The second C++ check-in, I gradually became proficient

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foreword

The last time our learning content was the standard input and output
Summary:
[The first check-in is here ( https://blog.csdn.net/m0_53590279/article/details/115217187 )

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1. Conversion of hexadecimal system

Hexadecimal is the basis of computer language, we must master it!
As we all know: binary is bin - octal is ocx - decimal is dec - hexadecimal is hex

    我们可以利用      cout<<ocx
           ——————————cout<<dec
           ——————————cout<<hex      来控制数据的输入输出

#include<iostream>
using namespace std;
main()
{
    
    
	//dec是十进制，OCT是八进制，HEX是十六进制
	int a=010,b=10,c=0X10;
	cout<<"DEC:";//默认十进制
	cout<<"a="<<a<<" ";
	cout<<"b="<<b<<" ";
	cout<<"c="<<c<<" \n";
	
	cout<<"OCT";
	cout<<oct; //调成八进制显示数据
	cout<<"a="<<a<<" ";
	cout<<"b="<<b<<" ";
	cout<<"c="<<c<<" \n";
	
	cout<<"HEX";
	cout<<hex;//调成十六进制 显示数据
	cout<<"a="<<a<<" ";
	cout<<"b="<<b<<" ";
	cout<<"c="<<c<<" \n";
	
    cout<<"a+b+c= ";//三种进制混合运算
    cout<<dec;//恢复十进制（不会主动恢复的）
    cout<<a+b+c<<" \n";
   
    cout<<"DEC:a=";cin>>a;
    cout<<"OCT:b=";cin>>b;
    cout<<"HEX（零X）:c=";cin>>c;
    cout<<"DEC:"<<dec<<"\n";//进制声明不用单独一行
	//转换为十进制输出
    cout<<"a="<<a<<"\n";
    cout<<"b="<<b<<"\n";
    cout<<"c="<<c<<"\n";
		 }

2. Input and output of floating-point and character variables

1. Floating point

The character type is declared as: float and double

name ------ float : single-precision floating-point number –||- double : double-precision floating-
point number bytes -------------- 4 --------- -----||-------- 8 -------------
Accuracy --------------- 6~7 -- -----------||----- 15~16 ----------

The code was eaten by me (QAQ / hiccup~)

2. Character type

The character type is declared as * char
character data can perform arithmetic/logic operations, and
cin can be used to assign values ​​to character variables. (Adding >> will report an error?!)

The code is as follows (example):

#include<iostream>
using namespace std;
main()//标题：字符型数据处理 
{
    
    
	char c1='A',c2;//格式化 
	
	c2=c1+32;//字符可以直接运算 
	
	cout<<"c1="<<c1<<endl;
	cout<<"c2="<<c2<<endl;
	
	cout<<c1<<':'<<int(c1)<<endl;//转换为ASCll 
	cout<<c2<<':'<<int(c2)<<endl;
	cout<<'$'<<':'<<int('$')<<endl;
	
	cout<<"c1 c2"<<endl;//字符的输入与输出 
	cin>>c1>>c2;//cin后加endl会报错 
	cout<<"c1="<<c1<<"  c2="<<c2<<endl;
	 
}

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three. const variable (DEFINE in C language?)

It can only be assigned before the program is executed, and the value of the const variable can
only be used when the program is executed . It is somewhat similar to the "DEFINE" of the C language.

#include<iostream>//标题：const变量使用 
using namespace std;
const double PI=3.1415926;
/*只能在程序执行前赋值，程序执行时只能使用
不可修改const变量的值*/ 
//类似于C语言的"DEFINE" 
main() 
{
    
    
double r,l,s;
cout<<"r=";
cin>>r;

l=2*PI*r;
s=PI*r*r;

cout<<"l="<<l<<endl;
cout<<"s="<<s<<endl;	
 } 
 

Four. Enum variables ("classes" in C?)

I accidentally wrote all the knowledge points into the code QAQ

#include<iostream>
using namespace std;
main()
{
    
    
	//枚举变量像C语言里的 类 
	//定义枚举变量 
	enum color{
    
    
		RED=3,
		YELLOW=6,
		BLUE=9
	};
	
	//声明枚举变量的两种方式： 
	enum color a=RED;
	/*此时的color就像是一种类型（int,char.etc） */
	color b;
	//C语言中只支持第一种 
	
	cout<<"RED="<<RED<<endl; 
	cout<<"YELLOW="<<YELLOW<<endl; 
	cout<<"BLUE="<<BLUE<<endl; 
	
	b=a;
	a=BLUE;
	cout<<endl<<"a=BLUE,b="<<a<<endl;
	cout<<"b=RED,b="<<b<<endl;
	//可以正常的对于新增变量赋值 
	//一个问题，新增到类里面的变量是存储在'color'中的吗？ 
	
	
	/*BLUE=66;报错原因 
	赋值语句的左边应该是变量，不能是表达式。
	而实际上 枚举变量中一斤存储的内容是表达式，
	不可修改*/ 
	
	b=BLUE;
	cout<<endl<<"b=BLUE;"<<endl;
	cout<<"a-b="<<a-b<<endl;
	
	/*cin>>RED;报错原因
	还是不可修改的表达式（耸肩）*/ 
}

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This is the end of today's check-in, I accidentally sat in front of the computer for more than two hours! The spring is beautiful outside, I suggest you go out for an outing. Thanks for watching. Next chapter **(Basic Operation Types(1))**. stay tuned