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c#decimal
Roger Far
Roger Far

Reputation: 2385

Accuracy of decimal

I use the decimal type for high precise calculation (monetary).

But I came across this simple division today:

1 / (1 / 37) which should result in 37 again

http://www.wolframalpha.com/input/?i=1%2F+%281%2F37%29

But C# gives me: 37.000000000000000000000000037M

I tried both these: 1m/(1m/37m); and Decimal.Divide(1, Decimal.Divide(1, 37))

but both yield the same results. How is the behaviour explainable?

Upvotes: 3

Views: 2809

Answers (2)

simba
simba

Reputation: 463

Decimal data type has an accuracy of 28-29 significant digits. So what you have to understand is when you consider 28-29 significant digits you are still not exact.

So when you compute a decimal value for (1/37) what you have to note is that at this stage you are only getting an accuracy of 28-29 digits. e.g 1/37 is 0.02 when you take 2 significant digits and 0.027 when you take 3 significant digits. Imagine you divide 1 with these values in each case. you get a 50 in first case and in second case you get 37.02...Considering 28-29 digits (decimal ) takes you to an accuracy of 37.000000000000000000000000037. If you have to get an exact 37 you simply need more than 28-29 significant digits than the decimal offers.

Always do computations with maximum significant digits and round off only your answer with Math.Round for desired result.

Upvotes: 1

Szymon
Szymon

Reputation: 43023

Decimal stores the value as decimal floating point with only limited precision. The result of 1 / 37 is not precicely stored, as it's stored as 0.027027027027027027027027027M. The true number has the group 027 going indefinitely in decimal representation. For that reason, you cannot get the precise numbers in decimal representation for every possible number.

If you use Double in the same calculation, the end result is correct in this case (but it does not mean it will always be better).

A good answer on that topic is here: Difference between decimal, float and double in .NET?

Upvotes: 5

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