c++学习笔记(2)

循环：

while循环计数控制循环 counter-controlled loop

标记控制的循环： sentinel-conrolled loop

在写循环的过程中容易出现多一次或者少一次的错误，即差一错误（off-by-one error）

计算减法--简单代码：

#include <iostream>
#include <cstdlib>    // rand and srand function
#include <ctime>      // for time function

using namespace std;

int main(int argc, char *argv[])
{
    srand(time(0));
	int number1 = rand() % 10;
	int number2 = rand() % 10;
	if(number1 < number2)
	{
		int temp = number1;
		number1 = number2;
		number2 = temp; 
	}    // swap	
	cout << "What is " << number1 << " - " << number2 << " ? " << endl;
	int answer;
	cin >> answer;
	while(answer != (number1 - number2))
	{
		cout << "You Entered a wrong answer !" << endl;
		cout << "Enter again: ";
		cin >> answer;
	}
	cout << "You git it!" << endl;
	return 0;
}

猜数字：

先随机生成一个[0, 100]之间的整数，由用户数输入猜测数字，每次给出猜测的结果(偏大还是偏小，直到猜中为止)

#include <iostream>
#include <cstdlib>    // rand and srand function
#include <ctime>      // for time function

using namespace std;

int main(int argc, char *argv[])
{
    srand(time(0));
	int number = 0 + rand() % (100 - 0 + 1);   // generate random number in [0, 100]
	cout << "Enter your guess([0, 100]): ";
	int guess;
	cin >> guess;
	while(guess != number)
	{
		if(guess - number > 0)
		{
			cout << "Your guess is too high." << endl;
		}
		else
		{
			cout << "Your guess is too low." << endl;
		}
		cout << "Enter your guess again: ";
		cin >> guess;
	}
	cout << "Yes, the number is " << guess << "!" << endl;
	return 0;
}

不要要浮点数作为循环的判断条件，因为浮点运算是一种近似的表示，程序可能会陷入死循环。

2.输入输出重定向

如果有许多数据需要输入，可以将文件存储在文件中，使用空格分开，如input.txt，运行：

SentineValue.exe < input.txt 输入重定向命令，从文件获取输入

输出重定向命令 SentineVAlue > output.txt , 可以将输出发送到文件中

输入输出重定向可以在同一个命令中：

SentineValue.exe < input.txt >output.txt

将input中的数据读取并发送到output中

(1)利用循环读取文件中的数据

#include <iostream>
#include <cstdlib>    // rand and srand function
#include <ctime>      // for time function
#include <fstream>    // for file operation

using namespace std;

int main(int argc, char *argv[])
{
    //srand(time(0));
    ofstream output;    // 先下入数据到txt文件中 
	output.open("number.txt");
	for(int count=0; count<10; count++)
	{
		output << count << " ";
	}
	output << endl;
	output << 3 << " " << 5 << endl;
	output.close();
	
	int number_1;
	int sum=0;
	// ifstream input("number.txt");
	ifstream input;
    input.open("number.txt");
    while(!input.eof())    // eof()[end of file]测试是否读到了文件末尾 
    {
	   input >> number_1;
	   sum += number_1;
	   cout << number_1 << endl; 	
    }
    
    cout << "The sum is " << sum << endl;
	return 0;
}

do-while 循环：while循环的变体

for 循环：简洁

先验循环： while, for 循环，循环的条件检验在循环体之前进行

后验循环： do while循环

#include <iostream>
#include <cstdlib>    // rand and srand function
#include <ctime>      // for time function
#include <fstream>    // for file operation
#include <iomanip>    // format outpit

using namespace std;

int main(int argc, char *argv[])
{
    //srand(time(0));
    cout << "       Multiplication Table         " << endl;
	cout << "    1   2   3  4  5  6  7  8  9"  << endl;
	cout << "---------------------------------" << endl;
	for(int i=1; i<= 10; i++)
	{
		cout << i << " | ";
		for(int j=1; j<=10; j++)
		{
		    cout << setw(3) << j*i << " ";	
		}
		cout << endl;
	}
	return 0;
}

运行结果：

求最大公约数

#include <iostream>
#include <cstdlib>    // rand and srand function
#include <ctime>      // for time function
#include <fstream>    // for file operation
#include <iomanip>    // for foemat output 

using namespace std;

int main(int argc, char *argv[])
{
    //srand(time(0));
    cout << "Enter a int number: ";
    int number1;
    cin >> number1;
    
    cout << "Enter another number: ";
    int number2;
    cin >> number2;
    
    int gcd = 1;
    int k = 2;    // 2-n1 2-n2
    while(k<number1 && k<number2)
    {
    	if(number1%k==0 && number2%k==0)
    	{
	    	gcd = k;
	    }
	    k++;
    }
    cout << "The gcd is "<< gcd << endl;

	return 0;
}

求最大公约数的其他方法：欧几里德算法，辗转相除法

欧几里德算法：

假设：

$x>y$

定义：

$x // y = a$

x % y =b，则有b<y成立。

x = a*y +b ---> b = x - a*y,则x,y的最大公约数也是b的约数，此时令x=y, y=b, 重复上述步骤，

知道x%y=0时，此时的y就是x, y的最大公约数(相当于不断缩小公约数的范围)

code:

#include <iostream>
#include <cstdlib>    // rand and srand function
#include <ctime>      // for time function
#include <fstream>    // for file operation
#include <iomanip>    // for foemat output 

using namespace std;

int main(int argc, char *argv[])
{
	// 欧几里得法求最大公约数 
    cout << "Enter a number1: ";
	int x;
	cin >> x;
	
	cout << "Enter another number: ";
	int y;
	cin >> y;
	
	if(x<y)
	{
		int temp;
		temp = x;
		x = y;
		y = temp;	
	} 
	cout << "x = " << x << endl;
	cout << "y = " << y << endl;
	
	while(x%y != 0)
	{
		int b = x % y;
		x = y;
		y = b;	
	}
	cout << "The gcd is " << y << endl;
	return 0;
}

预测未来的学费：

假设初始的学费为￥10000，每年以7%递增，多少年后学费会翻倍。

#include <iostream>
#include <cstdlib>    // rand and srand function
#include <ctime>      // for time function
#include <fstream>    // for file operation
#include <iomanip>    // for foemat output 
#include <cmath>      // for general math function 

using namespace std;

int main(int argc, char *argv[])
{
	float init_tuition = 10000;   // 初始学费
	float increase_rate = 0.07;
    int year=1;
    float tuition = init_tuition * pow((1+increase_rate), year);
	while(tuition < 2*init_tuition)
	{
		year++;
		tuition = init_tuition * pow((1+increase_rate), year);
	}
	cout << "After " << year << " year(s) " << " the tuition double" << endl;	 
	return 0;
}

蒙特卡洛模拟：

估计pi的值，作边长为1的正方形和它的内切圆。

#include <iostream>
#include <cstdlib>    // rand and srand function
#include <ctime>      // for time function
#include <fstream>    // for file operation
#include <iomanip>    // for foemat output 
#include <cmath>      // for general math function 

using namespace std;
const float RAND_MAX_float = static_cast<float>(RAND_MAX);

int main(int argc, char *argv[])
{
    const int trial_nums = 100000;
	int number_of_hits = 0;
	srand(time(0));
    for(int i=0; i<trial_nums; i++)
    {
    	double x = rand() / RAND_MAX_float;
    	double y = rand() / RAND_MAX_float;
    	if(sqrt(x*x + y*y)<1)
    	{
	    	number_of_hits++;
	    }
    }
    cout << "PI is " << 4.0 * number_of_hits / trial_nums << endl;
	return 0;
}

十进制数转换转换为十六进制：

十进制数连续除以16，直到商为零。

总结：将得到的字符串连接按照反序输出的方法：

code:

#include <iostream>
#include <cstdlib>    // rand and srand function
#include <ctime>      // for time function
#include <fstream>    // for file operation
#include <iomanip>    // for foemat output 
#include <cmath>      // for general math function 
#include <string>

using namespace std;
const float RAND_MAX_float = static_cast<float>(RAND_MAX);

int main(int argc, char *argv[])
{
	// decimal convert to
	 
    cout << "Enter a int: " << endl;
    int decimal;
    cin >> decimal;
    int tmp = decimal;
        
    string hex="";
    char hex_char;
    while(decimal!=0)
    {
    	int hexValue = decimal % 16;
    	hex_char = (hexValue>=0 && hexValue<=9)?static_cast<char>(hexValue+'0'): static_cast<char>(hexValue-10+'A');
        //hex = hex + hex_char;      // 这种写法输出的顺序是反的
		hex = hex_char +hex;         // 这种写法的输出是正确的 
		decimal = decimal / 16;	
    }
    
    cout << tmp << " to hex is " << hex << endl;
	return 0;
}

输出素数

#include <iostream>
#include <string>
#include <iomanip>
 
using namespace std;

const int number_of_prime = 50;
const int number_of_prime_per_line = 10;
	
int main(int argc, char *argv[])
{
    // 输出素数
	int counter = 0;
	int number = 2;   // 素数起始
	cout << "The first 50 primes are: " << endl;
	
	while(counter < number_of_prime)
	{
		bool isPrime = true;
		
		for(int devisor=2; devisor<=number/2; devisor++)
		{
			if(number % devisor == 0)
			{
			   isPrime = false;
			   break;
			}
		}
		
		if(isPrime)
		{
		   counter++;
		   if(counter%number_of_prime_per_line==0)
		   {
		   	   cout << setw(4) << number << endl;
		   }
		   else
		   {
		   	   cout << setw(4) << number;   // 固定宽度输出，不用考虑添加空格 
		   }
   		}
		   
		number++;
		
	}
		
	return 0;
}

posted @ 2018-09-07 11:05 Alpha205 阅读(74) 评论(0) 编辑收藏举报

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c++学习笔记(2)

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