IRB customization in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

IRB Prompt Customization Output

What will be the output of the following IRB prompt customization code snippet when you start IRB?

Ruby

IRB.conf[:PROMPT][:MY_PROMPT] = {
  PROMPT_I: "myirb> ",
  PROMPT_S: "myirb%l> ",
  PROMPT_C: "myirb* ",
  PROMPT_N: "myirb? ",
  RETURN: "=> %s\n"
}
IRB.conf[:PROMPT_MODE] = :MY_PROMPT

# Then start IRB

A The IRB prompt will show as 'myirb> ' for input lines.

B The IRB prompt will show as the default 'irb(main):001:0> '.

C The IRB prompt will show as '>> ' for input lines.

D The IRB prompt will be empty with no visible prompt.

💡 Hint

Look at the PROMPT_I key in the custom prompt hash and the PROMPT_MODE setting.

Setting IRB.conf[:PROMPT][:MY_PROMPT] defines a new prompt style named MY_PROMPT. Assigning IRB.conf[:PROMPT_MODE] = :MY_PROMPT activates it. PROMPT_I defines the main input prompt string, so the prompt will show as 'myirb> '.

🧠 Conceptual

intermediate

Purpose of IRB.conf[:SAVE_HISTORY]

What is the purpose of setting IRB.conf[:SAVE_HISTORY] = 1000 in IRB customization?

A It changes the prompt to show the last 1000 commands.

B It disables saving any command history in IRB.

C It sets the maximum number of concurrent IRB sessions to 1000.

D It limits the number of lines saved in the IRB command history to 1000.

💡 Hint

Think about what 'SAVE_HISTORY' might control in a command line tool.

IRB.conf[:SAVE_HISTORY] controls how many commands IRB remembers in its history file. Setting it to 1000 means IRB will save the last 1000 commands entered.

🔧 Debug

advanced

Fixing IRB Custom Prompt Syntax Error

This IRB customization code causes a syntax error. Which option fixes it correctly?

Ruby

IRB.conf[:PROMPT][:CUSTOM] = {
  PROMPT_I: '>> ',
  PROMPT_S: '>> ',
  PROMPT_C: '>> ',
  PROMPT_N: '>> ',
  RETURN: '=> %s\n'
}
IRB.conf[:PROMPT_MODE] = :CUSTOM

A Add a comma after the PROMPT_N line to separate the hash entries.

B Replace all single quotes with double quotes.

C Remove the RETURN key from the hash.

D Change the hash braces {} to square brackets [].

💡 Hint

Look carefully at the commas between hash entries in Ruby syntax.

In Ruby hashes, each key-value pair must be separated by a comma. The missing comma after PROMPT_N causes a syntax error. Adding it fixes the problem.

❓ Predict Output

advanced

IRB History File Location

Given this IRB configuration snippet, what will be the path of the history file used by IRB?

Ruby

IRB.conf[:HISTORY_FILE] = File.expand_path('~/.my_irb_history')

A The history file will be saved as 'history.txt' in the home directory.

B The history file will be saved in the current working directory as '.my_irb_history'.

C The history file will be saved as '.my_irb_history' in the user's home directory.

D IRB will not save any history file with this setting.

💡 Hint

File.expand_path with '~' expands to the user's home directory.

File.expand_path('~/.my_irb_history') resolves to the full path of the file named '.my_irb_history' inside the user's home directory. IRB will use this file to save command history.

🚀 Application

expert

Custom IRB Prompt with Dynamic Time

Which option correctly defines a custom IRB prompt that shows the current time before the prompt symbol?

A

IRB.conf[:PROMPT][:TIME_PROMPT] = {
  PROMPT_I: proc { "[Time.now] irb> " },
  PROMPT_S: 'irb%l> ',
  PROMPT_C: 'irb* ',
  PROMPT_N: 'irb? ',
  RETURN: '=> %s\n'
}
IRB.conf[:PROMPT_MODE] = :TIME_PROMPT

B

IRB.conf[:PROMPT][:TIME_PROMPT] = {
  PROMPT_I: -> { "[#{Time.now.strftime('%H:%M:%S')}] irb> " },
  PROMPT_S: 'irb%l> ',
  PROMPT_C: 'irb* ',
  PROMPT_N: 'irb? ',
  RETURN: '=> %s\n'
}
IRB.conf[:PROMPT_MODE] = :TIME_PROMPT

C

IRB.conf[:PROMPT][:TIME_PROMPT] = {
  PROMPT_I: "[#{Time.now.strftime('%H:%M:%S')}] irb> ",
  PROMPT_S: 'irb%l> ',
  PROMPT_C: 'irb* ',
  PROMPT_N: 'irb? ',
  RETURN: '=> %s\n'
}
IRB.conf[:PROMPT_MODE] = :TIME_PROMPT

D

IRB.conf[:PROMPT][:TIME_PROMPT] = {
  PROMPT_I: '[Time.now] irb> ',
  PROMPT_S: 'irb%l> ',
  PROMPT_C: 'irb* ',
  PROMPT_N: 'irb? ',
  RETURN: '=> %s\n'
}
IRB.conf[:PROMPT_MODE] = :TIME_PROMPT

💡 Hint

The prompt string must be a callable (like a lambda) to update dynamically each time.

To have a dynamic prompt showing current time, PROMPT_I must be a lambda or proc that returns the string with the current time. Option B uses a lambda with Time.now.strftime to format time dynamically. Other options use static strings or incorrect syntax.

Practice

1. What is the main purpose of customizing the ~/.irbrc file in Ruby's IRB?

easy

A. To create new Ruby classes

B. To install new Ruby gems automatically

C. To change how the IRB shell looks and behaves

D. To compile Ruby code faster

Solution

1. Step 1: Understand the role of ~/.irbrc

   The ~/.irbrc file is used to customize the IRB shell environment.

2. Step 2: Identify customization effects

   Customizations include changing the prompt, loading libraries, and saving history, which affect how IRB looks and works.

3. Final Answer:

   To change how the IRB shell looks and behaves -> Option C

4. Quick Check:

   IRB customization = change shell behavior [OK]

Hint: Remember ~/.irbrc customizes IRB shell behavior [OK]

Common Mistakes:

• Thinking it installs gems automatically
• Confusing with Ruby code compilation
• Assuming it creates Ruby classes

2. Which of the following is the correct way to load the 'date' library automatically when IRB starts by editing ~/.irbrc?

easy

A. require 'date'

B. load 'date'

C. import 'date'

D. include 'date'

Solution

1. Step 1: Recall Ruby syntax for loading libraries

   Ruby uses require 'library_name' to load external libraries.

2. Step 2: Check each option

   require 'date' is correct; load expects a file path, import and include are incorrect here.

3. Final Answer:

   require 'date' -> Option A

4. Quick Check:

   Load library = require 'library' [OK]

Hint: Use require to load libraries in ~/.irbrc [OK]

Common Mistakes:

• Using import which is not Ruby syntax
• Using include which mixes modules, not load libs
• Using load without full file path

3. Given this ~/.irbrc snippet:

IRB.conf[:PROMPT_MODE] = :SIMPLE
IRB.conf[:SAVE_HISTORY] = 1000

What effect does this have when you start IRB?

medium

A. Disables command history and uses default prompt

B. Changes prompt color but does not save history

C. Causes a syntax error and IRB won't start

D. Sets a simple prompt and saves last 1000 commands in history

Solution

1. Step 1: Understand IRB.conf[:PROMPT_MODE]

   Setting :PROMPT_MODE to :SIMPLE changes IRB prompt to a simple style.

2. Step 2: Understand IRB.conf[:SAVE_HISTORY]

   Setting :SAVE_HISTORY to 1000 means IRB saves the last 1000 commands in history.

3. Final Answer:

   Sets a simple prompt and saves last 1000 commands in history -> Option D

4. Quick Check:

   Prompt = simple, History saved = 1000 [OK]

Hint: Look for keys in IRB.conf to know prompt and history settings [OK]

Common Mistakes:

• Thinking it disables history
• Assuming syntax error from valid config
• Confusing prompt style with color only

4. You wrote this in your ~/.irbrc file:

IRB.conf[:SAVE_HISTORY] = '1000'

But IRB does not save history as expected. What is the problem?

medium

A. The key :SAVE_HISTORY is invalid

B. The value should be an integer, not a string

C. IRB does not support saving history

D. You must restart the computer for changes to apply

Solution

1. Step 1: Check the data type of the value

   :SAVE_HISTORY expects an integer number, not a string.

2. Step 2: Identify the error cause

   Using a string like '1000' causes IRB to ignore the setting, so history is not saved.

3. Final Answer:

   The value should be an integer, not a string -> Option B

4. Quick Check:

   Integer value needed for history count [OK]

Hint: Use numbers, not strings, for numeric IRB config values [OK]

Common Mistakes:

• Using quotes around numbers
• Thinking IRB can't save history
• Believing a full restart is needed

5. You want to customize IRB so that it automatically requires the 'json' library, sets a custom prompt showing the current time, and saves 500 commands in history. Which ~/.irbrc snippet correctly does this?

hard

A. require 'json'
IRB.conf[:PROMPT][:CUSTOM] = { PROMPT_I: "[#{Time.now.strftime('%H:%M:%S')}] > " }
IRB.conf[:PROMPT_MODE] = :CUSTOM
IRB.conf[:SAVE_HISTORY] = 500

B. require 'json'
IRB.conf[:PROMPT_MODE] = :SIMPLE
IRB.conf[:SAVE_HISTORY] = '500'

C. load 'json'
IRB.conf[:PROMPT_MODE] = :DEFAULT
IRB.conf[:SAVE_HISTORY] = 500

D. require 'json'
IRB.conf[:PROMPT_MODE] = :CUSTOM
IRB.conf[:SAVE_HISTORY] = 1000

Solution

1. Step 1: Load the 'json' library correctly

   Use require 'json' to load the library automatically.

2. Step 2: Define a custom prompt with current time

   Set IRB.conf[:PROMPT][:CUSTOM] with a prompt string showing current time using Ruby's Time.now.strftime.

3. Step 3: Set prompt mode and save history

   Assign IRB.conf[:PROMPT_MODE] = :CUSTOM to use the custom prompt and set IRB.conf[:SAVE_HISTORY] = 500 as an integer to save 500 commands.

4. Final Answer:

   require 'json', custom prompt with current time, :CUSTOM mode, SAVE_HISTORY = 500 -> Option A

5. Quick Check:

   Require json, custom prompt with time, save 500 history [OK]

Hint: Combine require, custom prompt hash, and integer history save [OK]

Common Mistakes:

• Using string instead of integer for history
• Not defining custom prompt hash properly
• Using load instead of require for libraries