C#: get decimal part of a float without rounding

Question

4.00/5 (1 vote)

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I would like to write function which give the decimal part of my float number. 
12.56 must return 56; 
90.2547 must return 2547; 
25 must return 0.

What I have tried:

C#

float_number - Math.Truncate(float_number);

But this solution does not return the exact value. 90.2547 can return 254692547. I just want 2547 as result.

Posted 4-Sep-19 9:30am

mart19

Updated 4-Sep-19 16:32pm

v3

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Nacoumblé Olivier Martial Soro · Accepted Answer · 2019-09-04T09:40:00

Solution 1

I think It will do the job:

C#

public int GetDecimalPart(float number)
        {
            var decimalNumber = Convert.ToDecimal(number);
            int decimalPart = int.Parse((decimalNumber % 1).ToString().Replace("0.", ""));

            return decimalPart;
        }

Posted 4-Sep-19 9:40am

Nacoumblé Olivier Martial Soro

Comments

BillWoodruff 4-Sep-19 22:47pm

Note that conversion to Decimal is going to round the value.

Nacoumblé Olivier Martial Soro 5-Sep-19 9:10am

When using Convert.Decimal the value is not rounded. The value is rounded with new Decimal (number).

BillWoodruff 5-Sep-19 17:57pm

You are incorrect.

decimal dec = Convert.ToDecimal(76790.25476f) // => 76790.26

BillWoodruff · Accepted Answer · 2019-09-04T16:33:00

See this for understanding the limitations of using decimal or float Types to express numeric values: [^].

private const int SIGN_MASK = ~Int32.MinValue;

public int GetDecimalRemainder(float fvalue)
{
    double dplaces;

    try
    {
        decimal dvalue = Convert.ToDecimal(fvalue);

        dplaces = (double) ((Decimal.GetBits(dvalue)[3] & SIGN_MASK) >> 16);

        return (int) ((dvalue - Math.Truncate(dvalue)) * (int) Math.Pow(10d, dplaces));
    }
    catch (Exception ex)
    {
        throw new TypeInitializationException(@"{fvalue} cannot be converted", ex);
    }
}

Notes:

0 note that conversion to decimal may round the value, leading to loss og precision in the mantissa.

1 this will return a negative number if the input float is negative

2 use of 'GetBits to find the number of powers of ten in a float mantissa iis based on: [^]