C++ ofstream cannot write to file....

Question

Hey I am trying to write some numbers to a file, but when I open the file it is empty. Can you help me out here? Thanks.

/** main function **/
int main(){

    /** variables **/
    RandGen* random_generator = new RandGen;
    int random_numbers; 
    string file_name;   

    /** ask user for quantity of random number to produce **/
    cout << "How many random number would you like to create?" << endl;
    cin >> random_numbers;

    /** ask user for the name of the file to store the numbers **/
    cout << "Enter name of file to store random number" << endl;
    cin >> file_name;

    /** now create array to store the number **/
    int random_array [random_numbers];

    /** file the array with random integers **/
    for(int i=0; i<random_numbers; i++){
        random_array[i] = random_generator -> randInt(-20, 20);
        cout << random_array[i] << endl;
    }

    /** open file and write contents of random array **/
    const char* file = file_name.c_str();
    ofstream File(file);

    /** write contents to the file **/
    for(int i=0; i<random_numbers; i++){
        File << random_array[i] << endl;
    }

    /** close the file **/
    File.close();   

    return 0;
    /** END OF PROGRAM **/
}

Answer 1

You can't declare an array of integers with a size known only at run-time on the stack. You can declare such an array on the heap however:

int *random_array = new int[random_numbers];

Don't forget to add delete [] random_array; at the end of main() (and delete random_generator; too) to deallocate the memory that you allocated using new. This memory is automatically freed when your program exits, but it's a good idea to release it anyway (if your program ever grows, it's easy to forget to add it in later).

Apart from that, your code looks fine.

Answer 2

If I just fill in your RandGen class to call rand, the program works fine on Mac OS X 10.6.

How many random number would you like to create?
10
Enter name of file to store random number
nums
55
25
44
56
56
53
20
29
54
57
Shadow:code dkrauss$ cat nums
55
25
44
56
56
53
20
29
54
57

Moreover, I see no reason for it not to work on GCC. What version and platform are you running on? Can you provide complete source?

Answer 3

There's no need to loop twice or keep an array or vector.

const char* file = file_name.c_str();
ofstream File(file);

for(int i=0; i<random_numbers; i++){
    int this_random_int = random_generator -> randInt(-20, 20);
    cout << this_random_int << endl;
    File << this_random_int << endl;
}

File.close();