Test-driven development workflow in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a failing test in TDD

What will be the output when running this RSpec test before implementing the method?

Ruby

RSpec.describe Calculator do
  describe '#add' do
    it 'adds two numbers' do
      calc = Calculator.new
      expect(calc.add(2, 3)).to eq(5)
    end
  end
end

class Calculator
  # add method not implemented yet
end

A SyntaxError: unexpected end-of-input, expecting keyword_end

B

Failure/Error: expect(calc.add(2, 3)).to eq(5)

RSpec::Expectations::ExpectationNotMetError:
  expected: 5
       got: nil

C

Failure/Error: expect(calc.add(2, 3)).to eq(5)

NoMethodError:
  undefined method `add' for #<Calculator:0x00007f...>

D Test passed successfully with 1 example, 0 failures

💡 Hint

Think about what happens if you call a method that does not exist yet on an object.

Since the add method is not defined in Calculator, Ruby raises a NoMethodError when the test tries to call it. This causes the test to fail with that error.

🧠 Conceptual

intermediate

Purpose of writing a failing test first in TDD

Why do developers write a failing test before implementing the actual code in Test-driven development?

A To ensure the test framework is working and to define the expected behavior clearly before coding

B To speed up coding by skipping tests and writing code directly

C To make the code run faster by pre-checking errors

D To avoid writing documentation for the code

💡 Hint

Think about how TDD helps clarify requirements and verify correctness.

Writing a failing test first helps clarify what the code should do and confirms that the test setup works. It guides the implementation and ensures tests actually check the intended behavior.

❓ Predict Output

advanced

Result of running a test after implementing minimal code

Given this test and implementation, what is the test output?

Ruby

RSpec.describe Calculator do
  describe '#add' do
    it 'adds two numbers' do
      calc = Calculator.new
      expect(calc.add(2, 3)).to eq(5)
    end
  end
end

class Calculator
  def add(a, b)
    a + b
  end
end

A NoMethodError: undefined method `add' for Calculator

B 1 example, 0 failures

C SyntaxError: undefined local variable or method `a' for Calculator

D

1 example, 1 failure
Failure/Error: expect(calc.add(2, 3)).to eq(5)

RSpec::Expectations::ExpectationNotMetError:
  expected: 5
       got: 0

💡 Hint

Check if the add method returns the sum of two numbers.

The add method correctly returns the sum of a and b, so the test passes with no failures.

🔧 Debug

advanced

Identify the error in this TDD cycle code snippet

What error will this test produce when run?

Ruby

RSpec.describe Calculator do
  describe '#subtract' do
    it 'subtracts two numbers' do
      calc = Calculator.new
      expect(calc.subtract(5, 3)).to eq(2)
    end
  end
end

class Calculator
  def subtract(a, b)
    a - b
  # missing end here
end

A SyntaxError: unexpected end-of-input, expecting keyword_end

B NoMethodError: undefined method `subtract' for Calculator

C 1 example, 0 failures

D RSpec::Expectations::ExpectationNotMetError: expected 2 but got 8

💡 Hint

Look carefully at the method definition and class structure.

The subtract method is missing an 'end' keyword, causing a syntax error when Ruby tries to parse the code.

🧠 Conceptual

expert

Why refactor after tests pass in TDD?

In Test-driven development, after making a failing test pass, why is refactoring the next important step?

A To rewrite the tests to match the new code instead of the other way around

B To add more features immediately without checking existing code

C To remove all tests since the code works now

D To improve code quality and maintainability without changing behavior, ensuring tests still pass

💡 Hint

Think about the purpose of refactoring in clean code practices.

Refactoring cleans up code structure and readability while keeping behavior the same. Tests ensure no changes break functionality during this process.

Practice

1. In Ruby's test-driven development (TDD) workflow, what is the first step you should do?

easy

A. Run all tests to check for errors

B. Write a failing test that defines the desired behavior

C. Refactor the existing code

D. Write the full implementation code

Solution

1. Step 1: Understand the TDD cycle

   The TDD cycle starts by writing a test that fails because the feature is not implemented yet.

2. Step 2: Identify the first action in TDD

   Writing a failing test first guides the development and ensures the code meets requirements.

3. Final Answer:

   Write a failing test that defines the desired behavior -> Option B

4. Quick Check:

   First step in TDD = Write failing test [OK]

Hint: Remember: Test first, code second in TDD [OK]

Common Mistakes:

• Starting by writing implementation code before tests
• Refactoring before any test is written
• Running tests before any code or test exists

2. Which Ruby command runs all tests in a typical TDD workflow using Minitest?

easy

A. rails server

B. rspec test_file.rb

C. ruby test_file.rb

D. bundle install

Solution

1. Step 1: Identify the test framework command

   Minitest tests are run by executing the Ruby test file directly with ruby test_file.rb.

2. Step 2: Eliminate unrelated commands

   rspec is for RSpec tests, rails server starts the server, and bundle install installs gems.

3. Final Answer:

   ruby test_file.rb -> Option C

4. Quick Check:

   Run Minitest with ruby test_file.rb [OK]

Hint: Run tests by executing test files with ruby [OK]

Common Mistakes:

• Using rspec command for Minitest tests
• Confusing server start with test run
• Running bundle install instead of tests

3. Given this Ruby test and code, what will be the test output?

require 'minitest/autorun'

class Calculator
  def add(a, b)
    a + b
  end
end

class TestCalculator < Minitest::Test
  def test_add
    calc = Calculator.new
    assert_equal 5, calc.add(2, 3)
  end
end

medium

A. 1 test, 0 failures

B. 1 test, 1 failure

C. SyntaxError

D. No tests run

Solution

1. Step 1: Analyze the test and method

   The add method returns the sum of two numbers. The test expects 5 from adding 2 and 3.

2. Step 2: Check if test passes

   Since 2 + 3 equals 5, the assertion passes and the test succeeds.

3. Final Answer:

   1 test, 0 failures -> Option A

4. Quick Check:

   Correct sum = test passes [OK]

Hint: If method matches test expectation, test passes [OK]

Common Mistakes:

• Thinking test fails when it actually passes
• Confusing syntax error with test failure
• Ignoring the assert_equal expected value

4. What is wrong with this TDD workflow snippet in Ruby?

class TestCalculator < Minitest::Test
  def test_multiply
    calc = Calculator.new
    assert_equal 6, calc.multiply(2, 3)
  end
end

class Calculator
  def multiply(a, b)
    a - b
  end
end

medium

A. The test method name is incorrect

B. assert_equal arguments are reversed

C. Calculator class is missing

D. The multiply method subtracts instead of multiplies

Solution

1. Step 1: Compare method implementation with test expectation

   The test expects multiply(2, 3) to return 6, but the method subtracts a - b.

2. Step 2: Identify the error cause

   Subtracting 2 - 3 gives -1, which will fail the test expecting 6.

3. Final Answer:

   The multiply method subtracts instead of multiplies -> Option D

4. Quick Check:

   Method logic must match test expectation [OK]

Hint: Check method logic matches test assertions [OK]

Common Mistakes:

• Confusing subtraction with multiplication
• Ignoring method implementation details
• Assuming test method name affects test result

5. You want to safely improve your Ruby code after passing tests. Which TDD step lets you do this without breaking functionality?

hard

A. Refactor the code while running tests frequently

B. Write new tests before changing code

C. Run tests only after all changes are done

D. Delete old tests to simplify code

Solution

1. Step 1: Understand the role of refactoring in TDD

   Refactoring means improving code structure without changing behavior, done safely with tests as a safety net.

2. Step 2: Identify the correct practice

   Running tests frequently during refactoring ensures no functionality breaks.

3. Final Answer:

   Refactor the code while running tests frequently -> Option A

4. Quick Check:

   Refactor + frequent tests = safe improvements [OK]

Hint: Refactor with tests running often to catch errors [OK]

Common Mistakes:

• Running tests only after all changes
• Deleting tests which protect code quality
• Writing tests after refactoring instead of before