CODEWITHSUNDEEP
c#.netmath
yonexbat
yonexbat

Reputation: 3012

decimal vs double again

I support a financial .net application. There is a lot of advice to use the decimal datatype for financial stuff.

Now I am stuck with this:

decimal price = 1.0m/12.0m;
decimal quantity = 2637.18m;
decimal result = price * quantity;  //results in 219.76499999999999999999999991

The problem is that the correct value to charge our customer is 219.77 (round function, MidpointRounding.AwayFromZero) and not 219.76.

If I change everything to double, it seems to work:

double price = 1.0/12.0;
double quantity = 2637.18;
double result = price * quantity;  //results in 219.765

Shall I change everything to double? Will there be other problems with fractions?

I think this question is different from Difference between decimal, float and double in .NET? because it does not really explain to me why the result with a more precise datatype decimal is less accurate (in the sample above) than the result with the double datatype which uses fewer bytes.

Upvotes: 0

Views: 4302

Answers (2)

Peter O.
Peter O.

Reputation: 32908

Usually, in financial calculations, multiplications and divisions are expected to be rounded to a certain number of decimal places and in a certain way. (Most currency systems use only base-10 amounts of money; in these systems, non-base-10 amounts of money are rare, if they ever occur.) Dividing a price by 12 without more is not always expected to result in a base 10 number; the business logic will dictate how that price will be rounded, including the number of decimal places the result will have. Depending on the business logic, such a result as 0.083333333333333333 might not be the appropriate one.

Upvotes: 1

Chris
Chris

Reputation: 27627

The reason decimal is recommended is that all numbers that can be represented as non-repeating decimals can be accurately represented in a decimal type. Units of money in the real world are always non-repeating decimals. Your problem as others have said is that your price is, for some reason, not representable as a non-repeating decimal. That is it is 0.083333333.... Using a double doesn't actually help in terms of accuracy - a double can not accurately represent 1/12 either. In this case the lack of accuracy is not causing a problem but in others it might.

Also more importantly the choice to use a double will mean there are many more numbers that you couldn't represent completely accurately. For example 0.01, 0.02, 0.03... Yeah, quite a lot of numbers you are likely to care about can't be accurately represented as a double.

In this case the question of where the price comes from is really the important one. Wherever you are storing that price almost certainly isn't storing 1/12 exactly. Either you are storing an approximation already or that price is actually the result of a calculations (or you are using a very unusual number storage system where you are storing rational numbers but this seems wildly unlikely).

What you really want is a price that can be represented as a double. If that is what you have but then you modify it (eg by dividing by 12 to get a monthly cost from an annual) then you need to do that division as late as possible. And quite possibly you also need to calculate the monthly cost as a division of the outstanding balance. What I mean by this last part is that if you are paying $10 a year in monthly instalments you might charge $0.83 for the first month. Then the second month you charge ($10-0.83)/11. This would be 0.83 again. On the fifth month you charge (10-0.83*4)/8 which now is 0.84 (once rounded). Then next month its (10-0.83*4-0.84)/7 and so on. This way you guarantee that the total charge is correct and don't worry about compounded errors.

At the end of the day you are the only one to judge whether you can re-architect your system to remove all rounding errors like this or whether you have to mitigate them in some way as I've suggested. Your best bet though is to read up on everything you can about floating point numbers, both decimal and binary, so that you fully understand the implications of choosing one over the other.

Upvotes: 6

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