How to convert fractional part of double to string or fractional part of double to integer using c without changing exact value

Question

Is there a way to convert double into string without changing the value? or is there a way to truncate fractional part of a number to certain decimal precision and store in long? convertions seem to round up or down which will not work in my case.

double t = 8.299877766666;

either string which holds exact same value or truncated to certain decimal precision for example string which holds "299877766666" or "0.29987"?

Answer 1

Not sure what you tried, but the naive thing works for me:

uint64_t frac_dig(double x, int digits)
{
    // 20 non-zero digits will overflow a 64bit integer
    assert(0 < digits && digits < 20);

    // remove non-fractional digits, and strip off sign
    x = fabs(fmod(x, 1));

    uint64_t result = 0;

    while (digits-- > 0) {
        x *= 10;
        result *= 10;
        int i = x;
        result += i;
        x -= i;
    }

    return result;
}

testing via

    double t = -8.299877766666;
    printf("%.24f  = %lu\n", t, frac_dig(t, 19));

prints out

-8.299877766666000411532877  = 2998777666660004115

note that floating point values are base-2 encoded not decimal. this might be causing some of your "rounding issues". for example, the nearest value that can be represented by a double is slightly larger than the value in your question, as shown when printing out extra digits via printf.