C++中文件的读取操作，如何读取多行数据，如何一个一个的读取数据

练习8.1：编写函数。接受一个istream&参数，返回值类型也是istream&。此函数必须从给定流中读取数据，直至遇到文件结束标识时停止。

#include <iostream>
#include <stdexcept>
using std::istream;
using std::cin;
using std::cout;
using std::cerr;
using std::endl;
using std::runtime_error;
istream &f(istream &in)
{
int v;
while(in >> v, !in.eof())
{
if(in.bad())
throw runtime_error("IO Stream error.");
if(in.fail())
{
cerr<<"Data error! Please try again."<<endl;
in.clear();
in.ignore(100, '\n');
continue;
}
cout<< v <<endl;
}
in.clear();
return in;
}
int main()
{
cout<<"Please input some numbers, enter Ctrl+Z to end"<<endl;
f(cin);
return 0;
}

练习8.3：什么情况下，下面的while循环会终止？

while (cin >> i) /*. . .*/

遇到文件结束符，或者遇到了IO流错误或者读入了无效数据。

练习8.4：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。

    #include <iostream>

    #include <string>

    #include <fstream>

    #include <vector>

    using std::cout;

    using std::endl;

    using std::string;

    using std::ifstream;

    using std::cerr;

    using std::vector;  

    int main()

    {

        ifstream in("data.txt");

        if(!in) {

            cerr<<"Can't open the file."<<endl;

            return -1;

        }  

        string line;

        vector<string> words;

        while(getline(in, line))

            words.push_back(line);  

        in.close();  

        vector<string>::const_iterator it = words.cbegin();

        while (it != words.cend())

        {

            cout<< *it <<endl;

            ++it;

        }

        return 0;

    }

练习8.5：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。

    #include <iostream>

    #include <string>

    #include <fstream>

    #include <vector>

    using std::cout;

    using std::endl;

    using std::string;

    using std::ifstream;

    using std::cerr;

    using std::vector;  

    int main()

    {

        ifstream in("data.txt");

        if(!in) {

            cerr<<"Can't open the file."<<endl;

            return -1;

        }  

        string line;

        vector<string> words;

        while(in >> line)

            words.push_back(line);  

        in.close();  

        vector<string>::const_iterator it = words.cbegin();

        while (it != words.cend())

        {

            cout<< *it <<endl;

            ++it;

        }

        return 0;

    }

练习8.6：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。

#include <iostream>
#include <fstream>
#include "Sales_data.h"
using std::cout;
using std::cerr;
using std::ifstream;
using std::endl;
int main(int argc, char *argv[])
{
if (argc != 2) {
cerr<< "Please give the file name."<<endl;
return -1;
}
ifstream in(argv[1]);
if (!in) {
cerr<<"Can't open the file."<<endl;
return -1;
}
Sales_data total;
if (read(in, total)) {
Sales_data trans;
while(read(in ,trans)) {
if(total.isbn() == trans.isbn())
total.combine(trans);
else {
print(cout, total) << endl;
total =trans;
}
}
print(cout, total)<<endl;
}
else {
cerr<<" No data?!"<<endl;
}
return 0;
}

练习8.7：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。

#include <iostream>
#include <fstream>
#include "Sales_data.h"
using std::cerr;
using std::ifstream;
using std::ofstream;
using std::endl;
int main(int argc, char *argv[])
{
if (argc != 3) {
cerr<< "Please give the input file name and out file name."<<endl;
return -1;
}
ifstream in(argv[1]);
if (!in) {
cerr<<"Can't open the file."<<endl;
return -1;
}
ofstream out(argv[2]);
if (!out) {
cerr<<"can't open output file."<<endl;
return -1;
}
Sales_data total;
if (read(in, total)) {
Sales_data trans;
while(read(in ,trans)) {
if(total.isbn() == trans.isbn())
total.combine(trans);
else {
print(out, total) << endl;
total =trans;
}
}
print(out, total)<<endl;
}
else {
cerr<<" No data?!"<<endl;
}
return 0;
}

练习8.8：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。

#include <iostream>
#include <fstream>
#include "Sales_data.h"
using std::cerr;
using std::ifstream;
using std::ofstream;
using std::endl;
int main(int argc, char *argv[])
{
if (argc != 3) {
cerr<< "Please give the input file name and out file name."<<endl;
return -1;
}
ifstream in(argv[1]);
if (!in) {
cerr<<"Can't open the file."<<endl;
return -1;
}
ofstream out(argv[2], ofstream::app);
if (!out) {
cerr<<"can't open output file."<<endl;
return -1;
}
Sales_data total;
if (read(in, total)) {
Sales_data trans;
while(read(in ,trans)) {
if(total.isbn() == trans.isbn())
total.combine(trans);
else {
print(out, total) << endl;
total =trans;
}
}
print(out, total)<<endl;
}
else {
cerr<<" No data?!"<<endl;
}
return 0;
}

练习8.3.1：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。

#include <iostream>
#include <sstream>
#include <string>
#include <stdexcept>
using std::istream;
using std::ostringstream;
using std::istringstream;
using std::string;
using std::cout;
using std::cerr;
using std::endl;
using std::runtime_error;
istream &f(istream &in)
{
int v;
while(in >> v, !in.eof())
{
if(in.bad())
throw runtime_error("IO Stream error.");
if(in.fail())
{
cerr<<"Data error! Please try again."<<endl;
in.clear();
in.ignore(100, '\n');
continue;
}
cout<< v <<endl;
}
in.clear();
return in;
}
int main()
{
ostringstream msg;
msg<<"C++ Primer 5th edition"<<endl;
istringstream in(msg.str());
f(in);
return 0;
}

练习8.10：编写程序，将来自一个文件中的行保存在一个vector<string>中。然后使用一个istringstream从vector读取数据元素，每次读取一个单词。

#include <iostream>
#include <fstream>
#include <sstream>
#include <string>
#include <vector>
using std::cerr;
using std::endl;
using std::cout;
using std::ifstream;
using std::istringstream;
using std::string;
using std::vector;
int main()
{
ifstream in("Data.txt");
if (!in) {
cerr<<" Can't open input file."<<endl;
return -1;
}
string line;
vector<string> words;
while (getline(in, line)) {
words.push_back(line);
}
in.close();
vector<string>::const_iterator it = words.begin();
while( it != words.end()) {
istringstream line_str(*it);
string word;
while(line_str >> word)
cout<< endl;
++it;
}
return 0;
}

练习8.11：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么样的修改？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。

#include <iostream>
#include <sstream>
#include <string>
#include <vector>
using std::cin;
using std::istringstream;
using std::string;
using std::vector;
struct PersonInfo {
string name;
vector<string> phones;
};
int main()
{
string line, word;
vector<PersonInfo> people;
istringstream record;
while (getline(cin,line)) {
PersonInfo info;
record.clear();
record.str(line);
record >> info.name;
while (record >> word)
info.phones.push_back(word);
people.push_back(info);
}
return 0;
}

练习8.12：我们为什么没有在PersonInfo中使用类内初始化？

由于每个人的电话号数量不固定，因此更好的方式不是通过类内初始化指定人名和所有电话号码，而是在缺省初始化之后，在程序中设置人名并逐个添加电话号码。

练习8.13：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。

#include <iostream>
#include <fstream>
#include <sstream>
#include <string>
#include <vector>
using std::cerr;
using std::endl;
using std::cout;
using std::ifstream;
using std::istringstream;
using std::ostringstream;
using std::string;
using std::vector;
struct PersonInfo {
string name;
vector<string> phones;
};
string format(const string &s) { return s; }
bool valid(const string &s)
{
return true;
}
int main(int argc, char *argv[])
{
string line, word;
vector<PersonInfo> people;
istringstream record;
if (argc != 2) {
cerr<<"Please give the file name."<<endl;
return -1;
}
ifstream in(argv[1]);
if(!in)
{
cerr<<"can't open input file"<<endl;
return -1;
}
while (getline(in, line)) {
PersonInfo info;
record.clear();
record.str(line);
record >> info.name;
while(record >> word)
info.phones.push_back(word);
people.push_back(info);
}
ostringstream os;
for (const auto &entry : people) {
ostringstream formatted, badNums;
for(const auto &nums : entry.phones) {
if (!valid(nums)) {
badNums << " "<< nums;
}
else
formatted << " " <<format(nums);
}
if (badNums.str().empty())
os <<entry.name<<" "<<formatted.str()<<endl;
else
cerr<<"input error: "<<entry.name<<" invalid number(s) "<<badNums.str()<<endl;
}
cout<<os.str()<<endl;
return 0;
}

练习8.14：我们为什么将entry和nums定义为const auto&？

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；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。

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