Round a float to a regular grid of predefined points

The standard ceil(), floor() functions don't have a precision, I guess could work around that by adding your own precision - but this may introduce errors - e.g.

double ceil(double v, int p)
{
  v *= pow(10, p);
  v = ceil(v);
  v /= pow(10, p);
}

I guess you could test to see if this is reliable for you?

As long as your grid is regular, just find a transformation from integers to this grid. So let's say your grid is

0.2  0.4  0.6  ...

Then you round by

float round(float f)
{
    return floor(f * 5 + 0.5) / 5;
    // return std::round(f * 5) / 5; // C++11
}