11. File Input/Output (Header <fstream>)

admin · 發表於 2016-1-1 20:29:26

11. File Input/Output (Header <fstream>)
The <fstream> header provides ifstream (input file stream) and ofstream (output file stream) for file input and output.
The steps for file input/output are:

Create a ifstream for input, or ofstream for output.
Connect the stream to an input or output file via open(filename).
Perform formatted output via stream insertion operator <<, or input via stream extraction operator >>, similar to cout << and cin >>.
Close the file and free the stream.

11.1 Example: File IO

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/* Test File I/O (TestFileIO.cpp)
Read all the integers from an input file and
write the average to an output file       */
#include <iostream>
#include <fstream> // file stream
#include <cstdlib>
using namespace std;

int main() {
ifstream fin; // Input stream
ofstream fout;  // Output stream

// Try opening the input file
fin.open("in.txt");
if (!fin.is_open()) {
   cerr << "error: open input file failed" << endl;
   abort();  // Abnormally terminate the program (in <cstdlib>)
}

int sum = 0, number, count = 0;
while (!(fin.eof())) {
   // Use >> to read
   fin >> number;
   sum += number;
   ++count;
}
double average = double(sum) / count;
cout << "Count = " << count << " average = " << average << endl;
fin.close();

// Try opening the output file
fout.open("out.txt");
if (!fout.is_open()) {
   cerr << "error: open output file failed" << endl;
   abort();
}
// Write the average to the output file using <<
fout << average;
fout.close();
return 0;
}

Input File: in.txt
12 15 35 26 68
Output File: out.txt
31.20
Program Notes:

Once the file is opened, you can use >> and << for input and output, similar to cin >> and cout <<. (Advanced note: ifstream is a subclass of istream, where cin belongs. ofstream is a subclass of ostream, where cout belongs.)
Similarly, IO manipulators, such as fixed, setprecision() and setw(), work on the file streams.

11.2 Exercises
[TODO]

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