Lecture 25

File

Reading

p500-506, 506-516

Appending to a file

File: lecture.txt 
Lecture 1: Introduction to C++
Lecture 2: Flow of Control
Lecture 3: Functions

Program: write to the file. 
#include<iostream>
#include<fstream> 
using namespace std;

int main() {
    ofstream fout; 
    fout.open("lecture.txt");
    fout << "Lecture 4: Pointer" << endl
         << "Lecture 5: File I/O" << endl;
    fout.close();

    return 0;
}

File lecture.txt after you execute the program. The original data are gone. 
Lecture 4: Pointer
Lecture 5: File I/O

Here is the new program. The new data are appended to the original file. 
#include<iostream>
#include<fstream> 
using namespace std;

int main() {
    ofstream fout; 
    fout.open("lecture.txt", ios::app);
    fout << "Lecture 4: Pointer" << endl
         << "Lecture 5: File I/O" << endl;
    fout.close();

    return 0;
}

File lecture.txt after you execute the program. The new data are appended to the original file. 
Lecture 1: Introduction to C++
Lecture 2: Flow of Control
Lecture 3: Functions
Lecture 4: Pointer
Lecture 5: File I/O

Check that a file was open successfully

#include<iostream>
#include<fstream>
#include<cstdlib> // for exit 
using namespace std;

int main() {
    ifstream fin;
    ofstream fout; 

    fin.open("input.txt");
    if(fin.fail()) { // fail to open the file
        cout << "Input file opening failed.\n";
        exit(1); // exit the program
    }

    fout.open("output.txt");
    if(fout.fail()) { // fail to open the file
        cout << "Output file opening failed.\n";
        exit(1); // exit program
    }

    int first, second, third;
    fin >> first >> second >> third;
    fout << "The sum of the first 3 numbers are "
         << (first + second + third) << endl;

    fin.close();
    fout.close();

    return 0;      
}

Suppose there is no input.txt. The program can't open the file. Here is the output 
Input file opening failed.

After you create the file input.txt 
25 35 45 55 65

After you run the program, output.txt will be created 
The sum of the first 3 numbers are 105

Checking for the end of file

#include<iostream>
#include<fstream>
#include<cstdlib> // for exit 
using namespace std;


int main() {
    ifstream fin;

    fin.open("score.txt");
    if(fin.fail()) { // fail to open the file
        cout << "Input file opening failed.\n";
        exit(1); // exit the program
    }

    int num=0; // number of scores 
    double sum=0, score=0;

    while(!fin.eof()) { // if it is not the end of the file
        fin >> score;
        sum+=score;
        num++;
    }

    cout.setf(ios::fixed);
    cout.setf(ios::showpoint);
    cout.precision(2);

    cout << "There are " << num << " scores.\n";
    cout << "The average is " << sum/num << ".\n";

    fin.close();

    return 0;      
}


File : score.txt 
11.5 33.6 99 93.2 42.5 64.1 88.35 99.25 33.54 
52 53.3 99 88 88.8 74.25

Here is the output on the screen 
There are 15 scores.
The average is 68.03.

Formatting output

You can rewrite the above program: 
#include<iostream>
#include<fstream>
#include<cstdlib> // for exit 
using namespace std;

int main() {
    ifstream fin;
    ofstream fout;
    fout.open("output.txt");

    fin.open("score.txt");
    if(fin.fail()) { // fail to open the file
        cout << "Input file opening failed.\n";
        exit(1); // exit the program
    }

    int num=0; // number of scores 
    double sum=0, score=0;

    while(!fin.eof()) { // if it is not the end of the file
        fin >> score;
        sum+=score;
        num++;
    }

    fout.setf(ios::fixed);
    fout.setf(ios::showpoint);
    fout.precision(2);

    fout << "There are " << num << " scores.\n";
    fout << "The average is " << sum/num << ".\n";

    fin.close();
    fout.close();

    return 0;      
}