assert_equal says <0.15> expected but was <0.15>, but only if the method computes 0.15 in a certain way

Question

So for this model method:

def tax_rate
  tax_rate = 0.0
  tax_rate += STATE_TAX if state_taxable?  #STATE_TAX = 0.1
  tax_rate += IMPORT_TAX if imported?      #IMPORT_TAX = 0.05
  tax_rate
end

This test fails:

@item.update_attributes({:state_taxable => true,
                         :imported => true,
                         :price => 32.19})
assert_equal 0.15, @item.tax_rate

I get this error:

<0.15> expected but was <0.15>.

However, this test will pass:

@item.update_attributes({:state_taxable => true,
                         :imported => false,
                         :price => 14.99})
assert_equal 0.1, @item.tax_rate

So I get the error when tax_rate does 0.0 + 0.1 + 0.05, but not when it does 0.0 + 0.1, or 0.0 + 0.05. Both 0.15s are Floats, so I don't see what could be causing this. I've spent too long mulling over this, hopefully someone can point out what the culprit is. Thanks in advance guys.

Answer 1

Floating-point numbers can't be represented exactly; what you need to do is use assert_in_delta to check you're within a specified range.

Something like assert_in_delta 0.15, @item.tax_rate, 0.001 should do it: it'll check you're within 0.001 of the expected value.