[CSHARP-1047] Losing precision in conversion from float to BsonDouble Created: 21/Aug/14 Updated: 22/Sep/14 Resolved: 22/Sep/14 |
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| Status: | Closed |
| Project: | C# Driver |
| Component/s: | BSON |
| Affects Version/s: | None |
| Fix Version/s: | None |
| Type: | Improvement | Priority: | Minor - P4 |
| Reporter: | Marc Cortada | Assignee: | Robert Stam |
| Resolution: | Done | Votes: | 0 |
| Labels: | bson | ||
| Remaining Estimate: | Not Specified | ||
| Time Spent: | Not Specified | ||
| Original Estimate: | Not Specified | ||
| Backwards Compatibility: | Fully Compatible |
| Description |
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We could improve the BsonDouble type converting values from a float type without losing precision like decimal types (decimal type in C# solves the losing precision converting some numeric values). Example: With the actual implementation, the function BsonTypeMapper.MapToBsonValue returns a BsonDouble with precision lost if the parameter passed is a float. See the following new test case: float value2 = 1.3f; The test case result is: MongoDB.Bson.Tests.BsonTypeMapperTests.TestMapBsonDouble: Expected: same as <1.3> But was: <1.2999999523162842> Also the comparison is not entirely accurate. See the following code of a console test code: float value = 1.3f; var bsonDouble = (BsonDouble)BsonTypeMapper.MapToBsonValue(value, BsonType.Double); bool result2 = (value == bsonDouble); Console.WriteLine("Case 2: Original: {0} , converted: {1}. AreSame: {2}. ¿?¿ ", value, bsonDouble, result2);Console.ReadKey(); Outputs: Case 1: Original: 1,3, converted: 1.2999999523162842. AreSame: True. ¿?¿ Case 2: Original: 1,3, converted: 1.2999999523162842. AreSame: True. ¿?¿ Proposed solution: Use the decimal type as an intermediate cast. This code converts values without losing precision. float f = 3.2f; double d = (double)(decimal)f; double d2 = (double)f; // Loss Console.WriteLine("Original float: {0} and converted double: {1} ", f, d); and converted double without intermediate decimal: {1}", f, d2); Output: I needed solve it and I have a valid implementation which passes the test cases and I’m able to pull request shortly. |
| Comments |
| Comment by Marc Cortada [ 22/Sep/14 ] | |||||||||||||
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Thank you Robert for the detailed explanation and lesson | |||||||||||||
| Comment by Robert Stam [ 22/Sep/14 ] | |||||||||||||
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The behavior you are observing is NOT a loss of precision, but rather just a reflection of the fact that base-2 floating point numbers can't accurately represent decimal fractions. The C# driver converts float to BsonDouble with NO loss of precision, which is true simply because the C# language (or rather, the representation of 32-bit and 64-bit floating point numbers) guarantees that floats can be converted to doubles with no loss of precision. Consider the following sample code:
which outputs:
The first result is True precisely because a float can be converted to a BsonDouble with NO loss of precision. The second result is False because 1.3d is NOT the same number as 1.3f. Neither constant is exactly equal to 1.3, because 1.3 does not have an exact representation in base-2 floating point numbers. The reason they are NOT the same value is that 1.3d is represented using 64 bits, while 1.3f is represented using 32 bits. So 1.3d is a closer approximation to the decimal number 1.3 than 1.3f is, and therefore is not the same value. The third result is False because converting from float to decimal and then to double doesn't actually accomplish what you intend, or rather, while it might accomplish what you want in one example, it is the wrong thing to do in general because the end result is not the same number you started out with. When you convert from float to decimal you introduce rounding errors as binary fractions are converted to decimal, and when you convert from decimal to double you introduce a second set of rounding errors. The final result is False just to illustrate that the compiler agrees with me that 1.3f and 1.3d are not the same number. I'm going to close this ticket as "Works as Designed". | |||||||||||||
| Comment by Craig Wilson [ 25/Aug/14 ] | |||||||||||||
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Thanks Marc. We are discussing the implications of doing this and will get back to you. | |||||||||||||
| Comment by Marc Cortada [ 25/Aug/14 ] | |||||||||||||
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Pull request is done! |