CODEWITHSUNDEEP
c++doubleprecision
Vinayak Garg
Vinayak Garg

Reputation: 6616

How to compare doubles with 5 digits of precision?

I am trying to compare 2 doubles which fall in [0.0, 1.0].

My function (taken from https://stackoverflow.com/a/17341 )-

inline bool isEqual(double x, double y)
{
    const double epsilon = 0.000001;
    return fabs(x - y) < epsilon;
}

Usage-

cerr << isEqual(1.000001, 1.000002) << endl;
cerr << isEqual(1.000010, 1.000020) << endl;

The output is-

0
0

Whereas I am expecting first to be true, second to be false. Please tell me where I am going wrong and how to fix it?

Upvotes: 0

Views: 1335

Answers (1)

Mark Ransom
Mark Ransom

Reputation: 308520

1.000001 when limited to the usual 64-bit IEEE floating point representation is actually 1.0000009999999999177333620536956004798412. Likewise 1.000002 is actually 1.0000020000000000575113290324225090444087. The two are ever more slightly apart than 0.000001.

You can use a very slightly larger comparison value to catch this:

    const double epsilon = 0.0000011;

It really isn't possible to completely eliminate any rounding problems with floating point numbers.

Upvotes: 6

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