Multiple rescue clauses in Ruby - Step-by-Step Execution

Concept Flow - Multiple rescue clauses

Start

↓

Try block

↓

Exception?

No→End

Yes↓

Rescue clause 1: matches?

No→Rescue clause 2: matches?

↓

Handle Exception

↓

End

The program tries code, checks for exceptions, and handles them with the first matching rescue clause, then ends.

Execution Sample

Ruby

begin
  1 / 0
rescue ZeroDivisionError
  puts "Zero division error caught"
rescue StandardError
  puts "Standard error caught"
end

This code tries to divide by zero, catches the ZeroDivisionError first, and prints a message.

Execution Table

Step	Action	Evaluation	Result
1	Execute begin block: 1 / 0	Raises ZeroDivisionError	Exception raised
2	Check rescue ZeroDivisionError	Matches exception	Rescue clause 1 runs
3	Execute rescue clause 1	puts message	Output: Zero division error caught
4	End of begin-rescue block	No more code	Program continues

💡 Exception handled by first matching rescue clause, program continues normally

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
exception	nil	ZeroDivisionError raised	Handled by rescue clause 1	Handled	nil

Key Moments - 2 Insights

Why does the first rescue clause run instead of the second?

Because the exception raised is ZeroDivisionError, which matches the first rescue clause exactly, so Ruby does not check further clauses (see execution_table step 2).

What happens if no rescue clause matches the exception?

The exception is unhandled and will cause the program to crash or propagate up (not shown in this example, but implied in concept_flow).

Visual Quiz - 3 Questions

Look at the execution table, what exception is raised at step 1?

A No exception

B StandardError

C ZeroDivisionError

D NameError

💡 Hint

Check the 'Evaluation' column in execution_table row 1

At which step does the program print the message 'Zero division error caught'?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look at the 'Result' column in execution_table row 3

If the exception was a RuntimeError, which rescue clause would run?

A Rescue clause 1 (ZeroDivisionError)

B Rescue clause 2 (StandardError)

C No rescue clause

D Both rescue clauses

💡 Hint

StandardError is a parent of RuntimeError, so rescue clause 2 matches (concept_flow and key_moments)

Concept Snapshot

begin
  # code that might raise
rescue ExceptionType1
  # handle ExceptionType1
rescue ExceptionType2
  # handle ExceptionType2
end

- Ruby tries rescue clauses in order.
- First matching rescue handles the exception.
- If none match, exception propagates.

Full Transcript

This example shows how Ruby handles multiple rescue clauses. The program tries to run code that divides by zero, which raises a ZeroDivisionError. Ruby checks the first rescue clause and finds it matches the exception, so it runs that rescue block and prints a message. It does not check further rescue clauses. If no rescue clause matched, the exception would be unhandled and cause an error. This step-by-step trace helps understand how Ruby picks the right rescue clause.

Practice

1. What is the purpose of using multiple rescue clauses in Ruby?

easy

A. To repeat the same error handling multiple times

B. To run code only when no errors occur

C. To handle different types of errors with specific responses

D. To skip error handling entirely

Solution

1. Step 1: Understand rescue clauses

   Multiple rescue clauses allow you to catch different error types separately.

2. Step 2: Purpose of specific error handling

   This helps you respond differently depending on the error type.

3. Final Answer:

   To handle different types of errors with specific responses -> Option C

4. Quick Check:

   Multiple rescue clauses = specific error handling [OK]

Hint: Multiple rescue clauses catch different errors separately [OK]

Common Mistakes:

• Thinking rescue clauses run only when no errors occur
• Assuming multiple rescue repeats same handling
• Believing rescue skips error handling

2. Which of the following is the correct syntax for multiple rescue clauses in Ruby?

easy

A. try
# code
rescue ZeroDivisionError
# handle zero division
rescue StandardError
# handle other errors
end

B. begin
# code
catch ZeroDivisionError
# handle zero division
catch StandardError
# handle other errors
end

C. begin
# code
rescue ZeroDivisionError, StandardError
# handle errors
end

D. begin
# code
rescue ZeroDivisionError
# handle zero division
rescue StandardError
# handle other errors
end

Solution

1. Step 1: Identify Ruby error handling syntax

   Ruby uses begin and rescue for error handling, not try or catch.

2. Step 2: Check multiple rescue clauses

   Multiple rescue clauses are written one after another inside begin...end.

3. Final Answer:

   begin # code rescue ZeroDivisionError # handle zero division rescue StandardError # handle other errors end -> Option D

4. Quick Check:

   Ruby uses begin-rescue-end with multiple rescue [OK]

Hint: Use begin-rescue-end blocks for multiple error types [OK]

Common Mistakes:

• Using try or catch instead of rescue
• Combining errors in one rescue without comma
• Incorrect block keywords

3. What will be the output of this Ruby code?

begin
  1 / 0
rescue ZeroDivisionError
  puts 'Zero division error caught'
rescue StandardError
  puts 'Standard error caught'
end

medium

A. Zero division error caught

B. Standard error caught

C. No output, program crashes

D. SyntaxError

Solution

1. Step 1: Identify the error raised

   The code 1 / 0 raises a ZeroDivisionError.

2. Step 2: Match error with rescue clauses

   The first rescue clause catches ZeroDivisionError and prints the message.

3. Final Answer:

   Zero division error caught -> Option A

4. Quick Check:

   ZeroDivisionError triggers first rescue [OK]

Hint: Match error type to first rescue clause that fits [OK]

Common Mistakes:

• Assuming StandardError rescue runs first
• Thinking program crashes without rescue
• Confusing syntax error with runtime error

4. Identify the error in this Ruby code with multiple rescue clauses:

begin
  puts 'Start'
rescue
  puts 'Rescue block'
rescue ZeroDivisionError
  puts 'Zero division error'
end

medium

A. Missing ensure block causes error

B. Multiple rescue clauses after a generic rescue cause syntax error

C. No error, code runs fine

D. Rescue clauses must be inside methods only

Solution

1. Step 1: Check for syntax errors

   In Ruby, a generic rescue clause must come after all specific rescue clauses. Placing a generic rescue before specific ones causes a syntax error.

2. Step 2: Analyze the code

   The code places a generic rescue first, then a specific rescue, which is invalid syntax.

3. Final Answer:

   Multiple rescue clauses after a generic rescue cause syntax error -> Option B

4. Quick Check:

   Generic rescue must be last [OK]

Hint: Generic rescue clause must come after specific rescue clauses [OK]

Common Mistakes:

• Thinking generic rescue before specific causes syntax error
• Believing ensure block is mandatory
• Assuming rescue clauses only work inside methods

5. Given this Ruby code, what will be printed?

def test
  begin
    raise ArgumentError, 'Wrong argument'
  rescue ZeroDivisionError
    puts 'Zero division error'
  rescue ArgumentError
    puts 'Argument error caught'
  ensure
    puts 'Always runs'
  end
end
test

hard

A. Argument error caught
Always runs

B. Zero division error
Always runs

C. Always runs
Argument error caught

D. No output, error not handled

Solution

1. Step 1: Identify raised error

   The code raises ArgumentError with message 'Wrong argument'.

2. Step 2: Match rescue clauses

   The ArgumentError rescue clause runs and prints 'Argument error caught'.

3. Step 3: Understand ensure block

   The ensure block always runs, printing 'Always runs'.

4. Final Answer:

   Argument error caught Always runs -> Option A

5. Quick Check:

   ArgumentError rescued, ensure always runs [OK]

Hint: Ensure block runs after rescue regardless of error [OK]

Common Mistakes:

• Thinking ensure runs before rescue
• Assuming wrong rescue clause runs
• Ignoring ensure block output