Question

So im writing c++ program, that takes a integer from input file, multiply it with 2 and outputs it on output file. So the code is –

#include <stdio.h>
#include <iostream>
using namespace std;

int main() {
    int n;
    FILE * inFile;
    FILE * outFile;
    inFile = fopen ("reiz.in","r");
    outFile = fopen ("reiz.out","r+");
    fscanf (inFile, "%s", n);
    int m = n * 2;
    fprintf (outFile, "%n", n);
    fclose (inFile);
    fclose (outFile);
    return 0;
}

But something is wrong. in reiz.in file there is number 2, after running program it should output 4 in reiz.out, but it just shows don’t send error. What exactly is wrong with my script?
Best regards,
Y2oK

EDIT 1: Ok now it looks like this –

#include <stdio.h>
#include <iostream>
using namespace std;

int main() {
    int n;
    FILE * inFile;
    FILE * outFile;
    inFile = fopen ("reiz.in","r");
    outFile = fopen ("reiz.out","r+");
    fscanf (inFile, "%d", &n);
    int m = n * 2;
    fprintf (outFile, "%d", m);
    fclose (inFile);
    fclose (outFile);
    return 0;
}

but still it gives same don’t send error when running reiz.exe file, and it doesn’t write anything on output file
I’m now a little bit confused, and don’t know who to chose as best answer, so I will chose the one who got most “+1”. But thanks to all!

Answer 1

This a C-program (apart from the using namespace std;). In C++ you should use streams and formatted I/O, like this:

#include <fstream>

int main() {
    std::ifstream input_file("reiz.in");
    int n;
    input_file >> n; // read one integer
    std::ostream output_file("reis.out");
    output_file << n * 2 << std::endl; // calculate n * 2 and write the result
                                       // to a file. std::endl adds a newline and
                                       // flushes the buffer
    return 0;
}