CSHARP.ARITH.FPEQUAL : Floating Point Equality (C#)

要旨

A comparison between non-integral values might be unreliable.

Floating point operations are by nature approximated. This can introduce bugs when precise, mathematical properties are expected from inherently imprecise floating point computations. Moreover, computations that might lose precision or overflow, whose result is stored in a larger type, are suspicious since, by computing on the larger type from the beginning, one could avoid approximations and overflows.

プロパティ

WARNING_FILTER += discard class="Floating Point Equality (C#)"

例

using System;

namespace DocumentationExamples
{
    public class Approximation
    {

        public static void Main(string[] args)
        {
            for (double d = 0.0; d != 5.0; d += 0.001) // Floating Point Equality (C#) warning issued here 
                                                       // due to floating point approximations, d != 5.0 will always be true and the loop will never terminate 
               Console.WriteLine(d);

            int days = int.Parse(args[0]);
            long milliseconds = days * 24 * 60 * 60 * 1000; // Cast: int Computation to long (C#) warning issued here 
                                                            // - multiplication is performed on integers; only the result is cast to long 
            Console.WriteLine("Inside %d days there are %d milliseconds", days, milliseconds);

        }

    }

}

To resolve the Floating Point Equality (C#) warning, replace d != 5.0 with d < 5.0 or replace floating point computations with (stable and more efficient) integral computations, as follows:

for (int d = 0; d != 5000; d++)
  Console.WriteLine(d / 1000.0);

To resolve the Cast: int Computation to long (C#) warning, perform the multiplication as an operation on long. Note that overflow can still occur if days is sufficiently large.

   int days = Convert.ToInt32(args[0]);
   long milliseconds = days * 24L * 60 * 60 * 1000;
   Console.WriteLine("Inside %d days there are %d milliseconds", days, milliseconds);

解決法

Use approximated comparisons (up to some epsilon) rather than exact comparisons. Use integral numbers instead of floating point values, whenever possible. Compute over integral numbers and translate into floating points only at the end, whenever possible. If a large type for the result of a computation is desired, compute from the beginning on that type instead of applying a type conversion at the end of the computation.

関連のある設定ファイルパラメータ

設定ファイルの以下のパラメータがこのワーニングクラスのチェックに影響します。

• REPORT_SIMILAR_WARNINGS
• WARNING_FILTER