sairate
c34823a3bd
- 新增现代 C++ 教程的 Preface 章节，包括英文和中文版本 - 添加 C++ Primer 练习代码 - 新增 Learn C++ 教程的 C++ 开发简介章节 - 添加头文件解析文档 - 更新 mkdocs.yml，包含新教程的目录结构 - 修改项目设置，使用 Python 3.10环境
418 lines
8.6 KiB
Markdown
418 lines
8.6 KiB
Markdown
---
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sort: 8
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---
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# IO库
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## 练习8.1
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> 编写函数，接受一个`istream&`参数，返回值类型也是`istream&`。此函数须从给定流中读取数据，直至遇到文件结束标识时停止。它将读取的数据打印在标准输出上。完成这些操作后，在返回流之前，对流进行复位，使其处于有效状态。
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解：
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```cpp
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std::istream& func(std::istream &is)
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{
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std::string buf;
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while (is >> buf)
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std::cout << buf << std::endl;
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is.clear();
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return is;
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}
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```
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## 练习8.2
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> 测试函数，调用参数为`cin`。
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解：
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```cpp
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#include <iostream>
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using std::istream;
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istream& func(istream &is)
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{
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std::string buf;
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while (is >> buf)
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std::cout << buf << std::endl;
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is.clear();
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return is;
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}
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int main()
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{
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istream& is = func(std::cin);
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std::cout << is.rdstate() << std::endl;
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return 0;
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}
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```
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## 练习8.3
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> 什么情况下，下面的`while`循环会终止？
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```cpp
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while (cin >> i) /* ... */
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```
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解：
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遇到文件结束符、或者IO流错误，或读入了无效数据。
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## 练习8.4
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> 编写函数，以读模式打开一个文件，将其内容读入到一个`string`的`vector`中，将每一行作为一个独立的元素存于`vector`中。
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解：
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```cpp
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void ReadFileToVec(const string& fileName, vector<string>& vec)
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{
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ifstream ifs(fileName);
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if (ifs)
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{
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string buf;
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while (getline(ifs, buf))
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vec.push_back(buf);
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}
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}
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```
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## 练习8.5
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> 重写上面的程序，将每个单词作为一个独立的元素进行存储。
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解：
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```cpp
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void ReadFileToVec(const string& fileName, vector<string>& vec)
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{
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ifstream ifs(fileName);
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if (ifs)
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{
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string buf;
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while (ifs >> buf)
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vec.push_back(buf);
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}
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}
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```
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## 练习8.6
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> 重写7.1.1节的书店程序，从一个文件中读取交易记录。将文件名作为一个参数传递给`main`。
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解：
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```cpp
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#include <fstream>
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#include <iostream>
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#include "../ch07/ex7_26.h"
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using std::ifstream; using std::cout; using std::endl; using std::cerr;
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int main(int argc, char **argv)
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{
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ifstream input(argv[1]);
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Sales_data total;
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if (read(input, total))
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{
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Sales_data trans;
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while (read(input, trans))
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{
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if (total.isbn() == trans.isbn())
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total.combine(trans);
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else
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{
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print(cout, total) << endl;
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total = trans;
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}
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}
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print(cout, total) << endl;
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}
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else
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{
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cerr << "No data?!" << endl;
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}
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return 0;
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}
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```
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## 练习8.7
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> 修改上一节的书店程序，将结果保存到一个文件中。将输出文件名作为第二个参数传递给`main`函数。
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解：
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```cpp
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#include <fstream>
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#include <iostream>
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#include "../ch07/ex7_26.h"
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using std::ifstream; using std::ofstream; using std::endl; using std::cerr;
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int main(int argc, char **argv)
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{
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ifstream input(argv[1]);
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ofstream output(argv[2]);
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Sales_data total;
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if (read(input, total))
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{
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Sales_data trans;
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while (read(input, trans))
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{
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if (total.isbn() == trans.isbn())
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total.combine(trans);
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else
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{
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print(output, total) << endl;
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total = trans;
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}
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}
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print(output, total) << endl;
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}
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else
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{
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cerr << "No data?!" << endl;
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}
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return 0;
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}
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```
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## 练习8.8
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> 修改上一题的程序，将结果追加到给定的文件末尾。对同一个输出文件，运行程序至少两次，检验数据是否得以保留。
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解：
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```cpp
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#include <fstream>
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#include <iostream>
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#include "../ch07/ex7_26.h"
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using std::ifstream; using std::ofstream; using std::endl; using std::cerr;
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int main(int argc, char **argv)
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{
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ifstream input(argv[1]);
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ofstream output(argv[2], ofstream::app);
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Sales_data total;
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if (read(input, total))
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{
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Sales_data trans;
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while (read(input, trans))
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{
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if (total.isbn() == trans.isbn())
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total.combine(trans);
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else
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{
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print(output, total) << endl;
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total = trans;
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}
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}
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print(output, total) << endl;
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}
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else
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{
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cerr << "No data?!" << endl;
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}
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return 0;
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}
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```
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## 练习8.9
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> 使用你为8.1.2节第一个练习所编写的函数打印一个`istringstream`对象的内容。
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解：
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```cpp
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#include <iostream>
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#include <sstream>
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using std::istream;
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istream& func(istream &is)
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{
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std::string buf;
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while (is >> buf)
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std::cout << buf << std::endl;
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is.clear();
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return is;
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}
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int main()
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{
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std::istringstream iss("hello");
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func(iss);
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return 0;
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}
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```
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## 练习8.10
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> 编写程序，将来自一个文件中的行保存在一个`vector`中。然后使用一个`istringstream`从`vector`读取数据元素，每次读取一个单词。
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解：
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```cpp
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#include <iostream>
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#include <fstream>
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#include <sstream>
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#include <vector>
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#include <string>
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using std::vector; using std::string; using std::ifstream; using std::istringstream; using std::cout; using std::endl; using std::cerr;
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int main()
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{
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ifstream ifs("../data/book.txt");
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if (!ifs)
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{
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cerr << "No data?" << endl;
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return -1;
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}
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vector<string> vecLine;
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string line;
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while (getline(ifs, line))
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vecLine.push_back(line);
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for (auto &s : vecLine)
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{
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istringstream iss(s);
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string word;
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while (iss >> word)
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cout << word << endl;
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}
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return 0;
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}
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```
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## 练习8.11
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> 本节的程序在外层`while`循环中定义了`istringstream`对象。如果`record`对象定义在循环之外，你需要对程序进行怎样的修改？重写程序，将`record`的定义移到`while`循环之外，验证你设想的修改方法是否正确。
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解：
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```cpp
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#include <iostream>
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#include <sstream>
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#include <string>
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#include <vector>
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using std::vector; using std::string; using std::cin; using std::istringstream;
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struct PersonInfo {
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string name;
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vector<string> phones;
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};
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int main()
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{
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string line, word;
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vector<PersonInfo> people;
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istringstream record;
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while (getline(cin, line))
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{
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PersonInfo info;
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record.clear();
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record.str(line);
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record >> info.name;
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while (record >> word)
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info.phones.push_back(word);
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people.push_back(info);
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}
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for (auto &p : people)
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{
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std::cout << p.name << " ";
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for (auto &s : p.phones)
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std::cout << s << " ";
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std::cout << std::endl;
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}
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return 0;
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}
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```
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## 练习8.12
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> 我们为什么没有在`PersonInfo`中使用类内初始化？
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解：
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因为这里只需要聚合类就够了，所以没有必要在`PersionInfo`中使用类内初始化。
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## 练习8.13
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> 重写本节的电话号码程序，从一个命名文件而非`cin`读取数据。
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解：
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```cpp
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#include <iostream>
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#include <sstream>
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#include <fstream>
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#include <string>
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#include <vector>
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using std::vector; using std::string; using std::cin; using std::istringstream;
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using std::ostringstream; using std::ifstream; using std::cerr; using std::cout; using std::endl;
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using std::isdigit;
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struct PersonInfo {
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string name;
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vector<string> phones;
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};
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bool valid(const string& str)
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{
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return isdigit(str[0]);
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}
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string format(const string& str)
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{
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return str.substr(0,3) + "-" + str.substr(3,3) + "-" + str.substr(6);
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}
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int main()
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{
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ifstream ifs("../data/phonenumbers.txt");
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if (!ifs)
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{
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cerr << "no phone numbers?" << endl;
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return -1;
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}
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string line, word;
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vector<PersonInfo> people;
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istringstream record;
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while (getline(ifs, line))
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{
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PersonInfo info;
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record.clear();
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record.str(line);
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record >> info.name;
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while (record >> word)
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info.phones.push_back(word);
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people.push_back(info);
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}
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for (const auto &entry : people)
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{
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ostringstream formatted, badNums;
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for (const auto &nums : entry.phones)
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if (!valid(nums)) badNums << " " << nums;
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else formatted << " " << format(nums);
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if (badNums.str().empty())
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cout << entry.name << " " << formatted.str() << endl;
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else
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cerr << "input error: " << entry.name
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<< " invalid number(s) " << badNums.str() << endl;
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}
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return 0;
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}
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```
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## 练习8.14
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> 我们为什么将`entry`和`nums`定义为`const auto&`？
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解：
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它们都是类类型，因此使用引用避免拷贝。
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在循环当中不会改变它们的值，因此用`const`。 |