How to solve binary to decimal with fraction in c#? Using winform, Im trying to convert binary numbers into decimal. I used the input function

Question

if (binarybtn.Checked == true)
{
    int decVal = 0, baseVal = 1, rem;
    binarytxtbox.Text = textBox1.Text;

    int input = Convert.ToInt32(textBox1.Text);

    while (input > 0)
    {
        rem     = input % 10;
        decVal  = decVal + rem * baseVal;
        input   = input / 10;
        baseVal = baseVal * 2;
    }

    decimaltxtbox.Text = Convert.ToString(decVal);

    ...

I tried using the input function, but it does not accept strings "."

Answer 1

If I've understood you right, you have a binary number with a fractional part, like

10.1 (binary)

and you want to convert it into a corresponding decimal number, in the case above it is

2.5 (decimal)

If it's your task, you can use

private static decimal BinaryToDecimal(string value, char decimalSeparator = '.') {
  value = value.Trim();

  int sign = value.StartsWith('-') ? -1 : 1;

  value = value.Trim('-');

  int index = value.IndexOf(decimalSeparator);

  string integerPart = index >= 0 ? value.Substring(0, index) : value;

  decimal result = string.IsNullOrEmpty(integerPart)
    ? 0
    : Convert.ToInt64(integerPart, 2);

  string fractionalPart = index >= 0 ? value.Substring(index + 1) : "";

  for (int i = 0; i < fractionalPart.Length; ++i)
    result += (decimal)(fractionalPart[i] - '0') / (1L << (i + 1));

  return sign * result;
}

Usage:

decimaltxtbox.Text = $"{BinaryToDecimal(binarytxtbox.Text)}";

Demo:

using System.Linq;

...

string[] tests = {
  "0",
  "-0",
  "100",
  "-100",
  "10.",
  "-10.",
  "1.1",
  "101.101",
  "-101.101",
  ".1011",
  "-.11011",
  "100.00",
  ".", // <- It seems, you treat it as a valid input
};

string report = string.Join(Environment.NewLine, tests
  .Select(test => $"{test,10} => {BinaryToDecimal(test)}"));

Console.Write(report);

Output:

         0 => 0
        -0 => 0
       100 => 4
      -100 => -4
       10. => 2
      -10. => -2
       1.1 => 1.5
   101.101 => 5.625
  -101.101 => -5.625
     .1011 => 0.6875
   -.11011 => -0.84375
    100.00 => 4
         . => 0  

If you are looking for reverse (i.e. from decimal to binary) conversion, have a look at

How to convert decimal fractions to hexadecimal fractions?