Significant Figures Formatting.00

Question

I wrote this for an SO question on formatting numbers to specific quantities of significant figures (after all, reinventing-the-wheel), anyway, all critiques welcome.

It's pretty easy to use, FormatSignificantFigures(0.123, 2) should return 0.12; FormatSignificantFigures(0.123, 5) should return 0.12300; FormatSignificantFigures(0.123456, 5) should return 0.12346, etc.

public static string FormatSignificantFigures(double number, int figures)
{
    int e = 0;

    while (number >= 10.0)
    {
        e += 1;
        number /= 10;
    }

    while (number < 1.0)
    {
        e -= 1;
        number *= 10;
    }

    figures--;

    number = Math.Round(number, figures);

    figures += 0 - e;
    while (e > 0)
    {
        number *= 10;
        e -= 1;
    }

    while (e < 0)
    {
        number /= 10;
        e += 1;
    }

    if (figures < 0)
    {
        figures = 0;
    }

    return number.ToString($"f{figures}");
}

Answer 1

int e = 0;

You're initializing e to default(int), which is redundant. This would be enough:

int e;

I'm not quite sure why you're incrementing e like this:

while (number >= 10.0)
{
    e += 1;
    number /= 10;
}

Why not just e++;?

And same thing here:

while (e > 0)
{
    number *= 10;
    e -= 1;
}

There's e--; to decrement.

But you know this already:

figures--;

Why? Why are you using both -= 1 and -- in the same method?

figures += 0 - e;

Looks like a pretty convoluted way to do figures -= e;.

And then we get to the return value...

return number.ToString($"f{figures}");

You're using C# 6.0 string interpolation here. A format string. The SO question you linked to has an accepted answer that uses a format string to do all the work: it seems string.Format("{0:G10}", number) would do the formatting part just right - I'd parameterize the length by making a method that builds a format string from the parameter (something like "{0:G" + digits + "}"), and uses that format string to return the formatted string.

I think it would be easier to read and to follow, than the whole logic you got there.

Or, don't use a format string. IMO using a formatted string here defeats the purpose of reinventing the wheel.