How to read data from files with C++
- Include the fstream header file with using std::ifstream;
- Declare a variable of type ifstream
- Open the file
- Check for an open file error
- Read from the file
- After each read, check for end-of-file using the eof() member function.
Example:
#include <iostream>
using std::cerr;
using std::cout;
using std::endl;
#include <fstream>
using std::ifstream;
#include <cstdlib> // for exit function
// This program reads values from the file 'example.dat'
// and echoes them to the display until a negative value
// is read.
int main()
{
ifstream indata; // indata is like cin
int num; // variable for input value
indata.open("example.dat"); // opens the file
if(!indata) { // file couldn't be opened
cerr << "Error: file could not be opened" << endl;
exit(1);
}
indata >> num;
while ( !indata.eof() ) { // keep reading until end-of-file
cout << "The next number is " << num << endl;
indata >> num; // sets EOF flag if no value found
}
indata.close();
cout << "End-of-file reached.." << endl;
return 0;
}
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