Question

the 3rd of the following tests fails:

  specify { (0.6*2).should eql(1.2) }
  specify { (0.3*3).should eql(0.3*3) }
  specify { (0.3*3).should eql(0.9) } # this one fails

Why is that? Is this a floating point issue or a ruby or rspec issue?

Answer 1

Don’t compare floating point numbers for equality

The problem is that neither 0.3 nor 0.9 has an exact representation¹ in the floating point format, and so when multiplying 0.3 * 3 you get a number that is very, very close to 0.9, and which will round to 0.9 for printing, but it isn’t 0.9.

And your 0.9 constant is also not precisely 0.9, and the two numbers are very slightly different.

Using exact equality comparisons for floating point numbers is usually a mistake in any language.

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^{1. All integers up to about 252 have exact FP representations, but the fractions are composed of a sequence of 1 / 2n terms. Most decimal string fractions repeat in base 2.}