Question

tl;dr: What’s wrong with my Cur (currency) structure?

tl;dr 2: Read the rest of the question please, before giving an example with float or double. 🙂

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I’m aware that this question has come up numerous times before all around the internet, but I have not yet seen a convincing answer, so I thought I’d ask again.

I fail to understand why using a non-decimal data type is bad for handling money. (That refers to data types that store binary digits instead of decimal digits.)

True, it’s not wise to compare two doubles with a == b. But you can easily say a - b <= EPSILON or something like that.

What is wrong with this approach?

For instance, I just made a struct in C# that I believe handles money correctly, without using any decimal-based data formats:

struct Cur
{
  private const double EPS = 0.00005;
  private double val;
  Cur(double val) { this.val = Math.Round(val, 4); }
  static Cur operator +(Cur a, Cur b) { return new Cur(a.val + b.val); }
  static Cur operator -(Cur a, Cur b) { return new Cur(a.val - b.val); }
  static Cur operator *(Cur a, double factor) { return new Cur(a.val * factor); }
  static Cur operator *(double factor, Cur a) { return new Cur(a.val * factor); }
  static Cur operator /(Cur a, double factor) { return new Cur(a.val / factor); }
  static explicit operator double(Cur c) { return Math.Round(c.val, 4); }
  static implicit operator Cur(double d) { return new Cur(d); }
  static bool operator <(Cur a, Cur b) { return (a.val - b.val) < -EPS; }
  static bool operator >(Cur a, Cur b) { return (a.val - b.val) > +EPS; }
  static bool operator <=(Cur a, Cur b) { return (a.val - b.val) <= +EPS; }
  static bool operator >=(Cur a, Cur b) { return (a.val - b.val) >= -EPS; }
  static bool operator !=(Cur a, Cur b) { return Math.Abs(a.val - b.val) < EPS; }
  static bool operator ==(Cur a, Cur b) { return Math.Abs(a.val - b.val) > EPS; }
  bool Equals(Cur other) { return this == other; }
  override int GetHashCode() { return ((double)this).GetHashCode(); }
  override bool Equals(object o) { return o is Cur && this.Equals((Cur)o); }
  override string ToString() { return this.val.ToString("C4"); }
}

(Sorry for changing the name Currency to Cur, for the poor variable names, for omitting the public, and for the bad layout; I tried to fit it all onto the screen so that you could read it without scrolling.) 🙂

You can use it like:

Currency a = 2.50;
Console.WriteLine(a * 2);

Of course, C# has the decimal data type, but that’s beside the point here — the question is about why the above is dangerous, not why we shouldn’t use decimal.

So would someone mind providing me with a real-world counterexample of a dangerous statement that would fail for this in C#? I can’t think of any.

Thanks!

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Note: I am not debating whether decimal is a good choice. I’m asking why a binary-based system is said to be inappropriate.

Answer 1

Floats aren’t stable for accumulating and decrementing funds. Here’s your actual example:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace BadFloat
{
    class Program
    {
        static void Main(string[] args)
        {
            Currency yourMoneyAccumulator = 0.0d;
            int count = 200000;
            double increment = 20000.01d; //1 cent
            for (int i = 0; i < count; i++)
                yourMoneyAccumulator += increment;
            Console.WriteLine(yourMoneyAccumulator + " accumulated vs. " + increment * count + " expected");
        }
    }

    struct Currency
    {
        private const double EPSILON = 0.00005;
        public Currency(double value) { this.value = value; }
        private double value;
        public static Currency operator +(Currency a, Currency b) { return new Currency(a.value + b.value); }
        public static Currency operator -(Currency a, Currency b) { return new Currency(a.value - b.value); }
        public static Currency operator *(Currency a, double factor) { return new Currency(a.value * factor); }
        public static Currency operator *(double factor, Currency a) { return new Currency(a.value * factor); }
        public static Currency operator /(Currency a, double factor) { return new Currency(a.value / factor); }
        public static Currency operator /(double factor, Currency a) { return new Currency(a.value / factor); }
        public static explicit operator double(Currency c) { return System.Math.Round(c.value, 4); }
        public static implicit operator Currency(double d) { return new Currency(d); }
        public static bool operator <(Currency a, Currency b) { return (a.value - b.value) < -EPSILON; }
        public static bool operator >(Currency a, Currency b) { return (a.value - b.value) > +EPSILON; }
        public static bool operator <=(Currency a, Currency b) { return (a.value - b.value) <= +EPSILON; }
        public static bool operator >=(Currency a, Currency b) { return (a.value - b.value) >= -EPSILON; }
        public static bool operator !=(Currency a, Currency b) { return Math.Abs(a.value - b.value) <= EPSILON; }
        public static bool operator ==(Currency a, Currency b) { return Math.Abs(a.value - b.value) > EPSILON; }
        public bool Equals(Currency other) { return this == other; }
        public override int GetHashCode() { return ((double)this).GetHashCode(); }
        public override bool Equals(object other) { return other is Currency && this.Equals((Currency)other); }
        public override string ToString() { return this.value.ToString("C4"); }
    }

}

On my box this gives $4,000,002,000.0203 accumulated vs. 4000002000 expected in C#. It’s a bad deal if this gets lost over many transactions in a bank – it doesn’t have to be large ones, just many. Does that help?