Week 10

File streams

Exercise 10.1

Write a program that reads from a file named input.txt containing numbers separated by newlines and writes the sum of the numbers at the end of the file. You may assume that each line of the file consists of exactly one number.

For example, if input.txt contains

1.1
2.3
3.1
0.2

then after running the program, input.txt should contain

1.1
2.3
3.1
0.2
6.7

Solution

Here is one possible solution.

#include <fstream>

using namespace std;

int main() {
    ifstream input("input.txt");

    double sum = 0;
    double number;

    while (input >> number) {
        sum += number;
    }

    ofstream output("input.txt", ofstream::app);

    output << sum << endl;

    return 0;
}

Remark: The opening mode flag ofstream::app could be written equivalently as ios_base::app, ios::app, or ostream::app.

Alternative solution

Here is another possible solution, using a single fstream instead of an ifstream and an ofstream.

#include <fstream>

using namespace std;

int main() {
    fstream file("input.txt");

    double sum = 0;
    double number;

    while (file >> number) {
        sum += number;
    }

    file.clear();
    file << sum << endl;

    return 0;
}

Note that clear is used to reset the stream’s error state flags so that the file can be written to after its end has been encountered.