第一章 C++编程基础——1.7文件读写

  本文是这一章的最后一小节，主要学习C++中文件的读写操作，最后案例依旧是上几个文章的更新。
  【代码已上传至自己github，供参考： github】
  本文的主要目的是学习文件读写，案例具体来说：(1)每次执行结束，将用户的姓名及程序(会话)的某些数据写入文件；(2)在程序打开另一个会话时，能够将数据从文件中读回。
  首先对文件读写时，要包含的头文件是： fstream

1、写入操作

  若是写文件，则创建ofstream对象，例如：

ofstream outfile(“seq_data.txt”);//以覆盖的方式写文件
ofstream outfile(“seq_data.txt”，ios_base::app);//以追加的方式进行写文件

  案例：

int main() {
	string name;
	int num_tries = 0;
	int num_rights = 0;
	///追加模式
	ofstream outfile("seq_data.txt",ios_base::app);
	cout << "please enter your name: ";
	cin >> name;
	cout << "please input num_tries、num_rights: ";
	cin >> num_tries>>num_rights;
	if (!outfile) //无法打开文件
		cerr << "Sorry!Unable to save session data!";
	else
		outfile << name << " "
				<< num_tries << " "
				<< num_rights
				<< endl;
	return 0;
}

2、读文件

  若是读文件，则创建ifstream对象，例如：

ifstream infile(“seq_data.txt”);//读文件

  案例：

int main(){
	string name;
	int num_tries = 0;
	int num_rights = 0;
	int nt;//用户猜过的总次数
	int nc;//用户猜对的总次数
	string usr_name;

	cout << "Please input your name: ";
	cin >> usr_name;

	ifstream infile("seq_data.txt");
	if (!infile)//无法打开文件
		cerr << "Sorry,the file can not open!";
	else
	{
		while (infile>>name)
		{
			infile >> nt >> nc;
			if (name==usr_name)
				cout << "Welcome back " << usr_name
					<< "\n Your current score is " << nc
					<< " out of " << nt << "\n Good luck!\n";
			num_tries = nt;
			num_rights = nc;
		}
	}
	return 0;
}

3、同时读写

  若是读写文件，则创建fstream对象，例如：

fstream iofile("seq_data.txt",ios_base::in|ios_base::app);

  案例：

int main(){
	fstream iofile("seq_data.txt",ios_base::in|ios_base::app);
	
	if (!iofile)
		cerr << "Sorry,the file can not open!";
	else
	{

		//写入
		int num_tries = 0;
		int num_rights = 0;
		string usr_name;
		iofile << "cqy2 " << 10 << ' '<< 7 << endl;
		
		//由于ios_base::app的原因，开始读取前将文件位置定位到起始
		iofile.seekg(0);
		//读
		iofile >> usr_name >> num_tries >> num_rights;
		cout << "name: " << usr_name << endl;
		cout << "num_tries: " << num_tries << endl;
		cout << "num_rightes: " << num_rights;
	}
	return 0;
}

  注意：其中由于ios_base::app的原因，开始读取前应该要将文件位置定位到起始位置，seekg()可以将iofile重新定位至文件起始处。

4、案例更新

int main() {
	int usr_guess;      //用户猜测的数字
	bool num_seq = true; //显示下一组数列
	bool guess_again = true; //用户想再猜一次
	int guess_num = 0;  //用户猜的总次数
	int guess_right = 0;//用户猜对次数
	char usr_rsp;      //用户的回答  内循环用
	char try_again;    //用户的回答  外循环用
	double usr_score = 0.0;//评分比值，采用double双精度类型
	const int max_tries = 3;//设置最多猜测次数

	const int seq_size = 18;//设置数列最大长度

	int elem_seq[seq_size] = { //每个数列存储前三个数字进行猜测
		1,2,3, //Fibonacci(斐波那契)
		3,4,7, //Lucas(卢卡斯)
		2,5,12, //Pell
		3,6,10, //Triangular
		4,9,16, //Square
		5,12,22 //Pentagonal
	};
	//将每个数列前三个数存入vector
	vector<int> fibonacci(elem_seq, elem_seq + 3);
	vector<int> lucas(elem_seq + 3, elem_seq + 6);
	vector<int> pell(elem_seq + 6, elem_seq + 9);
	vector<int> Triangular(elem_seq + 9, elem_seq + 12);
	vector<int> Square(elem_seq + 12, elem_seq + 15);
	vector<int> Pentagonal(elem_seq + 15, elem_seq + 18);

	const int max_seq = 6;
	string seq_names[seq_size] = {
		"Fibonacci",
		"Lucas",
		"Pell",
		"Triangular",
		"Square",
		"Pentagonal"
	};
	//将每个数列地址存入seq_addrs数组
	vector<int> *seq_addrs[max_seq] = {
		&fibonacci,&lucas,&pell,&Triangular,&Square,&Pentagonal
	};
	vector<int> *current_vec = 0;
	int seq_index;
	srand(max_seq);
	
//========================更新部分========================
	int nt = 0;//猜测总次数
	int nc = 0;//猜测对的次数
	string usr_name;


	cout << "Please input your name: ";
	cin >> usr_name;

	//读数据
	ifstream infile("seq_data.txt");
	if (!infile)
		cout << "Sorry,the file can not open!" << endl;
	else
	{
		int find=0;
		string name;
		while (infile >> name)
		{
			infile >> nt >> nc;
			if (name==usr_name)
			{
				find = 1;
				guess_num = nt;
				guess_right = nc;
			}
		}

		if (find == 1)
			cout << "Welcome back!" << usr_name
				<< "\n TOT [" << nt << "] "
				<< " of [" << nc << "]"
				<< "\n Good luck!\n";
		else
			cout << "Welcome," << usr_name << "!" << endl;
	}

//===================================================

	while (num_seq == true) {
		int try_cnt = 0;//猜的次数与最多次数比较
		bool got_it = false; //用户是否猜对

		//对数列进行随机化
		seq_index = rand() % max_seq;
		current_vec = seq_addrs[seq_index];//获取数列

		//开始猜测数字
		cout << "The first two elements of the squence are: "
			<< (*current_vec)[0] << ","
			<< (*current_vec)[1] << "."
			<< "\nWhat is the next element?\n";

		//用户猜错且想再次猜
		while (guess_again == true && got_it == false && (try_cnt++ <= max_tries))
		{

			std::cout << "please input your num:" << endl;
			std::cin >> usr_guess;
			guess_num++;

			//如果猜正确
			if (usr_guess == (*current_vec)[2]) {

				std::cout << "Your guess is right!"
					<< (*current_vec)[2]
					<< " is the next element in the "
					<< seq_names[seq_index]
					<< " sequence.\n";
				got_it = true;
				guess_right++;
			}
			//用户猜错
			else {
				//判断猜的次数  switch
				switch (try_cnt)
				{
				case(1):
					std::cout << "Oops!Nice guess but not quiye it! \n" << endl;
					break;
				case(2):
					std::cout << "Hmm.Sorry.Wrong a second time.\n" << endl;
					break;
				case(3):
					std::cout << "Ah,this is harder than it looks.\n" << endl;
					break;
				default:
					std::cout << "It must be getting pretty frustrating by now! \n" << endl;
					break;
				}
				//是否再试一次
				std::cout << "Error!Want to try again?(y/n):";
				std::cin >> usr_rsp;
				if (usr_rsp == 'N' || usr_rsp == 'n')
					guess_again = false;
			}
		}//内层循环结束

		std::cout << "want to try another sequence again?(y/n):";
		std::cin >> try_again;
		if (try_again == 'N' || try_again == 'n')
			num_seq = false;

	}//外循环结束

//=======================更新部分======================
	//将信息写入文件
	ofstream outfile("seq_data.txt", ios_base::app);
	outfile << usr_name << " "
		<< guess_num << " "
		<< guess_right << endl;
//====================================================
	return 0;
}

5、习题

  (1) 编写一个程序，能够询问用户的姓名，并读取用户所输人的内容。请确保用户输入的名称长度大于两个字符。如果用户的确输人了有效名称，就响应一些信息。请以两种方式实现:第一种使用C-style字符串，第二种使用string对象。
  第一种：C-style字符串

int main() {
	const int size = 18;
	char name[size];
	cout << "please input your name:" << endl;
	cin >> name;
	if (strlen(name) < 3)
		cout << "sorry,your name's size <3";
	else
		cout << "Welcome," << name << "!" << endl;
	return 0;
}

  第二种：string对象

int main(){
	string name;
	cout << "please input your name:"<<endl;
	cin >> name;
	if (name.size() < 3)
		cout << "sorry,your name's size <3";
	else
		cout << "Welcome," << name<<"!"<<endl;
	return 0;

}

  （2）编写一个程序，从标准输人设备读取一串整数,并将读人的整数依次放到array及vector,然后遍历这两种容器，求取数值总和。将总和及平均值输出至标准输出设备。

int main() {
	vector<int> vec_num;
	int num;
	int sum = 0;
	cout << "please input the num: "<<endl;
	while (cin>>num ){
		vec_num.push_back(num);
//=============这部分可以不要，设置输入数字最多个数 =============
		if (vec_num.size() > 5)
			break;
//=======================================================
	}

	for (int i = 0; i < vec_num.size(); i++)
	{
		sum += vec_num[i];
	}
	cout << "sum=" << sum << endl;
	cout << "average=" << sum / vec_num.size() << endl;
	return 0;
}

  (3)使用你最称手的编辑工具，输入两行(或更多)文字并存盘。然后编写-一个程序，打开该文本文件，将其中每个字都读取到一个vector对象中。遍历该vector, 将内容显示到cout。然后利用泛型算法sort()，对所有文字排序，再将排序后的结果输出到另一个文件。
  注意： sort()算法使用

#include 
sort(container.begin(),container.end()

int main() {
	vector<string> my_string;
	ifstream infile("string.txt");
	
	ofstream outfile("string_data.txt", ios_base::app);
	if (!infile || !outfile) 
		cout << "Error,the file can not open!" << endl;
	else
	{
		string txt;
		while (infile >> txt) 
			my_string.push_back(txt);

		cout << "Unsorted text:" << endl;
		for (int index = 0; index < my_string.size(); index++)
			cout << my_string[index] << endl;

		sort(my_string.begin(), my_string.end());
		for (int i = 0; i < my_string.size(); i++)
			outfile << my_string[i] << endl;
			
	}
	return 0;

}