Round a decimal number to the first decimal position that is not zero

Question

I want to shorten a number to the first significant digit that is not 0. The digits behind should be rounded.

Examples:

0.001 -> 0.001
0.00367 -&g         


        

Answer 1

Another approach

    decimal RoundToFirstNonNullDecimal(decimal value)
    {
        var nullDecimals = value.ToString().Split('.').LastOrDefault()?.TakeWhile(c => c == '0').Count();
        var roundTo = nullDecimals.HasValue && nullDecimals >= 1 ? nullDecimals.Value + 1 : 2;
        return Math.Round(value, roundTo);
    }

Result

        Console.WriteLine(RoundToFirstNonNullDecimal(0.001m));                0.001
        Console.WriteLine(RoundToFirstNonNullDecimal(0.00367m));              0.004
        Console.WriteLine(RoundToFirstNonNullDecimal(0.000000564m));          0.0000006
        Console.WriteLine(RoundToFirstNonNullDecimal(0.00000432907543029m));  0.000004
        Console.WriteLine(RoundToFirstNonNullDecimal(0.12m));                 0.12
        Console.WriteLine(RoundToFirstNonNullDecimal(1.232m));                1.23
        Console.WriteLine(RoundToFirstNonNullDecimal(7));                     7.00

Answer 2

I would declare precision variable and use a iteration multiplies that variable by 10 with the original value it didn't hit, that precision will add 1.

then use precision variable be Math.Round second parameter.

static decimal RoundFirstSignificantDigit(decimal input) {
    int precision = 0;
    var val = input;
    while (Math.Abs(val) < 1)
    {
        val *= 10;
        precision++;
    }
    return Math.Round(input, precision);
}

I would write an extension method for this function.

public static class FloatExtension
{
    public static decimal RoundFirstSignificantDigit(this decimal input)
    {
        int precision = 0;
        var val = input;
        while (Math.Abs(val) < 1)
        {
            val *= 10;
            precision++;
        }
        return Math.Round(input, precision);
    }
}

then use like

decimal input = 0.00001;
input.RoundFirstSignificantDigit();

c# online

Result

(-0.001m).RoundFirstSignificantDigit()                  -0.001
(-0.00367m).RoundFirstSignificantDigit()                -0.004
(0.000000564m).RoundFirstSignificantDigit()             0.0000006
(0.00000432907543029m).RoundFirstSignificantDigit()     0.000004

Answer 3

code is from R but the algo should be obvious

> x = 0.0004932
> y = log10(x)
> z = ceiling(y)
> a = round(10^(y-z),1)
> finally = a*10^(z)
> finally
[1] 5e-04

the following was basically already provided by Benjamin K

At the risk of being labelled a complete wacko, I would cheerfully announce that regexp is your friend. Convert your number to a char string, search for the location of the first char that is neither "." nor "0" , grab the char at that location and the next char behind it, convert them to a number, round, and (because you were careful), multiply the result by $10^{-(number of zeros you found between "." and the first number)}$

Answer 4

Something like that ?

    public decimal SpecialRound(decimal value) 
    {
        int posDot = value.ToString().IndexOf('.'); // Maybe use something about cultural (in Fr it's ",")
        if(posDot == -1)
            return value;

        int posFirstNumber = value.ToString().IndexOfAny(new char[9] {'1', '2', '3', '4', '5', '6', '7', '8', '9'}, posDot);

        return Math.Round(value, posFirstNumber);
    }

Answer 5

var value = 0.000000564;

int cnt = 0;
bool hitNum = false;
var tempVal = value;
while (!hitNum)
{
    if(tempVal > 1)
    {
        hitNum = true;
    }
    else
    {
        tempVal *= 10;
        cnt++;
    }
}

var newValue = (decimal)Math.Round(value, cnt);