Rescue modifier (inline form) in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the rescue modifier (inline form) in Ruby?

It is a way to handle exceptions in a single line using the rescue keyword after an expression. It returns the expression's value if no error occurs, or the rescue value if an error happens.

beginner

How does the rescue modifier differ from a full begin-rescue-end block?

The rescue modifier is shorter and used inline after an expression, while begin-rescue-end is a block that can handle multiple lines and more complex error handling.

beginner

Example: What does result = 10 / 0 rescue 'error' return?

It returns the string 'error' because dividing by zero raises an exception, which is caught by the rescue modifier.

intermediate

Can the rescue modifier catch specific exception types?

No, the rescue modifier catches all StandardError exceptions by default. For specific exceptions, use a full begin-rescue block.

beginner

Why use the rescue modifier instead of a full rescue block?

It makes code shorter and cleaner for simple error handling when you want to provide a fallback value quickly.

What does the rescue modifier do in Ruby?

A Handles exceptions inline and returns a fallback value if an error occurs

B Defines a new method

C Creates a loop

D Declares a variable

The rescue modifier catches exceptions inline and returns the fallback value if an error happens.

Which of these is a correct use of the rescue modifier?

A rescue value = 5 / 0 'fail'

B value = 5 / 0 rescue 'fail'

C begin value = 5 / 0 rescue 'fail' end

D value = rescue 5 / 0 'fail'

The rescue modifier is used after an expression, like value = 5 / 0 rescue 'fail'.

If no error occurs, what does the rescue modifier return?

A The original expression's value

B The rescue value

C nil

D An error message

If no error happens, the rescue modifier returns the original expression's value.

Can the rescue modifier specify which exception to catch?

A No, it catches all exceptions including system errors

B Yes, by adding the exception name after rescue

C Yes, by using a block

D No, it catches all StandardError exceptions

The rescue modifier catches all StandardError exceptions by default; specific exceptions require a full rescue block.

Which is a benefit of using the rescue modifier?

A Handles multiple exceptions separately

B Allows retrying the code automatically

C Shorter code for simple error handling

D Logs errors to a file

The rescue modifier makes code shorter and cleaner for simple fallback values.

Explain how the rescue modifier works in Ruby and when you might use it.

Think about how to handle errors quickly in one line.

Write a Ruby expression using the rescue modifier to divide two numbers and return 'error' if division fails.

Use the rescue keyword after the division.

Practice

1. What does the rescue modifier do in Ruby when used like value = 10 / x rescue 0?

easy

A. It ignores the division and returns nil

B. It always returns 10 divided by x

C. It returns 0 if dividing by x causes an error

D. It raises an error if x is zero

Solution

1. Step 1: Understand the rescue modifier usage

   The rescue modifier catches exceptions raised by the expression before it and returns the value after rescue if an error occurs.

2. Step 2: Apply to the example

   If 10 / x raises an error (like division by zero), the rescue modifier returns 0 instead of crashing.

3. Final Answer:

   It returns 0 if dividing by x causes an error -> Option C

4. Quick Check:

   rescue modifier returns fallback on error [OK]

Hint: Rescue modifier returns fallback if error happens [OK]

Common Mistakes:

• Thinking rescue modifier prevents all errors
• Assuming it always returns the original expression
• Confusing rescue modifier with if-else

2. Which of the following is the correct syntax for using the rescue modifier inline in Ruby?

easy

A. result = some_method rescue fallback_value

B. result = rescue some_method fallback_value

C. result = some_method ? rescue fallback_value

D. result = some_method else rescue fallback_value

Solution

1. Step 1: Recall the rescue modifier syntax

   The rescue modifier is placed after an expression, followed by the fallback value, like expression rescue fallback.

2. Step 2: Match with options

   Only result = some_method rescue fallback_value matches this syntax correctly: result = some_method rescue fallback_value.

3. Final Answer:

   result = some_method rescue fallback_value -> Option A

4. Quick Check:

   Correct rescue modifier syntax = result = some_method rescue fallback_value [OK]

Hint: Rescue modifier goes after expression, then fallback [OK]

Common Mistakes:

• Placing rescue before the expression
• Using question marks or else with rescue modifier
• Confusing rescue modifier with rescue block

3. What is the output of this Ruby code?

value = Integer('abc') rescue 'error'
puts value

medium

A. abc

B. error

C. 0

D. Raises an exception

Solution

1. Step 1: Understand Integer conversion with invalid string

   Calling Integer('abc') raises an ArgumentError because 'abc' is not a valid number.

2. Step 2: Apply rescue modifier

   The rescue modifier catches this error and returns the string 'error' instead of crashing.

3. Final Answer:

   error -> Option B

4. Quick Check:

   Invalid Integer conversion rescued to 'error' [OK]

Hint: Rescue returns fallback when conversion fails [OK]

Common Mistakes:

• Expecting the original string as output
• Thinking rescue returns nil by default
• Assuming code raises exception without rescue

4. Identify the error in this Ruby code snippet using rescue modifier:

result = 10 / 0 rescue puts 'Error!'

medium

A. No error; prints 'Error!' and result is nil

B. Syntax error because rescue modifier cannot follow puts

C. Runtime error because rescue modifier is misplaced

D. result will be 0 without error

Solution

1. Step 1: Analyze rescue modifier with puts

   The rescue modifier catches the division by zero error and executes puts 'Error!', which prints the message.

2. Step 2: Understand return value of puts

   puts returns nil, so result is assigned nil after rescue.

3. Final Answer:

   No error; prints 'Error!' and result is nil -> Option A

4. Quick Check:

   Rescue modifier runs puts and assigns nil [OK]

Hint: Rescue modifier returns last expression value after rescue [OK]

Common Mistakes:

• Expecting rescue modifier to return printed string
• Thinking rescue modifier causes syntax error here
• Assuming result keeps original expression value

5. You want to safely convert user input to integer but return -1 if conversion fails. Which code correctly uses rescue modifier for this?

hard

A. num = Integer(user_input) else rescue -1

B. num = rescue Integer(user_input) -1

C. num = Integer(user_input) ? rescue -1

D. num = Integer(user_input) rescue -1

Solution

1. Step 1: Understand rescue modifier placement

   The rescue modifier must come after the expression to catch exceptions and provide fallback.

2. Step 2: Check options for correct syntax and logic

   Only num = Integer(user_input) rescue -1 correctly places rescue after Integer(user_input) and returns -1 on failure.

3. Final Answer:

   num = Integer(user_input) rescue -1 -> Option D

4. Quick Check:

   Correct rescue modifier usage for fallback = num = Integer(user_input) rescue -1 [OK]

Hint: Put rescue after expression, then fallback value [OK]

Common Mistakes:

• Placing rescue before expression
• Using invalid syntax with question marks or else
• Forgetting rescue modifier for error handling