Debugging with pry and byebug in Ruby - Mini Project: Build & Apply

Debugging with pry and byebug

📖 Scenario: You are working on a simple Ruby program that calculates the total price of items in a shopping cart. Sometimes, the total price is not what you expect. To find the problem, you will use debugging tools pry and byebug to pause the program and check values step-by-step.

🎯 Goal: Learn how to use pry and byebug to debug a Ruby program by inserting breakpoints and inspecting variables.

📋 What You'll Learn

Create a hash called cart with specific items and prices

Add a variable discount_threshold to set a price limit

Use pry or byebug to pause the program inside a loop

Print the final total price after applying discount logic

💡 Why This Matters

🌍 Real World

Debugging tools like <code>pry</code> and <code>byebug</code> help developers find and fix problems in their code by letting them pause and check values step-by-step.

💼 Career

Knowing how to use debugging tools is essential for software developers to efficiently troubleshoot and improve code quality.

1

DATA SETUP: Create the shopping cart hash

Create a hash called cart with these exact entries: 'apple' => 2.5, 'banana' => 1.2, 'chocolate' => 3.0, 'bread' => 2.0

Ruby

# Create the cart hash with items and prices
# Your code here

Hint

Use curly braces {} to create a hash with keys and values separated by =>.

2

CONFIGURATION: Set the discount threshold

Create a variable called discount_threshold and set it to 5.0

Ruby

cart = { 'apple' => 2.5, 'banana' => 1.2, 'chocolate' => 3.0, 'bread' => 2.0 }
# Set the discount threshold value
# Your code here

Hint

Just assign the number 5.0 to the variable discount_threshold.

3

CORE LOGIC: Use byebug to debug the total calculation

Add require 'byebug' at the top. Then, create a variable total set to 0. Use a for loop with variables item and price to iterate over cart. Inside the loop, add byebug to pause execution. Add price to total only if price is less than discount_threshold.

Ruby

require 'byebug'
cart = { 'apple' => 2.5, 'banana' => 1.2, 'chocolate' => 3.0, 'bread' => 2.0 }
discount_threshold = 5.0

# Initialize total and loop through cart with byebug breakpoint
# Your code here

Hint

Use require 'byebug' to load the debugger. Insert byebug inside the loop to pause and inspect variables.

4

OUTPUT: Print the total price

Add a puts statement to print the string "Total price: " followed by the value of total

Ruby

require 'byebug'
cart = { 'apple' => 2.5, 'banana' => 1.2, 'chocolate' => 3.0, 'bread' => 2.0 }
discount_threshold = 5.0

total = 0
for item, price in cart
  byebug
  total += price if price < discount_threshold
end

# Print the total price
# Your code here

Hint

Use puts with string interpolation to show the total price.

Practice

1. What is the main purpose of using binding.pry or byebug in Ruby code?

easy

A. To pause the program and inspect variables at a specific point

B. To speed up the execution of the program

C. To automatically fix syntax errors

D. To compile Ruby code into machine code

Solution

1. Step 1: Understand the role of debugging tools

   Both binding.pry and byebug are used to pause program execution.

2. Step 2: Inspect variables and program state

   When paused, you can check variable values and program flow to find issues.

3. Final Answer:

   To pause the program and inspect variables at a specific point -> Option A

4. Quick Check:

   Debugging tools pause and inspect = A [OK]

Hint: They pause code to check variables and flow [OK]

Common Mistakes:

• Thinking they speed up code
• Believing they fix errors automatically
• Confusing with code compilation

2. Which of the following is the correct way to insert a breakpoint using Pry in Ruby code?

easy

A. debug.pry

B. breakpoint.pry

C. pry.binding

D. binding.pry

Solution

1. Step 1: Recall Pry syntax for breakpoints

   The correct syntax to pause execution with Pry is binding.pry.

2. Step 2: Check other options for correctness

   Other options like breakpoint.pry, pry.binding, and debug.pry are invalid method calls.

3. Final Answer:

   binding.pry -> Option D

4. Quick Check:

   Correct Pry breakpoint syntax = D [OK]

Hint: Use exactly 'binding.pry' to pause with Pry [OK]

Common Mistakes:

• Swapping method and object order
• Using non-existent methods
• Forgetting to require 'pry'

3. What will be the output when running this Ruby code with byebug inserted?

def add(a, b)
  byebug
  a + b
end

puts add(2, 3)

medium

A. 5

B. byebug error

C. nil

D. SyntaxError

Solution

1. Step 1: Understand what byebug does

   It pauses execution allowing inspection but does not change the return value.

2. Step 2: Trace the function call and output

   The method add(2, 3) returns 5, which is printed by puts.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   byebug pauses but output is 5 [OK]

Hint: byebug pauses but returns normal output [OK]

Common Mistakes:

• Expecting byebug to change output
• Thinking byebug causes errors
• Confusing pause with program stop

4. You inserted binding.pry in your Ruby code, but when you run it, the program does not pause. What is the most likely reason?

medium

A. You used byebug instead of binding.pry

B. You forgot to require the 'pry' gem in your code

C. The Ruby version does not support debugging

D. You placed binding.pry after the return statement

Solution

1. Step 1: Check if 'pry' gem is loaded

   Without require 'pry', binding.pry is undefined and won't pause.

2. Step 2: Verify placement and usage

   Using binding.pry correctly requires the gem loaded; placing after return stops execution before breakpoint.

3. Final Answer:

   You forgot to require the 'pry' gem in your code -> Option B

4. Quick Check:

   Missing require 'pry' stops pause [OK]

Hint: Always require 'pry' before using binding.pry [OK]

Common Mistakes:

• Not requiring the gem
• Placing breakpoint after return
• Confusing pry with byebug usage

5. You want to debug a method that processes a list of numbers and stops only when a number is negative. Which debugging tool and approach is best to use?

hard

A. Use puts statements to print all numbers

B. Insert byebug at the start of the method without any condition

C. Insert binding.pry inside the loop with a condition to pause only if the number is negative

D. Run the program without breakpoints and guess the error

Solution

1. Step 1: Understand conditional breakpoints

   Using binding.pry with a condition lets you pause only when needed, saving time.

2. Step 2: Compare other options

   Using byebug without condition pauses every time, puts is less interactive, guessing is ineffective.

3. Final Answer:

   Insert binding.pry inside the loop with a condition to pause only if the number is negative -> Option C

4. Quick Check:

   Conditional breakpoint with binding.pry = A [OK]

Hint: Use conditional binding.pry to pause only on negative numbers [OK]

Common Mistakes:

• Pausing unconditionally causing many stops
• Relying only on print statements
• Ignoring debugging tools