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Editorial Team
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Asked: 2026-05-15T12:46:51+00:00

I have a method that deals with some geographic coordinates in .NET, and I

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I have a method that deals with some geographic coordinates in .NET, and I have a struct that stores a coordinate pair such that if 256 is passed in for one of the coordinates, it becomes 0. However, in one particular instance a value of approximately 255.99999998 is calculated, and thus stored in the struct. When it’s printed in ToString(), it becomes 256, which should not happen – 256 should be 0. I wouldn’t mind if it printed 255.9999998 but the fact that it prints 256 when the debugger shows 255.99999998 is a problem. Having it both store and display 0 would be even better.

Specifically there’s an issue with comparison. 255.99999998 is sufficiently close to 256 such that it should equal it. What should I do when comparing doubles? use some sort of epsilon value?

EDIT: Specifically, my problem is that I take a value, perform some calculations, then perform the opposite calculations on that number, and I need to get back the original value exactly.

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  1. Editorial Team

    You could use the epsilon approach, but the epsilon is typically a fudge to get around the fact that floating-point arithmetic is lossy.

    You might consider avoiding binary floating-points altogether and use a nice Rational class.

    The calculation above was probably destined to be 256 if you were doing lossless arithmetic as you would get with a Rational type.

    Rational types can go by the name of Ratio or Fraction class, and are fairly simple to write

    Here’s one example.
    Here’s another

    Edit….

    To understand your problem consider that when the decimal value 0.01 is converted to a binary representation it cannot be stored exactly in finite memory. The Hexidecimal representation for this value is 0.028F5C28F5C where the “28F5C” repeats infinitely. So even before doing any calculations, you loose exactness just by storing 0.01 in binary format.

    Rational and Decimal classes are used to overcome this problem, albeit with a performance cost. Rational types avoid this problem by storing a numerator and a denominator to represent your value. Decimal type use a binary encoded decimal format, which can be lossy in division, but can store common decimal values exactly.

    For your purpose I still suggest a Rational type.

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