第八章 IO库

8.1&&8.2

 #include <iostream>
 #include <vector>
 #include <string>

 using namespace std;

 istream &func(istream &is)
 {
     int val;
     while (is >> val && !is.eof()) {
         cout << val << endl;
     }
     is.clear();        //复位所有错误标志位
 }

 int main()
 {
     func(cin);
     return ;
 }

8.3

读取类型不匹配；遇到文件结束符EOF；IO流错误

8.4

 #include <iostream>
 #include <fstream>
 #include <vector>
 #include <string>

 using namespace std;

 int main()
 {
     vector<string> vec;
     string ifile = "data.txt", ss;    //需在本文件夹下有data.txt这个文件
     ifstream in(ifile);
     while(getline(in, ss)) {
         vec.push_back(ss);
     }
     in.close();
     for (auto i : vec)
         cout << i << endl;
     return ;
 }

8.5

 #include <iostream>
 #include <fstream>
 #include <vector>
 #include <string>

 using namespace std;

 int main()
 {
     vector<string> vec;
     string ifile = "data.txt", ss;    //需在本文件夹下有data.txt这个文件
     ifstream in(ifile);
     while(in >> ss) {            //改动处
         vec.push_back(ss);
     }
     in.close();
     for (auto i : vec)
         cout << i << endl;
     return ;
 }

8.6

 #include <iostream>
 #include <fstream>
 #include <vector>
 #include <string>

 using namespace std;

 struct Sales_data {
     string bookNo;                //书的ISBN
     unsigned units_sold = ;    //售出的本数
     double revenue = 0.0;        //销售额
     Sales_data& combine(const Sales_data &);
     string isbn()    {    return bookNo;    }
 };

 Sales_data &Sales_data::combine(const Sales_data &rhs)
 {
     units_sold += rhs.units_sold;
     revenue += rhs.revenue;
     return *this;
 }

 istream &read(istream &is, Sales_data &item)
 {
     is >> item.bookNo >> item.units_sold >> item.revenue;
     return is;
 }

 ostream &print(ostream &os, const Sales_data &item)
 {
     os << item.bookNo << " " << item.units_sold << " " << item.revenue;
     return os;
 }

 Sales_data add(const Sales_data &lhs, const Sales_data &rhs)
 {
     Sales_data sum = lhs;
     sum.combine(rhs);
     return sum;
 }

 int main(int argc, char *argv[])
 {
     if (argc != ) {
         cerr<< "Please give the file name."<<endl;
         return -;
     }
     ifstream in(argv[]);
     if (!in) {
         cerr<<"Can't open the file."<<endl;
         return -;
     }
     Sales_data total;
     if (read(in, total)) {
         Sales_data trans;
         while (read(in, trans)) {
             if (total.isbn() == trans.isbn())    total = add(total, trans);
             else {
                 print(cout, total);
                 cout << endl;
                 total = trans;        //结构体赋值很方便
             }
         }
         cout << total.bookNo << " " << total.units_sold << " " << total.revenue << endl;
     }
     else {
         cerr << "No data?!" << endl;
         return -;
     }
     return ;
 }

文件名（book.txt）通过参数传给main函数：

8.7

 #include <iostream>
 #include <fstream>
 #include <vector>
 #include <string>

 using namespace std;

 struct Sales_data {
     string bookNo;                //书的ISBN
     unsigned units_sold = ;    //售出的本数
     double revenue = 0.0;        //销售额
     Sales_data& combine(const Sales_data &);
     string isbn()    {    return bookNo;    }
 };

 Sales_data &Sales_data::combine(const Sales_data &rhs)
 {
     units_sold += rhs.units_sold;
     revenue += rhs.revenue;
     return *this;
 }

 istream &read(istream &is, Sales_data &item)
 {
     is >> item.bookNo >> item.units_sold >> item.revenue;
     return is;
 }

 ostream &print(ostream &os, const Sales_data &item)
 {
     os << item.bookNo << " " << item.units_sold << " " << item.revenue;
     return os;
 }

 Sales_data add(const Sales_data &lhs, const Sales_data &rhs)
 {
     Sales_data sum = lhs;
     sum.combine(rhs);
     return sum;
 }

 int main(int argc, char *argv[])
 {
     if (argc != ) {
         cerr<< "Please give the input file name and output file name."<<endl;
         return -;
     }
     ifstream in(argv[]);
     ofstream out(argv[]);
     if (!in) {
         cerr<<"Can't open the file."<<endl;
         return -;
     }
     if (!out) {
         cerr<<"can't open output file."<<endl;
         return -;
     }
     Sales_data total;
     if (read(in, total)) {
         Sales_data trans;
         while (read(in, trans)) {
             if (total.isbn() == trans.isbn())    total = add(total, trans);
             else {
                 print(out, total) << endl;        //输出到指定文件中
                 total = trans;
             }
         }
         print(out, total) << endl;
     }
     else {
         cerr << "No data?!" << endl;
         return -;
     }
     return ;
 }

将上一题输出的内容输出到文件book1.txt中：

8.8

 #include <iostream>
 #include <fstream>
 #include <vector>
 #include <string>

 using namespace std;

 struct Sales_data {
     string bookNo;                //书的ISBN
     unsigned units_sold = ;    //售出的本数
     double revenue = 0.0;        //销售额
     Sales_data& combine(const Sales_data &);
     string isbn()    {    return bookNo;    }
 };

 Sales_data &Sales_data::combine(const Sales_data &rhs)
 {
     units_sold += rhs.units_sold;
     revenue += rhs.revenue;
     return *this;
 }

 istream &read(istream &is, Sales_data &item)
 {
     is >> item.bookNo >> item.units_sold >> item.revenue;
     return is;
 }

 ostream &print(ostream &os, const Sales_data &item)
 {
     os << item.bookNo << " " << item.units_sold << " " << item.revenue;
     return os;
 }

 Sales_data add(const Sales_data &lhs, const Sales_data &rhs)
 {
     Sales_data sum = lhs;
     sum.combine(rhs);
     return sum;
 }

 int main(int argc, char *argv[])
 {
     if (argc != ) {
         cerr<< "Please give the input file name and output file name."<<endl;
         return -;
     }
     ifstream in(argv[]);
     ofstream out(argv[], ofstream::app);        //唯一改变之处
     if (!in) {
         cerr<<"Can't open the file."<<endl;
         return -;
     }
     if (!out) {
         cerr<<"can't open output file."<<endl;
         return -;
     }
     Sales_data total;
     if (read(in, total)) {
         Sales_data trans;
         while (read(in, trans)) {
             if (total.isbn() == trans.isbn())    total = add(total, trans);
             else {
                 print(out, total) << endl;
                 total = trans;
             }
         }
         print(out, total) << endl;
     }
     else {
         cerr << "No data?!" << endl;
         return -;
     }
     return ;
 }

8.9

 #include <iostream>
 #include <vector>
 #include <string>
 #include <sstream> 

 using namespace std;

 int main()
 {
     string line, word;
     while(getline(cin, line)) {
         istringstream in(line);
         while (in >> word)
             cout << word << endl;
     }
     return ;
 }

8.10

 #include <iostream>
 #include <vector>
 #include <string>
 #include <fstream>
 #include <sstream> 

 using namespace std;

 int main()
 {
     vector<string> vec;
     string line, word;
     ifstream input("data.txt");
     while(getline(input, line)) {
         vec.push_back(line);
     }
     for (auto i : vec) {
         istringstream in(i);
         while (in >> word) {
             cout << word << endl;
         }
     }
     return ;
 }

8.11

 #include <iostream>
 #include <vector>
 #include <string>
 #include <fstream>
 #include <sstream> 

 using namespace std;

 struct PersonInfo {
     string name;
     vector<string> phones;
 };  

 int main()
 {
     string line, word;
     vector<PersonInfo> people;
     istringstream record;      //定义在循环之外
     while (getline(cin,line)) {
         PersonInfo info;
         record.clear();      //每次要解除上次绑定的string对象
         record.str(line);
         record >> info.name;
         while (record >> word)
             info.phones.push_back(word);
         people.push_back(info);
     }
     return ;
 }

8.12

因为此时我们需要其中的成员皆可被访问，所以 PersonInfo 是一个聚合类，不能在内部进行初始化

8.13

 #include <iostream>
 #include <vector>
 #include <string>
 #include <fstream>
 #include <sstream> 

 using namespace std;

 struct PersonInfo {
     string name;
     vector<string> phones;
 };  

 int main()
 {
     ifstream in("data.txt");         //从文件读取
     string line, word;
     vector<PersonInfo> people;
     istringstream record;
     while (getline(in,line)) {      //in为文件输入流
         PersonInfo info;
         record.clear();
         record.str(line);
         record >> info.name;
         while (record >> word)
             info.phones.push_back(word);
         people.push_back(info);
     }
     return ;
 }  

8.14

这两条语句分别适用范围for语句枚举people中所有项和每项的phones中的所有项。

使用const表明在循环中不会改变这些项的值；

使用auto是请求编译器依据vector元素类型来推断出entry和nums的类型，既简化代码又避免出错；

使用引用的原因是，people和phones的元素分别是结构对象和字符串对象，使用引用即可避免对象拷贝。