C / C++ / MFC

double roundToOneTenth(double number) {
    return 0.1*(int)(10.*number);
}

#include <sstream>
#include <iomanip>
double Round(double val, size_t decimal)
{
    std::ostringstream oss;
    oss << std::fixed << std::setprecision(decimal) << val << std::ends;
    std::istringstream(oss.str()) >> val;
    return val;
}

Luc Pattyn wrote:

double roundToOneTenth(double number) {
    return 0.1*(int)(10.*number);
}

double RoundToOneTenth(double nNumber) {
    return (int)(10.0 * nNumber + 0.5) * 0.1;
}

Hans Dietrich wrote:

double d = floor(1.333333 * 10.) / 10.;

double RoundToOneTenth(double nNumber) {
    return floor(nNumber * 10.0 + 0.5) * 0.1;
}

Alain Rist wrote:

#include <sstream>
#include <iomanip>
double Round(double val, size_t decimal)
{
    std::ostringstream oss;
    oss << std::fixed << std::setprecision(decimal) << val << std::ends;
    std::istringstream(oss.str()) >> val;
    return val;
}

Cedric Moonen wrote:

Are you aware that floating points number have a rounding error ? You'll never be able to represent perfectly a float in memory.

Andrew Brock wrote:

double RoundToOneTenth(double nNumber) 
{
    return floor(nNumber * 10.0 + 0.5) * 0.1;
}

This is the solution I would recommend. Note that I changed /10 to *0.1.

Alain Rist wrote:

-5.51 => -5.4

int RoundValue( double value )
{
    int result = 0;
    if( value < 0 )
        result = (int)( value - 0.5 );
    else
        result = (int)( value + 0.5 );
    return result;
}