Accuracy of "decimal" Data Type

C# Tutorials - Herong's Tutorial Examples

∟Accuracy of "decimal" Data Type

This section provides a tutorial example on how to compare the accuracy of 'decimal' data type operations with 'double' data type operations.

How much more accurate is "decimal" than "double"? Hope the following program will help us answer this question:

// Accuracy.cs
// Copyright (c) 2006 HerongYang.com. All Rights Reserved.

using System;
class Accuracy {
   static void Main() {
      long m = 1000;
      long n = 85;
      long i;
      double ds, df, da, dmin, dave, dmax;
      decimal ms, mf, ma, mmin, mave, mmax;
      dave = 0.0d;
      mave = 0.0m;
      dmin = double.MaxValue;
      mmin = decimal.MaxValue;
      dmax = 0.0d;
      mmax = 0.0m;
      Random r = new Random();
      for (i=0; i<m; i++) {
         ds = 0.5d + r.NextDouble()/2;
         df = 0.5d + r.NextDouble()/2;
         ms = (decimal)ds;
         mf = (decimal)df;
         da = LoopDouble(n, ref ds, ref df);
         ma = LoopDecimal(n, ref ms, ref mf);
         dave += da;
         mave += ma;
         if (da<dmin) dmin = da;
         if (ma<mmin) mmin = ma;
         if (da>dmax) dmax = da;
         if (ma>mmax) mmax = ma;
      }
      dave = dave/m;
      mave = mave/m;
      Console.WriteLine("# of tests: {0}", m);
      Console.WriteLine("# of operations: {0}", n);
      Console.WriteLine("Accuracy with double:");
      Console.WriteLine(" Average: {0}", dave);
      Console.WriteLine(" Minimum: {0}", dmin);
      Console.WriteLine(" Maximum: {0}", dmax);
      Console.WriteLine("Accuracy with decimal:");
      Console.WriteLine(" Average: {0}", mave);
      Console.WriteLine(" Minimum: {0}", mmin);
      Console.WriteLine(" Maximum: {0}", mmax);
   }
   private static double LoopDouble(long n, ref double s, ref double f) {
      long j;
      double o = s;
      for (j=1; j<=n; j++) {
         s = s*f;
      }
      for (j=1; j<=n; j++) {
         s = s/f;
      }
      return Math.Abs((s-o)/s);
   }
   private static decimal LoopDecimal(long n, ref decimal s,
      ref decimal f) {
      long j;
      decimal o = s;
      for (j=1; j<=n; j++) {
         s = s*f;
      }
      for (j=1; j<=n; j++) {
         s = s/f;
      }
      return Math.Abs((s-o)/s);
   }
}

Output:

# of tests: 1000
# of operations: 85
Accuracy with double:
 Average: 4.37517929465509E-16
 Minimum: 0
 Maximum: 1.97608339459022E-15
Accuracy with decimal:
 Average: 0.0000257496045277392923596642
 Minimum: 0
 Maximum: 0.0032269411262819481637932532

Well, the result is very surprising and disappointing. A value between 0.5 and 1.0 multiply by another value in the same range for 85 times, then divided by the second value 85 times. The resulting value should the same as the first value, if all the operations are 100% accurate. However, this program shows that the "double" values and operations did pretty well with relative errors in the order of 1.0E-16, comparing to 1.0E-5 for "decimal" values and operations.