Question

I’m running into something I’ve never seen before and would very much like to understand what is going on. I’m trying to round a double to 2 decimal places and cast that value to a string. Upon completion of the cast things go crazy on me.

Here is the code:

void formatPercent(std::string& percent, const std::string& value, Config& config)
{
    double number = boost::lexical_cast<double>(value);
    if (config.total == 0)
    {
        std::ostringstream err;
        err << "Cannot calculate percent from zero total.";
        throw std::runtime_error(err.str());
    }
    number = (number/config.total)*100;
    // Format the string to only return 2 decimals of precision
    number = floor(number*100 + .5)/100;
    percent = boost::lexical_cast<std::string>(number);

    return;
}

I wasn’t getting quite what I expected so I did some investigation. I did the following:

std::cout << std::setprecision(10) << "number = " << number << std::endl;
std::cout << "percent = " << percent << std::endl;

…and get the following:

number = 30.63
percent = 30.629999999999999

I suspect that boost is doing something funny. Does anyone have any insight here?

Seriously, how strange is this?!? I ask for 10 digit precision on a double and get 4 digits. I ask to cast those 4 digits to a string and get that mess. What is going on?

Answer 1

std::precision sets the maximum number of significant digits to display

On the default floating-point
notation, the precision field
specifies the maximum number of
meaningful digits to display in total
counting both those before and those
after the decimal point. Notice that
it is not a minimum and therefore it
does not pad the displayed number with
trailing zeros if the number can be
displayed with less digits than the
precision.

30.629999999999999 is the actually floating point representation of 30.63