Question

I have a function which should covert a double value into string one:

inline static string StringFromNumber(double val) // suppose val = 34.5678
{
 long integer = (long)val; // integer = 34
 long pointPart; // should be = 5678  how do I get it?
}

How do I get a long value for both integer and pointPart?

Add: I want a precision of 17 numbers, with discarding the zeros. More examples:

val = 3.14 integer = 3 pointPart = 14

val = 134.4566425814748 integer = 134 pointPart = 4566425814748

I have not got any solution so far. How can I get it?

Answer 1

You can use modf to separate the integer and fractional parts. You can then multiply the fractional part by 1.0e17, and call floor to properly round the results to it’s integer component, and then cast to a unsigned long (the fractional part will never be negative, and this allows you to maximize the number of bits in the integral type). Finally run though a loop to trim off the zeros on the unsigned long. For instance:

inline static string StringFromNumber(double val)
{
    double intpart, fracpart;
    fracpart = round((modf(val, &intpart)) * 1.0e17);

    long int_long = static_cast<long>(intpart);
    unsigned long frac_long = static_cast<long>(fracpart);

    //trim off the zeros
    for(unsigned long divisor = 10;;divisor *= 10)
    {
        if ((frac_long / divisor) * divisor != frac_long)
        {
            frac_long = frac_long / (divisor / 10);
            break;
        }
    }

    //...more code for converting to string
}

Note that this code will only work up to 17 decimal places if you are on a 64-bit platform and unsigned long is defined as a 64-bit integer-type. Otherwise you will want to change unsigned long to uint64_t. Also keep in mind that since floating point numbers are approximations, and there’s a multiplier by 1.0e17, the value of fracpart may not be exactly the value of the point-part of val … in other words there may be some additional digits after any necessary rounding.