Question

I would like to truncate the float to 4 digits.
Are there some efficient way to do that?
My current solution is:

double roundDBL(double d,unsigned int p=4)
{
 unsigned int fac=pow(10,p);
 double facinv=1.0/static_cast<double>(fac);
 double x=static_cast<unsigned int>(d*fac)*facinv;
 return x;
}

but using pow and delete seems to me not so efficient.

kind regards

Arman.

Answer 1

round(d*10000.0)/10000.0;

or if p must be variable;

double f = pow(10,p);
round(d*f)/f;

round will usually be compiled as a single instruction that is faster than converting to an integer and back. Profile to verify.

Note that a double may not have an accurate representation to 4 decimal places. You will not truly be able to truncate an arbitrary double, just find the nearest approximation.