Question

Say I have a method returning a double, but I want to determine the precision after the dot of the value to be returned. I don’t know the value of the double varaible.

Example:

double i = 3.365737;
return i;

I want the return value to be with precision of 3 number after the dot

Meaning: the return value is 3.365.

Another example:

double i = 4644.322345;
return i;

I want the return value to be: 4644.322

Answer 1

What you want is truncation of decimal digits after a certain digit. You can easily do that with the floor function from <math.h> (or std::floor from <cmath> if you’re using C++):

double TruncateNumber(double In, unsigned int Digits)
{
    double f=pow(10, Digits);
    return ((int)(In*f))/f;
}

Still, I think that in some cases you may get some strange results (the last digit being one over/off) due to how floating point internally works.

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On the other hand, most of time you just pass around the double as is and truncate it only when outputting it on a stream, which is done automatically with the right stream flags.