You can try this... But I don't guarantee anything... Written and tested in 20 minutes and based on Pyrolistical's code from https://stackoverflow.com/a/1581007/613130
There is a big difference in that he uses a long
for the shifted
variable (because a double
has a precision of 15-16 digits, while a long
has 18-19, so a long
is enough), while I use a decimal
(because decimal
has a precision of 28-29 digits).
public static decimal RoundToSignificantFigures(decimal num, int n)
{
if (num == 0)
{
return 0;
}
// We are only looking for the next power of 10...
// The double conversion could impact in some corner cases,
// but I'm not able to construct them...
int d = (int)Math.Ceiling(Math.Log10((double)Math.Abs(num)));
int power = n - d;
// Same here, Math.Pow(10, *) is an integer number
decimal magnitude = (decimal)Math.Pow(10, power);
// I'm using the MidpointRounding.AwayFromZero . I'm not sure
// having a MidpointRounding.ToEven would be useful (is Banker's
// rounding used for significant figures?)
decimal shifted = Math.Round(num * magnitude, 0, MidpointRounding.AwayFromZero);
decimal ret = shifted / magnitude;
return ret;
}
If you don't trust the (int)Math.Ceiling(Math.Log10((double)
you could use this:
private static readonly decimal[] Pows = Enumerable.Range(-28, 57)
.Select(p => (decimal)Math.Pow(10, p))
.ToArray();
public static int Log10Ceiling(decimal num)
{
int log10 = Array.BinarySearch(Pows, num);
return (log10 >= 0 ? log10 : ~log10) - 28;
}
I have written it in another 20 minutes (and yes, I have tested all the Math.Pow((double), p)
for all the values -28 - +28). It seems to work, and it's only 20% slower than the C# formula based on double
s). It's based on a static array of pows and a BinarySearch
. Luckily the BinarySearch
already "suggests" the next element when it can't find one :-), so the Ceiling
is for free.
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