C language - Introduction - Foundation - syntax - [variable, constant light, scope] (V)

What is constant ?

Constant is a fixed value , Will not change during program execution . These fixed values , Also called literal quantity .
Constants can be any basic data type , For example, integer constants 、 Floating-Point Literals 、 character constants , Or string literals , There are also enumeration constants . Constants are like regular variables , It's just that the value of a constant cannot be modified after it's defined .

It's like having boys and girls in real life , Born a boy, always a boy , Born a girl, you will always be a girl , therefore
Gender is a constant in real life

The type of constant

Integer constant

Integer constants can be decimal 、 A constant in octal or hexadecimal . Prefix specifies cardinality ：0x or 0X For hexadecimal ,0 For octal , Without prefix, it means decimal by default .

Integer constants can also have a suffix , Suffix is U and L The combination of ,U Represents an unsigned integer （unsigned）,L For long integers （long）. Suffixes can be uppercase , It can also be lowercase ,U and L In any order .

Here are a few examples of integer constants ：

212         /*  legal  */
215u        /*  legal  */
0xFeeL      /*  legal  */
078         /*  Illegal ：8  It's not an octal number  */
032UU       /*  Illegal ： You can't repeat the suffix  */

Here are examples of various types of integer constants ：

85         /*  Decimal system  */
0213       /*  octal  */
0x4b       /*  Hexadecimal  */
30         /*  Integers  */
30u        /*  Unsigned integer  */
30l        /*  long integer  */
30ul       /*  Unsigned long integers  */

Floating-Point Literals

Floating point constants consist of integer parts 、 decimal point 、 The fractional part and the exponential part make up . You can use decimal or exponential forms to represent floating-point constants .

When expressed in decimal form , Must contain decimal point 、 Index , Or both . When expressed in exponential form , Must contain integer part 、 The fractional part , Or both . The index with sign is to use e or E Introduced .

Here are some examples of floating-point constants ：

3.14159       /*  legal  */
314159E-5L    /*  legal  */
510E          /*  Illegal ： Incomplete index  */
210f          /*  Illegal ： There are no decimals or exponents  */
.e55          /*  Illegal ： Lack of integers or fractions  */

character constants

• Character constants are all in ’'( Single quotation marks ) The enclosed . for example :‘a’、‘b’、‘c’
• A character constant can have only one character in a single quotation mark
• A special case : If it's an escape character , There can be two characters in a single quotation mark . for example :‘\n’、‘\t’

String constant

• Character constants are all in ""( Double quotes ) The enclosed . for example :“a”、“abc”、“lnj”
• The system will automatically add a character... To the end of the string constant ’\0’ As a string end flag

Define constants

stay C in , There are two simple ways to define constants ：

• Use #define The preprocessor .
• Use const keyword .

Please note that , Define constants as capital letters , Is a good programming practice .

Use #define The preprocessor defines the form of the constant ：#define identifier value

#include <stdio.h>

#define LENGTH 10 
#define WIDTH 5
#define NEWLINE '\n'

int main()
{
    

   int area;  
  
   area = LENGTH * WIDTH;
   printf("value of area : %d", area);
   printf("%c", NEWLINE);

   return 0;
}

When the above code is compiled and executed , It will produce the following results ：

value of area : 50

You can also use it const Prefixes declare constants of the specified type ：const type variable = value; See the following example for details ：

#include <stdio.h>
const int  LENGTH = 10;
const int  WIDTH  = 5;
const char NEWLINE = '\n';
int main()
{
    
    int area;
    area = LENGTH * WIDTH;
    printf("value of area : %d", area);
    printf("%c", NEWLINE);

    return 0;
}

When the above code is compiled and executed , It will produce the following results ：

value of area : 50

#define Vs const

1. How the compiler handles

• define – Replacement in the preprocessing stage
• const – Determine its value at compile time

2. Type checking

• define – No type , No type safety check , Unexpected errors may occur
• const – There are data types , Type checking occurs at compile time

3. Memory space

• define – Don't allocate memory , What is given is an immediate number , Replace as many times as you use them , There will be multiple copies in memory , Large memory consumption
• const – Allocate space in static storage , There is only one copy in memory while the program is running

4. debugging

• define You can't debug , Because it has been replaced in the precompile phase .
• const Constants can be debugged .

I just want to emphasize here define It's very powerful , Although it has no type detection , Can't debug , Also consider the boundary effect , But just because there is no type detection , Precompiling is done , To make its use more flexible , More powerful , If we can make good use of define, Can often play an unexpected effect .

What is a variable ？

" The amount " According to the data . Variable , It means some unfixed data , That is, the data that can be changed is like the height of people in real life 、 Same weight , It will change with age , So height 、 Weight is an embodiment of variables in real life , There is also the storage grid in the supermarket in real life , The same grid is used by different people in different periods , The items stored in the grid can be changed . When Zhang San uses this grid, it may contain diapers , But when Li Si used this grid, it might be bread

How to define variables

Why define variables ？, Any variable before it's used , It has to be defined first , Storage space is allocated only when variables are defined , Only then has the space to store the number

Why qualify types ？, Used to constrain the type of data stored in variables . Once the type is specified for the variable , Then this variable can only store this type of data , Memory space is extremely limited , Different types of variables take up different sizes of storage space

Why specify a variable name ？, The space to store data means nothing to us , What we need is the value stored in space , Only , With a name , We can get the value in space , like , Your name is Zhang San , His name is Li Si , If there is no name, how to let the other party know that we are talking to him , Is it just feed feed , When there are many people in the mall , You try , Look back

Format 1： Variable type Variable name ;

int a; 
float b; 
char ch;

Format 2： Variable type Variable name , Variable name ...; Use commas between multiple variables (,) The number separated

int a,b,c;

The variable name belongs to the identifier , Therefore, the naming principle of identifiers must be strictly observed

How to use variables ？

stay C In language , utilize = Number Store data into variables , We call it assigning values to variables

int value;
value = 998; //  assignment 

Be careful ：
there = Number , It's not in mathematics “ equal ”, It is C The prestige operator in language , The function is to set the integer constant on the right 998 Assign to the integer variable on the left value , When you assign ,= The left side of the sign must be a variable , For easy code reading , Used to = Add a space on each side of

Initialization of a variable

C In language , The first assignment of a variable , We call it “ initialization , Two forms of initialization :
Define first and then initialize

int value;
value=998;// initialization 

When defined, it is initialized at the same time

int a = 10;

int b = 4,c=2;

How to modify variable values ?

Multiple assignment is enough , Each assignment will override the original value

int i = 10;
i = 20; //  Change the value of the variable 

Value transfer between variables

You can assign the value stored in one variable to another variable

int a = 10; 
int b = a; //  Is equivalent to a Stored in the 10 Made a copy to b

How to view the value of a variable ?

Use printf Output the value of one or more variables

int a = 10, c = 11; 
printf("a=%d, c=%d", a, c);

Output the values of other types of variables

double height 1.75;
char blood 'A';
printf（"hei ght=%.2f, What's the blood type %c",height,blood）;

Scope of variable ( important )

In any kind of programming , Scope is the area where variables defined in a program exist , Beyond that area variables cannot be accessed . C All variables in a language have their own scope , Variables are defined in different locations , It also has a different scope , According to the scope, it can be divided into two types , Local variables and global variables

local variable

Local variables are also called internal variables , Local variables are defined within code blocks , Its scope is limited to code blocks , Cannot use... After leaving this code block , Let's look at a few cases :

int main(){
     
	int i = 998; //  Scope start  
	return 0;//  Scope end  
}

Global variables

Global variables are also called external variables , It is a variable defined outside the code block

int i =666;
int main(){
    
    printf("i = %d\n",i);// have access to 
    return 0;
}
int call(){
    
    printf("i = %d\n",i);// have access to 
    return 0;
}

Be careful : Variables with the same name cannot be in the same scope

Variables with the same name can be in different scopes

Variable memory analysis

Byte and address

• In order to better understand the storage details of variables in memory , Let's first get to know the memory “ byte ” and “ Address ”
• Each cell represents a byte
• Each byte has its own memory address
• Memory address is continuous

The storage space occupied by a variable , It is related to the type declared when defining variables and the current compilation environment

Determine how much storage space needs to be opened up according to the type declared when defining variables and the current compilation environment , When developing, start with a large memory address （ Memory addressing from large to small ）, Save the data to the corresponding memory space that has been opened up


Don't worry , First contact C Language , It's enough for me to know so much first . The details of storage will be explained in more depth again later
section .

give the thumbs-up - Collection - Focus on - Easy to review and receive the latest content in the future There are other questions to discuss in the comments section - Or believe me - I will reply as soon as I receive it thank , coordination , I hope my efforts will help you ^_^

disclaimer ： Part of the material in this paper comes from the Internet , The copyright belongs to the creator , If articles exist / picture / Improper use of audio and video , Please feel free to contact me by private letter .