C++ input and output files

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;
}

Answer 2

Pass the address of n to fscanf() (and it's an integer not a string):

fscanf (inFile, "%d", &n);

All that stuff is a bit long in the tooth, unless you are actually writing 'C'. Look at the iostream family of classes if you are writing 'C++'

Answer 3

There are multiple issues with your code:

#include <stdio.h>

This is a C include, not a C++ include. Instead, you should use:

#include <cstdio>

---

fscanf (inFile, "%s", n);

The format %s is for strings, of type char*. You want to read a decimal number, so use %d instead. Furthermore, you have to pass the address of the destination:

fscanf (inFile, "%d", &n);

---

fprintf (outFile, "%n", n);

The format %n is used to query the number of bytes written so far, and requires a pointer. Since you want to write a decimal number, use %d again:

fprintf (outFile, "%d", n);

Answer 4

Go use iostream (Bjorns answer)

But I think

outFile = fopen ("reiz.out","r+");

should be

outFile = fopen ("reiz.out","w+");

Also

fscanf (inFile, "%d", n);

should be

fscanf (inFile, "%d", &n);

For ease of reference, a fully working (C) program:

#include <stdio.h>

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

Answer 5

There are two problems with fscanf. First, your format string is wrong -- you're taking a string, where you should be taking a (signed?) decimal. Second, you are passing n by-value, but you should be passing a pointer to n instead:

fscanf (inFile, "%d", &n);

You are also #includeing the now-deprecated <stdio.h> file. You should:

#include <cstdio>

Finally, you claim to be writing C++ code, but the code you actually wrote is decidedly not C++-ish, which is to say it is type-unsafe. I'd recommend you either go C++, or don't -- don't pussyfoot around.

Here is a more C++ way of doing what you're trying to do (you should add error-checking and writing to the output file):

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

int main() {
    ifstream in("reiz.in", std::ios::in);
    int n = 0;
    in >> n;
    return 0;
}

Answer 6

There are several issues which will prevent your code from working properly.

1. There is no need to open outfile as r+. You can just open it with mode w.

   outFile = fopen ("reiz.out","w");
   

2. When you read from the file with fscanf, you are passing the integer n instead of the pointer to the integer n. fscanf is expecting to receive the location to write the integer it read. You are passing the integer itself (which will, in the best case, crash your program. Worst case, weird stuff will start happening).

   fscanf (inFile, "%s", &n);
   

3. You are using %n when writing to outfile. This does not print the integer n but rather it reads the number of bytes written so far. You want to use %d instead.

   fprintf (outFile, "%d", m);
   

Also, as a side-note, you seem to be using C instead of C++. In C++, you would usually use file stream objects, like ifstream and ofstream, to read from and write to files. You are using things like fopen which are decidedly more C-like.

This code should work better:

#include <stdio.h>

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