\0'

1.3 The use of one-dimensional arrays

before this , We are going to invite an old friend (づ￣ 3￣)づ [] Subscript reference operator .

#include <stdio.h>
int main()
{
    
    int arr[10] = {
     0 };// Incomplete initialization 
    int sz = sizeof(arr) / sizeof(arr[0]);// Find the number of elements of the array 
    int i = 0;
    // Assign values to the array by input 
    for (i=0; i<10; ++i)
    {
    
        scanf("%d", &arr[i]);
    }
    // Output each element of the array 
    for (i=0; i<sz; ++i)
    {
    
        printf("%d ", arr[i]);
    }
    
    return 0;
}

Summary ：

• The array is accessed by subscript , Subscript from 0 Start
• The number of elements of the array can be calculated

2. The creation and initialization of two-dimensional array

2.1 The creation of two-dimensional array

int arr1[3][4];
char arr2[3][4];
double arr3[][4];

When creating a two-dimensional array , Lines can be omitted , Columns cannot be omitted , That is, the second [] There must be specific Constant expression

2.2 Initialization of 2D array

int arr1[3][4] = {
     1,2,3,4 };// Incomplete initialization 
int arr2[3][4] = {
     {
    1,2},{
    3,4} };
int arr3[][4] = {
     {
    1,2},{
    3,4},{
    5,6} };

reflection ：

• arr2 How is it stored ？

It's not hard to see. , In the use of { } When separating arrays , every last { } Separation will divide the internal into different groups

2.3 The use of two-dimensional arrays

Two dimensional arrays are also accessed through subscripts .

#include <stdio.h>
int main()
{
     
    int arr[3][4] = {
     0 };// Incomplete initialization 
    int i = 0;// That's ok 
    int j = 0;// Column 
    
    // Assign a value to a two-dimensional array 
    for (i=0; i<3; ++i)
    {
    
        for (j=0; j<4; ++j)
        {
    
            // Assign a value to a two-dimensional array 
            sacnf("%d", &arr[i][j]);
        }
    }
    
    for (i=0; i<3; ++i)
    {
    
        for (j=0; j<4; ++j)
        {
    
            printf("%d ", arr[i][j]);
        }
        printf("\n");// Line break 
    }
    
    return 0;
}

3. Array storage in memory

3.1 One dimensional array storage in memory

Look at the following code

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

	return 0;
}

By debugging , We can see , The type is int A string of elements , In memory is Continuous storage from low address to high address

3.2 Two dimensional array storage in memory

We probably know the storage of one-dimensional arrays in memory , Can you expand it , How are two-dimensional arrays stored ？

below , Let's take a look through the following code

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

Compare the two display modes , You can see , Two dimensional arrays are also stored continuously in memory , Then we can also regard arrays as such an arrangement .

that , How can we determine how many elements are placed in a row ？

This problem , That explains why when defining a two-dimensional array , The reason why the following figures cannot be missing

4. Array access beyond bounds

Array of Subscript Of Range The rule is 0 ~ n-1 ,C The language itself does not check the bounds of array subscripts , The compiler also doesn't report errors , But that doesn't mean it's right ,

As a programmer , It's best to check the number crossing by yourself

5. Arrays as arguments to functions

Often when we write code , The array will be used as part of the function parameters ,

for example ： Using functions to achieve a bubble sort ( Sort an integer array )

#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 i = 0;
    int arr[] = {
     1,1,4,5,1,4,1,9,1,9,8,1,0 };
    int sz = sizeof(arr)/sizeof(arr[0]);// Determine the elements of the array 
    
    // Sort 
    bubble_sort(arr, sz);
    
    // Print 
    for (i=0; i<sz; ++i)
    {
    
        printf("%d ", arr[i]);
    }
    
    return 0;
}

reflection

• If you will bubble_sort Is it OK to change the function to the following ？

void bubble_sort(int arr[])
{
    
    int sz = sizeof(arr)/sizeof(arr[0]);
    // Ditto below 
}

You can see it , If the number of elements of the function is required , It doesn't work inside a function .

The main reason is ,** Array parameters ** The message is The address of the first element of the array , What the first element accesses is actually the first digit of the array .
#FF6A6A size = 4> reflection

• If you will bubble_sort Is it OK to change the function to the following ？

void bubble_sort(int arr[])
{
    
    int sz = sizeof(arr)/sizeof(arr[0]);
    // Ditto below 
}

You can see it , If the number of elements of the function is required , It doesn't work inside a function .

The main reason is , Array parameters The message is The address of the first element of the array , What the first element accesses is actually the first digit of the array .