C language -- structs, unions, enumerations, and custom types

C Language allows users to construct the same or different basic types into a custom according to their own needs
Special types of , That is, structure and common body .

Structure

Definition of structure type

The definition of structure determines the following two points ：

1. Define the structure type , Determine the name and type of member items in the structure .
2. Indicates the organization of variables of this structure type in memory .

That is, defining a structure type only specifies the memory allocation mode of the structure type , Did not open up memory space . Only defined Structure variable or structure array after , Only according to the memory allocation mode of the structure type, can the storage space be opened .

• There are two steps to use the structure type ：

1. By keyword struct And user-defined structure name （ It is commonly called structure type name ） To define the type .
2. Then, the structure type Define the structure variable name

Declaration of structure variables

• Defining a structure only determines the name of the structure type, the composition of its member items, and the class of its member items
type . The structure variable must be specified by the defined structure type , Before opening up the corresponding memory space for
Use . There are three ways to describe structural variables ：

1. Define variables with the already defined structure type name
2. Define structure variables while defining structure types
3. Do not define the structure type name , Define structure variables directly

1 Define variables with the already defined structure type name .
2. Define structure variables while defining structure types

3. Do not define the structure type name , Define structure variables directly

struct {
    
	int id;
	char name[31];
	char gender;
	char address[200];
	double scores[3];
} alice, bob, clark;

explain ：

1. Pay attention to the difference between types and variables .
2. Member items can be used alone ,bob.id=140101
3. The member items of a structure can also be structural variables .

4. Nested definition of structure type

struct Birthday{
    
	int year;
	int month;
	int day;
};

struct Student{
    
	int id;
	char name[30];
	struct Birthday birth;
	char gender;
	double scores[3];
};

Initialization of structure variables

Structural variables can be assigned initial values when describing , Called initialization . Curly braces can be used to enclose the initial values corresponding to these structural type variables as a whole .

1. The number of initial values is allowed to be less than the number of members in the structure variable , However, the number of initial values is not allowed to be more than the number of members in the structure variable .

2. In the former case , When the initialization statement is executed, the initial value symbol in the initial value table is used to assign values to the members in front of the structure variable , The remaining members without corresponding initial value symbols are implicitly initialized . namely ： Members of all arithmetic types are assigned 0, Members of all pointer types are given null pointer constants

struct Birthday{
    
	int year;
	int month;
	int day;
};

struct Student{
    
	int id;
	char name[30];
	struct Birthday birth;
	char gender;
	double scores[3];
};

struct Student alice = {
    
	200250,
	"Alice", 
	{
    2002, 4},
	'm',
	 {
    77, 88, 69.5}
};

struct Student alice = {
    
200250,
"Alice", {
    2002, 4},
'm', {
    77}};

that , Structural variables alice Member of the alice.birth.day Is implicitly initialized to 0 ,alice.scores[1] and alice.scores[2] It's also initialized to 0.

References to structure variables

1. Reference structure members

• Because the types of each member in the structure variable are different , Generally, you can only reference members of structural variables , Do not reference structural variables as a whole .
• Without nesting , The way to reference structural variable members is ：

 Structure variable name . Member item name 
alice.id;

explain ：
“.” be called “ Struct member operator ”, The structure member referenced in this way is equivalent to a common variable , It has the same properties as ordinary variables .

for example ：

 alice.id = 200250

effect ： Reference structure variables alice Members of the id, Equivalent to an integer variable .

lucy.name

effect ： Structural variable alice Members of name, Equivalent to one Character array name .C Some library functions of the language can access the character array through the first address , for example ：puts(lucy.name);
So it is legal to reference the name of character array .

lucy.score[0]

effect ： Reference structure variables alice Members of the score[0], because score yes int Type of the array , Can't quote... As a whole , So you must reference array elements , Equivalent to one Variable of integer type .

2. Structural variables can perform various operations , It's fine too The address of the reference member or the address of the structure variable .

 for example ：scanf("%d",&alice.id);

3. Structural variables can be referenced as a whole .

Structural variables of the same type can be assigned as a whole . Structure type members embedded in structure variables are also the same .

struct Student alice = {
    
	200250,
	"Alice", 
	{
    2002, 4},
	'm', 
	{
    77}};
	
struct Student bob = alice;

bob.birth = alice.birth;

Example ： Define a structure type , The member item includes the student number 、 full name 、 Birthday 、 Gender 、 English 、 mathematics 、 Scores of three Chinese courses , And define the structural variable of this type . Programming the input of structural variable member items 、 Output . And calculate the average score of the three courses

#include <stdio.h>
struct Birthday{
    
	int year;
	int month;
	int day;
};

struct Student{
    
	int id;
	char name[30];
	struct Birthday birth;
	char gender;
	double scores[3];
};

struct Student alice;

scanf("%d", &alice.id);
getchar();
gets(alice.name);
scanf("%d %d %d", &alice.birth.year, &alice.birth.month, &alice.birth.day);
getchar();
alice.gender = getchar();
for(int i = 0; i < 3; i ++){
    
	scanf("%lf", &alice.scores[i]);
}

Output

printf("id: %d\nname: ", alice.id);
puts(alice.name);
printf("birthday: %d-%d-%d\n",
alice.birth.year,
alice.birth.month,
alice.birth.day);
printf("math\tenglish\tliterature\n");
for(int i = 0; i < 3; i++){
    
printf("%.1f\t", alice.scores[i]);
}
putchar('\n');

Find the average score of the three courses

double average = 0;
for(int i = 0; i < 3; i ++){
    
	average += alice.scores[i];
}
printf("%f", average);

Array of structs

Structural variables can also be constructed into arrays , It's called a structure array . Each array element is a structural variable , Both contain structure member items . Their addresses in memory are continuous .

The definition of structure array

struct  Structure type name   Structure array name  [（ Constant ） expression ] ;

#include <stdio.h>
#define N 50

struct Birthday {
    
	int year;
	int month;
	int day;
};

struct Student {
    
	int id;
	char name[30];
	struct Birthday birth;
	char gender;
	double scores[3];
};

int main() {
    
	struct Student students[N];
}

Initialization of structure array

struct Student students[3] = {
    
	{
    140101, "lucy", {
    1991, 1, 2}, 'M', {
    78, 89, 91}},
	{
    140102, "lily", {
    1991, 2, 3}, 'F', {
    87, 60, 89}},
	{
    140103, "tom", {
    1991, 3, 4}, 'M', {
    82, 91, 92}}
};

explain ：

1. All three structural array elements contain struct Student All member items of .
2. Structure array name students, Represents the first address of the structure array .

Use of structure array

Similar to the reference of general array , A reference to a structure array is a reference to a structure array element .

1. In addition to initialization , Assign a constant value to the structure array 、 Input and output 、 All operations are performed on the members of the array elements of the structure .

The members of structure array elements are represented as ： Structure array name [ Subscript ]. Member name

 for example ：students[0].id  and  scanf("%d", &students[0].id);

In the case of nesting ：

 Structure array name [ Subscript ]. Structure member name … Structure member name .  Basic member name 

2. Structure array elements can be assigned to each other as a whole .

 for example ：students[0] = students[1];

Define a structure , The member item includes the student number 、 full name 、 Birthday 、 Gender 、 English 、 mathematics 、 Scores of three courses in politics .
Programming to realize 2 Input of student information 、 Output . Find out the average score of each student , And sort .

#include <stdio.h>
#define N 2
struct Birthday {
    
	int year;
	int month;
	int day;
};
struct Student {
    
	int id;
	char name[30];
	struct Birthday birth;
	char gender;
	int scores[3];
	double aver;
};

struct Student students[N];

for(int i = 0; i < N; i ++){
    
	printf(" Please enter the first %d Student information \n", i + 1);
	printf(" Please enter the student number ：");
	scanf("%d", &students[i].id);
	printf(" Please enter a name ：");
	fflush(stdin);
	gets(students[i].name);
	printf(" Please enter the date of birth ：");
	scanf("%d %d %d", &students[i].birth.year, &students[i].birth.month, &students[i].birth.day);
	printf(" Please enter gender ：");
	fflush(stdin);
	students[i].gender = getchar();
	printf(" Please enter the grades of the three courses ：");
	for(int j = 0; j < 3; j++){
    
		scanf("%d", &students[i].scores[j]);
	}
	putchar('\n');
}

 Calculate the average score 
for(int i = 0; i < N; i ++){
    
	students[i].aver = 0;
	for(int j = 0; j < 3; j ++){
    
		students[i].aver += students[i].scores[j];
	}
	students[i].aver /= 3.0; 
}

 Sort by average 
for(int i = 0; i < N - 1; i ++){
    
	for(int j = 0; j < N - 1 - i; j++){
    
		if(students[j].aver < students[j + 1].aver){
    
			struct Student tmp;
			tmp = students[j];
			students[j] = students[j + 1];
			students[j + 1] = tmp; 
		} 
	}
}

printf("***** %d The information of students is as follows  *****\n", N);
printf(" Student number \t%-10s\t Year of birth   month   Japan \t Gender \t English \t mathematics \t Chinese language and literature \t average \n"," full name ");
for(int i = 0; i < N; i ++) {
    
	printf("%d\t", students[i].id);
	printf("%-10s\t", students[i].name);
	printf("%6d%3d%3d\t", students[i].birth.year,
	students[i].birth.month, students[i].birth.day);
	if (students[i].gender == 'M') printf(" male \t");
	else if (students[i].gender == 'F') printf(" Woman \t");
	else printf(" nothing \t");
	for (int j = 0; j < 3; j++) {
    
		printf("%d\t", students[i].scores[j]);
	}
	printf("%6.2f\n", students[i].aver);
}

Structure pointer

The pointer to the structure variable is the first address of the memory unit occupied by the structure variable

Pointer variables to struct variables

struct Student alice;
struct Student *ptr = &alice;

 explain ：
1. ptr  That is to point to structural variables  alice  Pointer variable for .
2.  Structure pointer variable plus 1, Address to add 1 The number of bytes occupied by structural variables .

Use the pointer variable pointing to the structure variable to represent the members of the structure variable . Format ：

• Mode one ：(*p). Member item name
• Mode two ： p-> Member item name

• example ： If you define a variable that points to a structure alice Pointer variable for ptr,*ptr Express
ptr The structure variable pointed to alice, Its members alice.id Can be expressed as ：

(*ptr).id  perhaps  ptr->id

• The two representations are equivalent , Operator “->” And “.” Operators have the same precedence , Having the highest
priority

Example ： Define a structure , The member item includes the student number 、 full name 、 Birthday 、 Gender 、 English 、 mathematics 、 Scores of three courses in politics , Programming the output of structural variable member items , Use a pointer to achieve .

struct Student alice = {
    200250, "Alice", {
    2002, 2, 3}, 'M', {
    78, 89, 75}};
struct Student *ptr = &alice;
printf("%d\n", ptr->id);
puts(ptr->name);
printf("%d-%d-%d\n", ptr->birth.year,ptr->birth.month, ptr->birth.day);
printf("%c\n", ptr->gender);
for(int i = 0; i < 3; i ++){
    
	printf("%5d", ptr->scores[i]);
}

Structure array and structure pointer variable

The array name of a one-dimensional array represents the first address of the array , One dimensional structure array is the same .

You can assign the array name of one-dimensional structure to the pointer variable pointing to the structure , The pointer variable will point to the subscript
by 0 The elements of , It can move between array elements

Example ： Define a structure , The member item includes the student number 、 full name 、 Date of birth 、 Gender 、 English 、 mathematics 、 Scores of three Chinese courses , Programming to realize N A student , The average grade of each student , Then the output .

struct Student students[N] = {
    
	{
    140101, "Alice", {
    1991, 1, 2}, 'M', {
    78, 89, 91}},
	{
    140102, "Bob", {
    1991, 2, 3}, 'F', {
    87, 60, 89}},
	{
    140103, "Clark", {
    1991, 3, 4}, 'M', {
    82, 91, 92}}
};

struct Student *p;
for(p = students; p < students + N; p++){
    
	p->aver = 0;
	for(int j = 0; j < 3; j ++){
    
		p->aver += p->scores[j];
	}
	p->aver /= 3;
	printf("%s's average score is %f\n", p->name, p->aver);
}

Structures and functions

Anonymous structure

struct names{
    
	char first[20];
	char last[20];
};


 Nested structure members 
struct person
{
    
	int id;
	struct names name;
};

• Anonymous structure

struct person
{
    
	int id;
	struct {
    char first[20]; char last[20];};
}

Both initialization methods are the same ：

struct person jack = {
    1000, {
    “Jack”, “Zhang”}};

The two access methods are different ：

jack.name.first 
jack.first   omitted  name