First knowledge of C language -- constants and variables

Some values in life are constant （ such as ： PI , Gender , Id card number , Blood type and so on ）
Some values are variable （ such as ： Age , weight , Salary ）.
Constant value ,C The concept of constant is used in language to express , Become value C In languages, variables are used to represent .

1. How to define variables

grammar ： data type Variable name = The value to initialize

int age = 150;
float weight = 45.5f;
char ch = 'w';

2. Variable naming requirements

1. Only letters （ Including uppercase and lowercase ）、 Numbers and underscores （ _ ） form .
2. Cannot start with a number .
3. The length cannot exceed 63 Characters .
4. Variable names are case sensitive .
5. Variable names cannot use keywords .

#include <stdio.h>
int main()
{
    
	int d_9 = 8;
	//int 6_p = 7;
	int a = 3;
	int A = 4;
	printf("a=%d A=%d", a, A);
	//int if = 9;
	return 0;
}

3. Classification of variables

Variables are divided into Local and global variables , We can think of it this way ：

1. Global variables are defined in { } External variables , If the global variable is not initialized, the default value is 0.
   

2. Local variables are defined in { } External variables .
   

Then think about a problem , Local and global variables can The same name Do you ？
The answer is yes ！
Look at this piece of code ：

#include <stdio.h>
int global = 2019;// Global variables 
int main()
{
    
    int local = 2018;// local variable 
    // As defined below global Will there be a problem ？
    int global = 2020;// local variable 
    printf("global = %d\n", global);
    return 0; }

however , We don't recommend writing the names of local variables and global variables the same .

4. The scope and life cycle of variables

First of all, we need to know what the scope is ？
Scope （scope） It's a programming concept , Generally speaking , The names used in a program are not always valid / The scope of available code that limits the availability of the name is the scope of the name .
1. The scope of a local variable is the local scope of the variable .
2. The scope of global variables is the whole project .

Look at a piece of code ：

void test()
{
    
	int i = 9;
	printf("%d\n", p);
}
#include <stdio.h>
int p = 8;
int main()
{
    
	test();
	printf("%d\n", i);
	printf("%d\n", p);
	return 0;
}

Then let's discuss it next , What is life cycle ？
The life cycle of a variable is the period between the creation of a variable and its destruction
1. The life cycle of a local variable is ： Enter the scope lifecycle begins , Out of scope life cycle ends .
2. The life cycle of global variables is ： The whole life cycle of the program .

5. Constant

C There are differences in the form of definitions between constants and variables in languages .
C Constants in a language can be divided into the following categories ：
Literal constants
const Modified constant variable
#define Defined identifier constant
Enumeration constants

1. First, we introduce literal constants ：
It's like 3.14,100,‘a’ , “abcdf”. These are literal constants

int main()
{
    
	3.14;
	5;
	300;
	's';
	"adsfis";// These are literal constants 
	"hello world";
	return 0;
}

2. const Modified constant variable
const The modified constant is C In language, it is only restricted at the grammatical level that this variable cannot be changed directly , But it's essentially a variable , So it's called a constant variable .
for instance ：

#include <stdio.h>
int main()
{
    
	const int i = 9;
	i = 0;
	return 0;
}

3.#define Defined identifier constant
grammar ：#define Constant names value
//#define Defined identifier constant

#define max 100
int main()
{
    
	max = 99;
	int arr[max]={
    0};
	return 0;
}

4. Enumeration constants
Enumeration constants are constants that can be enumerated one by one , Enumeration keywords are required to define enumeration types enum.
enum Is a data type in a computer programming language . Enumeration type ： In practical terms , Some variables are limited in a limited range . for example , There are only seven days in a week , There are only twelve months in a year , A class has six courses a week and so on . If we describe these quantities as integers , Character type or other types are obviously inappropriate . So ,C Language provides a language called “ enumeration ” The type of . stay “ enumeration ” All possible values are listed in the definition of type , Described as the “ enumeration ” The variable value of type cannot exceed the defined range . It should be noted that , Enumeration type is a basic data type , Not a construction type , Because it can no longer be broken down into any basic type .
for instance , For example, gender , For gender , Men and women , We can list them one by one , Code up ：

#include <stdio.h>
int main()
{
    
	enum Sex	//enum Enumerate keywords 
	{
    
		MALE,
		FEMALE,
		SECRET		// Possible values of gender enumeration types 
	};
	printf("%d\n", MALE);
	printf("%d\n", FEMALE);
	printf("%d\n", SECRET);
	enum Sex s = MALE;
	// notes ： The default of enumeration constants is from 0 Start , Increase in descending order 1 Of 
	return 0;
}

That's right C Introduction of constants and variables in language ！