be_within matcher

Normal equality expectations do not work well for floating point values. Consider this irb session:

  > radius = 3
    => 3
  > area_of_circle = radius * radius * Math::PI
    => 28.2743338823081
  > area_of_circle == 28.2743338823081
    => false

Instead, you should use the be_within matcher to check that the value is within a delta of your expected value:

    expect(area_of_circle).to be_within(0.1).of(28.3)

Note that the difference between the actual and expected values must be smaller than your delta; if it is equal, the matcher will fail.

Basic usage

Given a file named “bewithinmatcher_spec.rb” with:

RSpec.describe 27.5 do
  it { is_expected.to be_within(0.5).of(27.9) }
  it { is_expected.to be_within(0.5).of(28.0) }
  it { is_expected.to be_within(0.5).of(27.1) }
  it { is_expected.to be_within(0.5).of(27.0) }

  it { is_expected.not_to be_within(0.5).of(28.1) }
  it { is_expected.not_to be_within(0.5).of(26.9) }

  # deliberate failures
  it { is_expected.not_to be_within(0.5).of(28) }
  it { is_expected.not_to be_within(0.5).of(27) }
  it { is_expected.to be_within(0.5).of(28.1) }
  it { is_expected.to be_within(0.5).of(26.9) }
end

When I run rspec be_within_matcher_spec.rb

Then the output should contain all of these:
10 examples, 4 failures
expected 27.5 not to be within 0.5 of 28
expected 27.5 not to be within 0.5 of 27
expected 27.5 to be within 0.5 of 28.1
expected 27.5 to be within 0.5 of 26.9