How to replace a number in a file with its sum?-CodePudding

I'd like to write a program that gets an integer in a file, sums it with a input number and replace the previous integer in the file with the result of the sum. I thought the following code would work, but there's a 0 written in the file that remains 0, no matter the integer I input. What am I doing wrong?

#include <iostream>
#include <fstream>
using namespace std;

int main() {
    fstream arq;
    arq.open("file.txt");
    int points, total_points;
    cin >> points;

    arq >> total_points;
    total_points  = points;
    arq << total_points; 
        
}

CodePudding user response：

You can try reading and writing the input file separately as shown below:

#include <iostream>
#include <fstream>
using namespace std;

int main() {
    ifstream arq("file.txt");
    int points=0, total_points=0;
    cin >> points;

    arq >> total_points;
   
    total_points  = points;
    arq.close();
    ofstream output("file.txt");
    
    output  << total_points;
    output.close();
    
        
}

The output of the above program can be seen here.

CodePudding user response：

Since you've opened the file for both reading and writing you need to set the output position indicator to position 0 before you write the new value to the file.

Example:

#include <cerrno>
#include <fstream>
#include <iostream>

int main() {
    const char* filename = "file.txt";
    if(std::fstream arq(filename); arq) {
        if(int total_points; arq >> total_points) {
            if(int points; std::cin >> points) {
                total_points  = points;
                // rewind the output position pointer to the start of the file
                arq.seekp(0);
                arq << total_points;
            }
        }
    } else {
        std::perror(filename);
    }
}

Note though that if you want to write a shorter value to the file, you may end up with a file looking like this:

Before:

After writing 45 to the file:

So, I recommend the approach in Anoop Rana's answer which truncates the file before writing to it.