RSpec expectations and matchers in Ruby - Mini Project: Build & Apply

RSpec Expectations and Matchers

📖 Scenario: You are writing tests for a simple Ruby method that calculates the square of a number. Testing helps you check if your code works as expected before using it in real projects.

🎯 Goal: Build a basic RSpec test suite using expectations and matchers to verify that a square method returns correct results for given inputs.

📋 What You'll Learn

Create a Ruby method called square that returns the square of a number

Write an RSpec test file with a describe block for the square method

Use expect with the eq matcher to check the output for input 3

Add a test using the be_a matcher to confirm the output is an Integer

Use the not_to matcher to verify the output is not zero for input 5

💡 Why This Matters

🌍 Real World

Testing code with RSpec is common in Ruby projects to catch bugs early and ensure code quality.

💼 Career

Many Ruby developer jobs require writing and understanding RSpec tests to maintain reliable software.

1

Create the square method

Write a Ruby method called square that takes one parameter n and returns n * n.

Ruby

# Your code here

Hint

Define a method with def and return the number multiplied by itself.

2

Set up RSpec describe block

Write an RSpec describe block for the square method. Inside it, write an it block with description 'returns the square of a number'.

Ruby

def square(n)
  n * n
end

# Your code here

Hint

Use RSpec.describe 'square' do and inside it an it block with the given description.

3

Add expectation with eq matcher

Inside the it block, write an expectation using expect(square(3)) and the eq(9) matcher to check the output.

Ruby

def square(n)
  n * n
end

RSpec.describe 'square' do
  it 'returns the square of a number' do
    # Your code here
  end
end

Hint

Use expect with the method call and to eq(9) to check the result.

4

Add more matchers and print test results

Add two more tests inside the describe block: one using expect(square(4)).to be_a(Integer) to check the output type, and another using expect(square(5)).not_to eq(0) to confirm the output is not zero. Then run the tests to see the results.

Ruby

def square(n)
  n * n
end

RSpec.describe 'square' do
  it 'returns the square of a number' do
    expect(square(3)).to eq(9)
  end

  # Your code here
end

Hint

Use be_a(Integer) matcher to check type and not_to eq(0) to check value is not zero.

Practice

1. What does the RSpec matcher expect(value).to eq(5) check for?

easy

A. It checks if value is less than 5.

B. It checks if value is greater than 5.

C. It checks if value is not equal to 5.

D. It checks if value is equal to 5.

Solution

1. Step 1: Understand the eq matcher

   The eq matcher tests for equality between the expected and actual values.

2. Step 2: Analyze the expectation

   expect(value).to eq(5) means the test expects value to be exactly 5.

3. Final Answer:

   It checks if value is equal to 5. -> Option D

4. Quick Check:

   eq matcher = equality check [OK]

Hint: Remember eq means exact equality check [OK]

Common Mistakes:

• Confusing eq with greater_than or less_than
• Thinking eq checks for inequality
• Mixing up expect().to and expect().not_to

2. Which of the following is the correct syntax to check that an array includes the number 3 in RSpec?

easy

A. expect(array).to has(3)

B. expect(array).to include(3)

C. expect(array).to contains(3)

D. expect(array).to has_key(3)

Solution

1. Step 1: Identify the correct matcher for inclusion

   The include matcher checks if a collection contains a specific element.

2. Step 2: Match syntax with RSpec standards

   expect(array).to include(3) is the correct syntax to check if 3 is in the array.

3. Final Answer:

   expect(array).to include(3) -> Option B

4. Quick Check:

   include matcher = check element presence [OK]

Hint: Use include to check if collection has an element [OK]

Common Mistakes:

• Using has or contains which are not valid RSpec matchers
• Using has_key which is for hashes, not arrays
• Forgetting to use expect().to syntax

3. What will be the output of this RSpec test snippet?

describe 'Number test' do
  it 'checks if number is odd' do
    number = 7
    expect(number.odd?).to be true
  end
end

medium

A. The test passes because 7 is odd.

B. The test fails because 7 is not odd.

C. SyntaxError due to incorrect matcher usage.

D. The test passes but warns about deprecated syntax.

Solution

1. Step 1: Understand the code logic

   The variable number is set to 7, which is an odd number.

2. Step 2: Analyze the expectation

   expect(number.odd?).to be true checks if number.odd? returns true, which it does for 7.

3. Final Answer:

   The test passes because 7 is odd. -> Option A

4. Quick Check:

   odd? returns true for odd numbers [OK]

Hint: odd? returns true for odd numbers, expect that [OK]

Common Mistakes:

• Assuming 7 is even
• Confusing be true with eq true
• Thinking syntax is wrong here

4. Identify the error in this RSpec expectation:

expect(user.name).to eq nil

medium

A. Should use not_to eq nil to check for nil.

B. Missing parentheses around eq(nil).

C. Should use be_nil matcher instead of eq nil.

D. No error; the syntax is correct.

Solution

1. Step 1: Understand nil checking in RSpec

   While eq nil works, the preferred matcher for nil is be_nil for clarity and style.

2. Step 2: Identify best practice

   Using expect(user.name).to be_nil is clearer and idiomatic in RSpec.

3. Final Answer:

   Should use be_nil matcher instead of eq nil. -> Option C

4. Quick Check:

   Use be_nil for nil checks [OK]

Hint: Use be_nil matcher for nil checks, not eq nil [OK]

Common Mistakes:

• Thinking eq nil is a syntax error
• Adding unnecessary parentheses
• Using not_to eq nil to check for nil

5. You want to test that a method calculate returns a value greater than 10 but less than 20. Which RSpec expectation correctly checks this?

hard

A. expect(calculate).to satisfy { |v| v > 10 && v < 20 }

B. expect(calculate).to be > 10 and be < 20

C. expect(calculate).to be_between(10, 20).exclusive

D. expect(calculate).to be_between(11, 19).inclusive

Solution

1. Step 1: Understand the range requirement

   The value must be greater than 10 and less than 20, excluding 10 and 20 themselves.

2. Step 2: Analyze each option

   expect(calculate).to be > 10 and be < 20: expect(calculate).to be > 10 and be < 20 is invalid syntax; correct chaining uses (be > 10).and(be < 20).
   Options C and D: be_between is not a built-in RSpec matcher.
   expect(calculate).to satisfy { |v| v > 10 && v < 20 }: expect(calculate).to satisfy { |v| v > 10 && v < 20 } correctly checks the condition using a standard flexible matcher.

3. Step 3: Choose the best matcher

   expect(calculate).to satisfy { |v| v > 10 && v < 20 } is the correct and explicit choice using standard RSpec.

4. Final Answer:

   expect(calculate).to satisfy { |v| v > 10 && v < 20 } -> Option A

5. Quick Check:

   satisfy matcher = custom condition check [OK]

Hint: Use satisfy with block for complex conditions [OK]

Common Mistakes:

• Using non-standard be_between matcher
• Trying to chain matchers with 'and' incorrectly
• Ignoring boundary conditions in range checks