第八章 IO库
8.1&&8.2
1 #include <iostream> 2 #include <vector> 3 #include <string> 4 5 using namespace std; 6 7 istream &func(istream &is) 8 { 9 int val; 10 while (is >> val && !is.eof()) { 11 cout << val << endl; 12 } 13 is.clear(); //复位所有错误标志位 14 } 15 16 int main() 17 { 18 func(cin); 19 return 0; 20 }
8.3
读取类型不匹配；遇到文件结束符EOF；IO流错误
8.4
1 #include <iostream> 2 #include <fstream> 3 #include <vector> 4 #include <string> 5 6 using namespace std; 7 8 int main() 9 { 10 vector<string> vec; 11 string ifile = "data.txt", ss; //需在本文件夹下有data.txt这个文件 12 ifstream in(ifile); 13 while(getline(in, ss)) { 14 vec.push_back(ss); 15 } 16 in.close(); 17 for (auto i : vec) 18 cout << i << endl; 19 return 0; 20 }
8.5
1 #include <iostream> 2 #include <fstream> 3 #include <vector> 4 #include <string> 5 6 using namespace std; 7 8 int main() 9 { 10 vector<string> vec; 11 string ifile = "data.txt", ss; //需在本文件夹下有data.txt这个文件 12 ifstream in(ifile); 13 while(in >> ss) { //改动处 14 vec.push_back(ss); 15 } 16 in.close(); 17 for (auto i : vec) 18 cout << i << endl; 19 return 0; 20 }
8.6
1 #include <iostream> 2 #include <fstream> 3 #include <vector> 4 #include <string> 5 6 using namespace std; 7 8 struct Sales_data { 9 string bookNo; //书的ISBN 10 unsigned units_sold = 0; //售出的本数 11 double revenue = 0.0; //销售额 12 Sales_data& combine(const Sales_data &); 13 string isbn() { return bookNo; } 14 }; 15 16 Sales_data &Sales_data::combine(const Sales_data &rhs) 17 { 18 units_sold += rhs.units_sold; 19 revenue += rhs.revenue; 20 return *this; 21 } 22 23 istream &read(istream &is, Sales_data &item) 24 { 25 is >> item.bookNo >> item.units_sold >> item.revenue; 26 return is; 27 } 28 29 ostream &print(ostream &os, const Sales_data &item) 30 { 31 os << item.bookNo << " " << item.units_sold << " " << item.revenue; 32 return os; 33 } 34 35 Sales_data add(const Sales_data &lhs, const Sales_data &rhs) 36 { 37 Sales_data sum = lhs; 38 sum.combine(rhs); 39 return sum; 40 } 41 42 int main(int argc, char *argv[]) 43 { 44 if (argc != 2) { 45 cerr<< "Please give the file name."<<endl; 46 return -1; 47 } 48 ifstream in(argv[1]); 49 if (!in) { 50 cerr<<"Can't open the file."<<endl; 51 return -1; 52 } 53 Sales_data total; 54 if (read(in, total)) { 55 Sales_data trans; 56 while (read(in, trans)) { 57 if (total.isbn() == trans.isbn()) total = add(total, trans); 58 else { 59 print(cout, total); 60 cout << endl; 61 total = trans; //结构体赋值很方便 62 } 63 } 64 cout << total.bookNo << " " << total.units_sold << " " << total.revenue << endl; 65 } 66 else { 67 cerr << "No data?!" << endl; 68 return -1; 69 } 70 return 0; 71 }
文件名（book.txt）通过参数传给main函数：
8.7
1 #include <iostream> 2 #include <fstream> 3 #include <vector> 4 #include <string> 5 6 using namespace std; 7 8 struct Sales_data { 9 string bookNo; //书的ISBN 10 unsigned units_sold = 0; //售出的本数 11 double revenue = 0.0; //销售额 12 Sales_data& combine(const Sales_data &); 13 string isbn() { return bookNo; } 14 }; 15 16 Sales_data &Sales_data::combine(const Sales_data &rhs) 17 { 18 units_sold += rhs.units_sold; 19 revenue += rhs.revenue; 20 return *this; 21 } 22 23 istream &read(istream &is, Sales_data &item) 24 { 25 is >> item.bookNo >> item.units_sold >> item.revenue; 26 return is; 27 } 28 29 ostream &print(ostream &os, const Sales_data &item) 30 { 31 os << item.bookNo << " " << item.units_sold << " " << item.revenue; 32 return os; 33 } 34 35 Sales_data add(const Sales_data &lhs, const Sales_data &rhs) 36 { 37 Sales_data sum = lhs; 38 sum.combine(rhs); 39 return sum; 40 } 41 42 int main(int argc, char *argv[]) 43 { 44 if (argc != 3) { 45 cerr<< "Please give the input file name and output file name."<<endl; 46 return -1; 47 } 48 ifstream in(argv[1]); 49 ofstream out(argv[2]); 50 if (!in) { 51 cerr<<"Can't open the file."<<endl; 52 return -1; 53 } 54 if (!out) { 55 cerr<<"can't open output file."<<endl; 56 return -1; 57 } 58 Sales_data total; 59 if (read(in, total)) { 60 Sales_data trans; 61 while (read(in, trans)) { 62 if (total.isbn() == trans.isbn()) total = add(total, trans); 63 else { 64 print(out, total) << endl; //输出到指定文件中 65 total = trans; 66 } 67 } 68 print(out, total) << endl; 69 } 70 else { 71 cerr << "No data?!" << endl; 72 return -1; 73 } 74 return 0; 75 }
将上一题输出的内容输出到文件book1.txt中：
8.8
1 #include <iostream> 2 #include <fstream> 3 #include <vector> 4 #include <string> 5 6 using namespace std; 7 8 struct Sales_data { 9 string bookNo; //书的ISBN 10 unsigned units_sold = 0; //售出的本数 11 double revenue = 0.0; //销售额 12 Sales_data& combine(const Sales_data &); 13 string isbn() { return bookNo; } 14 }; 15 16 Sales_data &Sales_data::combine(const Sales_data &rhs) 17 { 18 units_sold += rhs.units_sold; 19 revenue += rhs.revenue; 20 return *this; 21 } 22 23 istream &read(istream &is, Sales_data &item) 24 { 25 is >> item.bookNo >> item.units_sold >> item.revenue; 26 return is; 27 } 28 29 ostream &print(ostream &os, const Sales_data &item) 30 { 31 os << item.bookNo << " " << item.units_sold << " " << item.revenue; 32 return os; 33 } 34 35 Sales_data add(const Sales_data &lhs, const Sales_data &rhs) 36 { 37 Sales_data sum = lhs; 38 sum.combine(rhs); 39 return sum; 40 } 41 42 int main(int argc, char *argv[]) 43 { 44 if (argc != 3) { 45 cerr<< "Please give the input file name and output file name."<<endl; 46 return -1; 47 } 48 ifstream in(argv[1]); 49 ofstream out(argv[2], ofstream::app); //唯一改变之处 50 if (!in) { 51 cerr<<"Can't open the file."<<endl; 52 return -1; 53 } 54 if (!out) { 55 cerr<<"can't open output file."<<endl; 56 return -1; 57 } 58 Sales_data total; 59 if (read(in, total)) { 60 Sales_data trans; 61 while (read(in, trans)) { 62 if (total.isbn() == trans.isbn()) total = add(total, trans); 63 else { 64 print(out, total) << endl; 65 total = trans; 66 } 67 } 68 print(out, total) << endl; 69 } 70 else { 71 cerr << "No data?!" << endl; 72 return -1; 73 } 74 return 0; 75 }
8.9
1 #include <iostream> 2 #include <vector> 3 #include <string> 4 #include <sstream> 5 6 using namespace std; 7 8 int main() 9 { 10 string line, word; 11 while(getline(cin, line)) { 12 istringstream in(line); 13 while (in >> word) 14 cout << word << endl; 15 } 16 return 0; 17 }
8.10
1 #include <iostream> 2 #include <vector> 3 #include <string> 4 #include <fstream> 5 #include <sstream> 6 7 using namespace std; 8 9 int main() 10 { 11 vector<string> vec; 12 string line, word; 13 ifstream input("data.txt"); 14 while(getline(input, line)) { 15 vec.push_back(line); 16 } 17 for (auto i : vec) { 18 istringstream in(i); 19 while (in >> word) { 20 cout << word << endl; 21 } 22 } 23 return 0; 24 }
8.11
1 #include <iostream> 2 #include <vector> 3 #include <string> 4 #include <fstream> 5 #include <sstream> 6 7 using namespace std; 8 9 struct PersonInfo { 10 string name; 11 vector<string> phones; 12 }; 13 14 int main() 15 { 16 string line, word; 17 vector<PersonInfo> people; 18 istringstream record; //定义在循环之外 19 while (getline(cin,line)) { 20 PersonInfo info; 21 record.clear(); //每次要解除上次绑定的string对象 22 record.str(line); 23 record >> info.name; 24 while (record >> word) 25 info.phones.push_back(word); 26 people.push_back(info); 27 } 28 return 0; 29 }
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
1 #include <iostream> 2 #include <vector> 3 #include <string> 4 #include <fstream> 5 #include <sstream> 6 7 using namespace std; 8 9 struct PersonInfo { 10 string name; 11 vector<string> phones; 12 }; 13 14 int main() 15 { 16 ifstream in("data.txt"); //从文件读取 17 string line, word; 18 vector<PersonInfo> people; 19 istringstream record; 20 while (getline(in,line)) { //in为文件输入流 21 PersonInfo info; 22 record.clear(); 23 record.str(line); 24 record >> info.name; 25 while (record >> word) 26 info.phones.push_back(word); 27 people.push_back(info); 28 } 29 return 0; 30 }
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。
