TDD Basics : Testing computation involving floating-point numbers


  • Floating-point numbers are not real.


They are approximation of real numbers, there is inevitably a little error present. This error, called roundoff, can lead to surprising results. You should not use floating-point numbers for financial applications. They are intended for efficient scientific computation.

Tip by Chuck Allison from the book 97 Things a Programmer Should Know

We can compare adding two floats as follows:

 > (0.25 + 0.79)
 => 1.04 
 > (0.25 + 0.79) == 1.04
  => true

You see that the result of adding these two numbers is a Float:

(139.25 + 74.79).class
 => Float 

Let's now consider this example:

 > 139.25 + 74.79
 => 214.04000000000002 
 > (139.25 + 74.79)
 => 214.04000000000002 
 > (139.25 + 74.79) == 214.04
 => false 

Floats cannot store decimal numbers precisely. If we are interested only in comparing upto two decimal places, we can do so in our test like this:

describe 'Float accuracy' do
  it 'should be within a given decimal places' do
    x = 139.25
    y = 74.79

    expect(x + y).to be_within(0.001).of(214.04)

Run the spec:

rspec float_spec.rb

It will pass.


Ruby Float quirks
be_within matcher

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