Question

Is there any straightforward way to get the mantissa and exponent from a double in c# (or .NET in general)?

I found this example using Google, but I’m not sure how robust it would be. Could the binary representation for a double change in some future version of the framework, etc?

The other alternative I found was to use System.Decimal instead of double and use the Decimal.GetBits() method to extract them.

Any suggestions?

Answer 1

The binary format shouldn’t change – it would certainly be a breaking change to existing specifications. It’s defined to be in IEEE754 / IEC 60559:1989 format, as Jimmy said. (C# 3.0 language spec section 1.3; ECMA 335 section 8.2.2). The code in DoubleConverter should be fine and robust.

For the sake of future reference, the relevant bit of the code in the example is:

public static string ToExactString (double d) {     …      // Translate the double into sign, exponent and mantissa.     long bits = BitConverter.DoubleToInt64Bits(d);     // Note that the shift is sign-extended, hence the test against -1 not 1     bool negative = (bits & (1L << 63)) != 0;     int exponent = (int) ((bits >> 52) & 0x7ffL);     long mantissa = bits & 0xfffffffffffffL;      // Subnormal numbers; exponent is effectively one higher,     // but there's no extra normalisation bit in the mantissa     if (exponent==0)     {         exponent++;     }     // Normal numbers; leave exponent as it is but add extra     // bit to the front of the mantissa     else     {         mantissa = mantissa | (1L << 52);     }      // Bias the exponent. It's actually biased by 1023, but we're     // treating the mantissa as m.0 rather than 0.m, so we need     // to subtract another 52 from it.     exponent -= 1075;      if (mantissa == 0)      {         return negative ? '-0' : '0';     }      /* Normalize */     while((mantissa & 1) == 0)      {    /*  i.e., Mantissa is even */         mantissa >>= 1;         exponent++;     }      … } 

The comments made sense to me at the time, but I’m sure I’d have to think for a while about them now. After the very first part you’ve got the ‘raw’ exponent and mantissa – the rest of the code just helps to treat them in a simpler fashion.