c++学习笔记（1）

常用的c++库：

#include <iostream>   // 输入输出
#include <cstdlib>    // 随机函数 
#include <cctype>     // 字符函数 
#include <string>     // 字符串函数 
#include <iomanip>    // 格式化输出
#include <fstream>    // 文件读写操作 
#include <cmath>      // 数学函数相关的函数

字符串读取：

#include <iostream>
using namespace std;
int main()
{
    char ch;
    cout<<"Enter a char: ";
    cin >> ch;
    cout << "You input " << ch << endl;
    cout << "He said \"program is interetsing\"." << endl;
    int c = static_cast<int>(ch);    //类型转换
    cout << "num is " << c << endl;
    return 0;
}

字符大小写转换：

#include <iostream>
using namespace std;
int main()
{
   cout << "Enter a lowercase char: ";
   char lowercase;
   cin >> lowercase;
   char uppercase = static_cast<char>('A' + lowercase - 'a');    //原理   'A'-'a'='C'-'c'
   cout << "The corresponding letter is " << uppercase << endl;
   return 0;
}

生成一个随机字母：

生成固定范围内的整数： 公式 a+rand()%(b-a+1) ----> (a, a+b)之间的随机数

#include <iostream>
#include <cstdlib>    // 随机函数 
int main()
{
   int a = static_cast<int>('a');
   int b = static_cast<int>('z');
   srand(time(0));
   char random_char = a + rand() % (b-a+1); //公式 a+rand()%(b-a+1) ----> (a, a+b)之间的随数 
   cout << "The random char is " << random_char << endl; 
   return 0;
}

随机在两个字符之间生成一个字符(ASCII 码  America standard code for information interchange)

#include <iostream>
#include <cstdlib>    // 随机函数 
int main()
{
   cout << "input a char: ";
   char ch1;
   cin >> ch1;
   
   cout << "Input a char: ";
   char ch2;
   cin >> ch2;
	
   int a = static_cast<int>(ch1);   // 类型转换
   int b = static_cast<int>(ch2);
	
   if(a<b)
   {
	char random_char = a + rand() % (b - a + 1);
	cout << "The random char between " << ch1 << " and " << ch2 << " is " << random_char << endl;
    }
	
    else if(a>b)
    {
	char random_char = b + rand() % (a - b + 1);
	cout << "The random char between " << ch2 << " and " << ch1 << " is " << random_char << endl;
    }
 
   else
   {
	cout << "You Inpit two same char!" << endl;
   }
   return 0;
}

猜生日code:

简单原理：

0~31之间的数字，可以用二进制11111表示，可以看到没一张表中的第一个数字分别是1，2，4，8，16，用二进制可以表示为

00001， 00010， 00100， 01000， 10000。五张表中的数字分布是有一定的规律的。0-31之间的任何一个数字都可以由前面

五个数字组成，将能够组成生日的数字所在的表中填入即可，直接判断自己的生日在不在表中，将得到的表第一个数字相加集

即可得到正确的数字。

#include <iostream>
using namespace std;
int main()
{
        int day = 0;
	char answer;
	cout << "Is your birthday in set1" << endl;
	cout << "1 3 5 7\n" << 
	        "9 11 13 15\n" << 
		"17 19 21 23\n" << 
		"25 27  29 31\n" << endl;
	cout << "Enter your answer(y/n): ";
	cin >> answer;
	if(answer == 'y' || answer == 'Y')
	{
		day += 1;
	}
	
	cout << "\nIs your answer in set2: " << endl;
	cout << "2 3 6 7\n" <<
	        "10 11 14 15\n" <<
		"18 19 22 23\n" << 
		"26 27 30 31\n" << endl;
	cout << "ENter your answer(y/n): ";
	cin >> answer;
	if(answer == 'y' || answer == 'Y')
	{
		day += 2;
	}
	
	
	cout << "\nIs your answer in set3: " << endl;
	cout << "4 5 6 7\n" <<
	        "12 13 14 15\n" <<
		"20 21 22 23\n" << 
		"28 29 30 31\n" << endl;
	cout << "ENter your answer(y/n): ";
	cin >> answer;
	if(answer == 'y' || answer == 'Y')
	{
		day += 4;
	}
	
	
	cout << "\nIs your answer in set4: " << endl;
	cout << "8 9 10 11\n" <<
	        "12 13 14 15\n" <<
		"24 25 26 27\n" << 
		"28 29 30 31\n" << endl;
	cout << "ENter your answer(y/n): ";
	cin >> answer;
	if(answer == 'y' || answer == 'Y')
	{
		day += 8;
	}
	
	cout << "\nIs your answer in set5: " << endl;
	cout << "16 17 18 19\n" <<
	        "20 21 22 23\n" <<
		"24 25 26 27\n" << 
		"28 29 30 31\n" << endl;
	cout << "ENter your answer(y/n): ";
	cin >> answer;
	if(answer == 'y' || answer == 'Y')
	{
		day += 16;
	}
	
	cout << "Your birth day is " << day << endl;
        return 0;
}

大小写转换(cctype 里的函数)

如果输入是大写，则转换为小写，输入小写，转换为大写， 如果不是字母给出提示；

字符常用函数：

isdigit()  

isalpha()  

isalnum() : 是否为数字或者字母

islower(): 是否为小写

isupper()

isspace() : 是否是空白字符

tolower()

toupper()

#include <iostream>
#include <cstdlib>    // 随机函数 
#include <cctype>     // 字符函数 
using namespace std;
int main()
{
        cout << "Enter a character: ";
	char ch;
	cin >> ch;
	cout << "You Entered " << ch << endl;
	
	if(islower(ch))
	{
		char temp;
		cout << "It is a lowercase." << endl;
		temp = static_cast<char>(toupper(ch));
		cout << "The equilent is " << temp << endl;
	} 
	else if(isupper(ch))
	{
		char temp;
		cout << "It is a uppercase" << endl;
		temp = static_cast<char>(tolower(ch));
		cout << "The equilent is " << temp << endl;
	}
	else
	{
		cout << "It is not a alpha" << endl;
	}
        return 0;
}

字符串操作：

常用字符串函数入下：

length(), size() : 字符串长度

at(index): 字符串索引

#include <iostream>
#include <cstdlib>    // 随机函数 
#include <cctype>     // 字符函数 
#include <string>     // 字符串函数 
using namespace std;
int main()
{
        string message = "Hello python"; 
	string str1 = "ABCDEF";
	cout << "string len is " << str1.length() << endl;
	cout << "C is at " << str1.at(2) << endl;   
        return 0;
}

彩票：

生成两位的随机数与输入的两位数比较：

完全相同

次序相同

只有一个数字相同

涉及到：（1）随机数字生成， 字符串比较

#include <iostream>
#include <cstdlib>    // 随机函数 
#include <cctype>     // 字符函数 
#include <string>     // 字符串函数 
using namespace std;
int main()
{
        // 彩票代码 
	string lottery;    // string类型的变量可以直接用+连接
	srand(time(0));    // time(0)返回格林尼治时间起经过的秒数，作为随机数的种子
	int digit = rand() % 10;   // 生成一位随机整数
	lottery += static_cast<char>(digit + '0');
	digit = rand() % 10;
	lottery += static_cast<char>(digit + '0');   // 两个随机整数链接为字符串
	
	cout << "Enter a guess(two numbers): ";
	string guess;
	cin >> guess;
	
	if(lottery == guess)
	{
		cout << "You win $10 000" << endl;
	}
	else if(lottery[0]==guess[1] && lottery[1]==guess[0])
	{
		cout << "You win $3000" << endl;
	}
	else if(lottery[0]==guess[0] || lottery[0]==guess[1] || lottery[1]==guess[0]
	        || lottery[1]==guess[1])
	        {
        		cout << "You win $1000" << endl;
        	}
	else
	{
		cout << "You loss" << endl;
	}
	return 0; 
}

格式化输出：

常用的格式化输出函数

setprecision(n)

fixed()

showpoint()

setw(width)

left() 左对齐

right()   右对齐

#include <iostream>
#include <cstdlib>    // 随机函数 
#include <cctype>     // 字符函数 
#include <string>     // 字符串函数 
#include <iomanip>    // 格式化输出
using namespace std;
int main()
{
        double amount = 3342.3456;
	double rate = 0.0087;
	double interest = amount * rate;
	cout << "Interest is " << fixed << setprecision(3) <<interest << endl;
	return 0;
}
 
 

文件读写：

写文件： ofstream output ;

读文件： ifstream  input ;

程序从文件中读取数据相当于input，写入数据到文件中相当于output

#include <iostream>
#include <cstdlib>    // 随机函数 
#include <cctype>     // 字符函数 
#include <string>     // 字符串函数 
#include <iomanip>    // 格式化输出
#include <fstream>    // 文件读写操作 
#include <cmath>      // 数学函数相关
using namespace std;
int main()
{
        //文件写操作 
	ofstream output;
	output.open("numbers.txt");   // 创建文件
	output << 23 << " " << 90 << " " << 56 << endl;
	//output << 98 << " " << "hello" << " " << 98 <<endl; 
	output.close();
	
	// 读取文件
	ifstream input;
	// 用Input对象的open()函数打开文件
	input.open("numbers.txt");
	int score1, score2, score3;
	input >> score1;
	input >> score2;
	input >> score3;
	cout << "Read from file: \n" << "score1== " << score1 << endl;
	cout << "score2== " << score2 << endl;
	cout << "score3== " << score3 << endl;  
	input.close(); 
        return 0;
}