Question

I’ve got homework, to take phone numbers from a text file, and put it into an array…
here is what I wrote, though it puts nothing on it…
the loop is that as long as theres text in the .txt files, read it into the array…

void get_phones(int *phones)
{
    FILE *fp;
    fp = fopen("phones.txt", "rt");
    if (fp == NULL)
    {
        printf ("Error\n");
    }
    else
    {
        while (fscanf(fp, "%d\n", &phones) > 0)
        {
            fscanf(fp, "%d\n", &phones);
        }
    } 
}

Answer 1

Lastest update:

For testing:
please put the following code in your main(),and change the number 10 to the # of lines you have in the file. To compute the upper bound automatically you can try to change the return type of get_phones to int and put a counter in the while-loop.

int i=0;
for(i=0;i<10;i++){
   printf("%lld\n",*(phones+i));
}

You need deference a pointer in order to print the actual value. Otherwise you are printing the address stored in the pointer. And phones, besides being the name of an array, it is a pointer pointing to the first element in the array in its nature. For more info on pointer and array, see this tutorial.

Assume that you are using a 32-bit machine
The phone numbers I put in my textfile is the following:
22121222345
678139199
111111111

Note that 22,121,222,345>=2,147,483,647, which is the maximum number an int can represent. (For unsigned int it is 2^32-1). Now if we try to run the following code:

void get_phones(int *phones){
    FILE *fp;
    fp = fopen("phones.txt", "rt");

    if (fp == NULL)
        printf ("Error\n");
    else
        while (fscanf(fp, "%d\n", phones++) !=EOF){}

}

The list of number printed on terminal are:
646385865
678139199
111111111

The first number saved in the array is totally random! Why? because it is too large for an integer and it overflows.

Now if we try the following version:

void get_phones(long long* phones){
    FILE *fp;
    fp = fopen("phones.txt", "rt");
        if (fp == NULL)
            printf ("Error\n");
        else
            while (fscanf(fp, "%lld\n", phones++) !=EOF){}

}

The list of number printed on terminal are:
22121222345
678139199
111111111

Why does it work? Because the type long long can store number up to 2^63-1 = 9,223,372,036,854,775,807

Tested working version. Change the data type to long long if you want.

!! The problem with your code is that you are passing the wrong thing to the fscanf. phones is already a pointer — it is pointed to the starting point of the array. when you write phone++, it passes the current pointer to fscanf, and increase the pointer by one, which moves the pointer to the next slot in the integer array.

Also, although fscanf will return the number of items it scanned, in your while loop condition, the scanning is already performed. So you don’t need to call it second time in the body of the while loop

5195551234 this number is still too large for unsigned long, unfortunately. Because for 32-bit machine, both int and unsigned long has the max value $2^{32}-1$. Check out wikipedia if you need more information on this. Instead you need long long if you are running your code on 32-bit machine, otherwise it will overflow and store incorrect data.