CODEWITHSUNDEEP
c#.netfloating-pointprecision
Arthur
Arthur

Reputation: 3473

Can't increment value of a floating-point variable

I am newbie in C# and I can't understand, why my variable doesn't want to increment to value, bigger than 16777215.

I have code:

float fullPerc = 100;
float bytesRead = 1;
long fileSize = 1;
ProgressBar pb1;
int blockSizeBytes = 0;
float counter = 0;

using (FileStream inFs = new FileStream(inFile, FileMode.Open))
{
    fileSize = inFs.Length;
    do
    {
        counter = counter + 1;
        if (counter > 16777215) //&& counter < 16777230)
        {
            counter = counter + 10;
            //Console.WriteLine(((counter + 10) /100));
        }
        count = inFs.Read(data, 0, blockSizeBytes);
        offset += count;
        outStreamEncrypted.Write(data, 0, count);

        bytesRead += blockSizeBytes;

   }
   while (count > 0);
   inFs.Close();
}

https://bitbucket.org/ArtUrlWWW/myfileencoderdecoder/src/f7cce67b78336636ef83058cd369027b0d146b17/FileEncoder/Encrypter.cs?at=master&fileviewer=file-view-default#Encrypter.cs-166

When value of the counter is equal 16777216, code of the variable increment counter = counter + 1; doesn't work, but this code 

if (counter > 16777215) //&& counter < 16777230)
                                    {
                                        counter = counter + 10;
                                        //Console.WriteLine(((counter + 10) /100));
                                    }

is works fine.

Ie, if I comment this if code, my counter will grow up to 16777216 value and will stop on this value. Only increment by 10 will grow this variable, when this variable >=16777216 .

Why?

Upvotes: 1

Views: 624

Answers (1)

Dmitrii Bychenko
Dmitrii Bychenko

Reputation: 186668

You have mantissa overflow and as the result a presision loss. float (or Single) type has 24 bits mantissa (up to 16777216):

https://en.wikipedia.org/wiki/Single-precision_floating-point_format

Let's see what's going on:

  private static String MakeReport(float value) {
    return String.Join(" ", BitConverter
     .GetBytes(value)
     .Select(b => Convert.ToString(b, 2).PadLeft(8, '0')));
  }

...

  float f = 16777215;

  // Mantissa (first 3 bytes) is full of 1's except the last one bit
  // 11111111 11111111 01111111 01001011
  Console.Write(MakeReport(f)); 

  // Overflow! Presision loss
  // 00000000 00000000 10000000 01001011  
  Console.Write(MakeReport(f + 1)); 

  // Overflow! Presision loss
  // 00000000 00000000 10000000 01001011  
  Console.Write(MakeReport(f + 2)); 

  // Overflow! Presision loss
  // 00000100 00000000 10000000 01001011  
  Console.Write(MakeReport(f + 10));

Remedy: do not use floating points as counter but integer:

 int counter = 0;

To avoid integer division cast value into double

  float x = (float) ((((double)counter * blockSizeBytes) / fileSize) * fullPerc);

Upvotes: 6

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