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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 "be_within_matcher_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 |