Why is this simple calculation of two doubles inaccurate?

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C# (4): double minus double giving precision problems

86.25 - 86.24 = 0.01

generally, I would think the above statement is true right?

However, if I enter this

        double a = 86.24;
        double b = 86.25;
        double c = b - a;

I find that c = 0.010000000000005116

Why is this?

i'm asking for WHY it's there, like what is happening behind the scene for it to be giving this? — Diskdrive, Nov 23 '10 at 05:05

score 7 · Answer 1 · edited Nov 23 '10 at 05:18

7

Floating point numbers (in this case doubles) cannot represent decimal values exactly. I would recommend reading David Goldberg's What every computer scientist should know about floating-point arithmetic

This applies to all languages that deal with floating point numbers whether it be C#, Java, JavaScript, ... This is essential reading for any developer working with floating point arithmetic.

As VinayC suggests, if you need an exact (and slower) representation, use System.Decimal (aka decimal in C#) instead.

edited Nov 23 '10 at 05:18

Kobi

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answered Nov 23 '10 at 05:09

James Kovacs

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Since you mention various programming languages, I must point out that Ada is unique in allowing you to specify the desired precision of your reals (e.g. type Happiness is digits 7 range 0.0 .. 1.0`), and also in allowing fixed precision real values, such as `type Compass_Direction is delta 0.1 0 .. 359.9` which don't vary their precision at all across their range. – TamaMcGlinn Oct 13 '20 at 15:48

score 1 · Answer 2 · answered Nov 23 '10 at 05:00

1

Double is incorrect data type for decimal calculations, use Decimal for that. Main reason lies in how double stores the number - its essentially a binary approximation of the decimal number.

answered Nov 23 '10 at 05:00

VinayC

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Why is this simple calculation of two doubles inaccurate?

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