<> If you know the true face of Lushan Mountain

C There are several kinds of constants in language ：

*
Literal constant

*
const Modified constant variable

*
#define Defined identifier constant

*
enumeration constant

<> Literal constant

That is, the literal meaning can not be changed . as 1 namely 1, You can't say let me 1 be equal to 2; For example, there are several fixed blood groups （A,B,O,AB）; For example, the gender of people is only divided into men , female sex , And a deeper form .

stay C In language ：1,3.14,‘a’,“hello”… These are called constants .

<>const Modified constant variable

It can be understood by a piece of code const Modified constant variable ：
int num = 10; printf("%d\n", num); //num=10 num = 5; printf("%d\n", num);
//num=5
In the code above num It's a variable , Through you num Assign a new value ,num It's constantly changing .

But when you add const,num There's a subtle change .（ I can use it on the compiler ）
const int num = 10; printf("%d\n", num); // Compile error num = 5; printf("%d\n", num);
When you compile , As a result, an error will be reported ：

Because at this time num stay const It has become a constant variable , Variables cannot be modified .

however num It can't be called a constant , It's a variable after all . For example, it cannot be used when defining arrays .
// See the problem by defining an array //int arr[10] = { 0 };------ Normal definition array //int num = 10; //int
arr[num] = { 0 };----- The result is an error //const int num = 10; //int arr[num] = { 0
};---- The result is an error
<>#define Defined identifier constant （ Also called pretreatment ）

This is C Language definition of array size is often used , You can use it by yourself and feel it .

Use format ：#define < identifier > < constant value / expression >
#include <stdio.h> #define MAX 10 int main() { int arr[MAX] = { 0 };
// By changing MAX Can change the size of the array printf("%d",MAX); //MAX=10 return 0; }
Here's a thought , You can think about the result ：
#include <stdio.h> #define MAX 5+5 int main() { printf("%d", 3 * MAX); return 0
; }
The output is 20, It's not 30. So understand #define MAX 5+5,MAX It's not the same 10.

Now that you can assign an expression to an identifier , Can I assign some parameters to this identifier ?

#include <stdio.h> #define Add(a,b) a+b int main() { int sum = Add(3,2); printf
("%d\n",sum); return 0; }
here , Come first sum=a+b, Recurrence sum=3+2, So output 5

We are just going to talk about the most basic constant problem , Therefore, there are not many derived macro definitions , Later, we will focus on macro definition .

<> enumeration constant

If you've learned about structure , The definition of enumeration is similar to that of enumeration .

enum How to use it ：

1. In the definition enum At the same time , Declare variables :
enum Day { Mon,Tue,Wed,Thus,Fri,Sat,Sun }Workday;
2. End of definition enum Then declare the variable ：
enum Day { Mon,Tue,Wed,Thus,Fri,Sat,Sun }; enum Day Workday;
3. Define anonymous enumeration variables ：（ If the whole program uses only one enumeration , be enum There is no need to add an identifier after it , However, the enumeration structure can no longer be defined ）
enum { Mon,Tue,Wed,Thus,Fri,Sat,Sun }Workday;
Through a section of code to analyze some details of enumeration structure ：
#include <stdio.h> enum Day { //enum-- Enumeration type keyword Day-- Enumeration type label enum Day-- The type of the enumeration Mon=1
, Tue, //{ } Inside are enumeration values Wed, Thus, Fri, Sat, Sun }Workday; //Workday-- Enumerating variables
Here are some explanations ：

1. If Mon No assignment , The default value is 0, After that, increase in turn , as Tue=1,Wed=2…

2. If Mon Assign to 3, Then it will increase in turn , as Tue=4,Wed=5…

3. If it is assigned from the middle , as Thus=7, be Thus The following ones increase in turn , Previous from Mon Start with 0

Start to increase

4. Enumeration values are constants , It's not a variable . You cannot assign a value to it again in a program with an assignment statement .

as ：Tue=7,Sun=Wed. These are all wrong .

5. You can only assign an enumeration value to an enumeration variable , The value of the enumeration value cannot be assigned to an enumeration variable

as ：Workday=Tue---- correct

​ Workday=2---- error

More about enumeration , Share later . I hope you like it .

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