C++从文件中读取数据

13.1 using cin

1 You must include the iostream header file.
2 The iostream header file defines an istream class for handling input.
3 The iostream header file declares an istream variable, or object, called cin.
4 You must account for the std namespace; for example, you can use the using directive
or the std:: prefix for elements such as cin.
5 You can use cin with the >> operator to read a variety of data types.
6 You can use cin with the get() method to read individual characters and with the
getline() method to read a line of characters at a time.
7 You can use cin with methods such as eof() and fail() to monitor the success of
an input attempt.
8 The object cin itself, when used as a test condition, is converted to the Boolean
value true if the last read attempt succeeded and to false otherwise.

13.2 using ifstream

1 You must include the fstream header file.
2 The fstream header file defines an ifstream class for handling input.
3 You need to declare one or more ifstream variables, or objects, which you can
name as you please, as long as you respect the usual naming conventions.
4 You must account for the std namespace; for example, you can use the using directive
or the std:: prefix for elements such as ifstream.
5 You need to associate a specific ifstream object with a specific file; one way to do
so is to use the open() method.
6 When you’re finished with a file, you should use the close() method to close the file.
7 You can use an ifstream object with the >> operator to read a variety of data types.
8 You can use an ifstream object with the get() method to read individual characters
and with the getline() method to read a line of characters at a time.
9 You can use an ifstream object with methods such as eof() and fail() to monitor
the success of an input attempt.
10 An ifstream object itself, when used as a test condition, is converted to the
Boolean value true if the last read attempt succeeded and to false otherwise.

13.3 实例

code:

// File name:           sumafile
// Last modified Date:  2021年11月21日16点51分
// Last Version:        V1.0
// Descriptions:        从文件中读取数据

// sumafile.cpp -- functions with an array argument
#include 
#include  // file I/O support
#include  // support for exit()
const int SIZE = 60;
int main()
{
	using namespace std;
	char filename[SIZE];
	ifstream inFile; // object for handling file input
	cout << "Enter name of data file: ";
	cin.getline(filename, SIZE);
	inFile.open(filename); // associate inFile with a file

	if (!inFile.is_open()) // failed to open file
	{
		cout << "Could not open the file " << filename << endl;
		cout << "Program terminating.n";
		exit(EXIT_FAILURE);
	}
	double value;
	double sum = 0.0;
	int count = 0; // number of items read
	inFile >> value; // get first value
	while (inFile.good()) // while input good and not at EOF  在倒数第二个数据读取到时就会为假，所以最后一个数据没有读取到
	{
		++count; // one more item read
		sum += value; // calculate running total
		inFile >> value; // get next value
	}
	//因此在这加两句读取最后一个数据
	++count; // one more item read
	sum += value; // calculate running total

	if (inFile.eof())
		cout << "End of file reached.n";
	else if (inFile.fail())
		cout << "Input terminated by data mismatch.n";
	else
		cout << "Input terminated for unknown reason.n";
	if (count == 0)
		cout << "No data processed.n";
	else
	{
		cout << "Items read: " << count << endl;
		cout << "Sum: " << sum << endl;
		cout << "Average: " << sum / count << endl;
	}
	inFile.close(); // finished with the file
	return 0;
}

运行结果：

Enter name of data file: scores.txt
End of file reached.
Items read: 12
Sum: 204.5
Average: 17.0417

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文件内容为：

18 19 18.5 13.5 14 16 19.5 20 18 12 18.5
17.5