Am I incorrectly using atoi?

Question

I was having some trouble with my parsing function so I put some cout statements to tell me the value of certain variables during runtime, and I believe that atoi is incorrectly converting characters.

heres a short snippet of my code thats acting strangely:

c = data_file.get();
if (data_index == 50)
    cout << "50 digit 0 = '" << c << "' number = " << atoi(&c) << endl;

the output for this statement is: 50 digit 0 = '5' number = 52

I'm calling this code within a loop, and whats strange is that it correctly converts the first 47 characters, then on the 48th character it adds a 0 after the integer, on the 49th character it adds a 1, on the 50th (Seen here) it adds a two, all the way up to the 57th character where it adds a 9, then it continues to convert correctly all the way down to the 239th character.

Is this strange or what?

Just to clarify a little more i'll post the whole function. This function gets passed a pointer to an empty double array (ping_data):

int parse_ping_data(double* ping_data)
{
    ifstream data_file(DATA_FILE);

    int pulled_digits [4];
    int add_data;
    int loop_count;
    int data_index = 0;

    for (char c = data_file.get(); !data_file.eof(); c = data_file.get())
    {
        if (c == 't' && data_file.get() == 'i' && data_file.get() == 'm' && data_file.get() == 'e' && data_file.get() == '=')
        {
            loop_count = 0;
            c = data_file.get();
            if (data_index == 50)
                    cout << "50 digit 0 = '" << c << "' number = " << atoi(&c) << endl;
            pulled_digits[loop_count] = atoi(&c);

            while ((c = data_file.get()) != 'm')
            {
                loop_count++;
                if (data_index == 50)
                    cout << "50 digit " << loop_count << " = '" << c << "' number = " << atoi(&c) << endl;
                pulled_digits[loop_count] = atoi(&c);
            }
            add_data = 0;
            for (int i = 0; i <= loop_count; i++)
                add_data += pulled_digits[loop_count - i] * (int)pow(10.0,i);

            if (data_index == 50)
                cout << "50 index = " << add_data << endl;
            ping_data[data_index] = add_data;
            data_index++;

            if (data_index >= MAX_PING_DATA)
            {
                cout << "Error parsing data. Exceeded maximum allocated memory for ping data." << endl;
                return MAX_PING_DATA;
            }   
        }
    }

    data_file.close();

    return data_index;
}

Answer 1

atoi takes a string, i.e. a null terminated array of chars, not a pointer to a single char so this is incorrect and will get you unpredictable results.

char c;
//...
/* ... */ atoi(&c) /* ... */

Also, atoi doesn't provide any way to detect errors, so prefer strtol and similar functions.

E.g.

char *endptr;
char c[2] = {0}; // initalize c to all zero

c[0] = data.file.get(); // c[1] is the null terminator

long l = strtol(c, &endptr, 10);

if (endptr == c)
    // an error occured

Answer 2

Nevermind, it was simply that I needed to convert the character into a string terminated by \0. I changed it to this code:

char buffer [2];

buffer[1] = '\0';

buffer[0] = data_file.get();

if (data_index == 50)

cout << "50 digit 0 = '" << buffer[0] << "' number = " << atoi(buffer) << endl;

and it worked.

Answer 3

atoi expects a null-terminated string as an input. What you are supplying is not a null-terminated string.

Having said that, it is always worth adding that it is very difficult (if at all possible) to use atoi properly. atoi is a function that offers no error control and no overflow control. The only proper way to perform string-representation-to-number conversion in C standard library is functions from strto... group.

Actually, if you need to convert just a single character digit, using atoi or any other string conversion function is a weird overkill. As it has already been suggested, all you need is to subtract the value of 0 from your character digit value to get the corresponding numerical value. The language specification guarantees that this is a portable solution.