Begin/rescue/end blocks in Ruby

Introduction

Begin/rescue/end blocks help your program handle errors without crashing. They let you try code and catch problems if they happen.

When reading a file that might not exist.

When dividing numbers and the divisor could be zero.

When connecting to the internet and the connection might fail.

When converting user input to a number that might be invalid.

Syntax

Ruby

begin
  # code that might cause an error
rescue SomeErrorClass
  # code to handle the error
end

You put the risky code inside begin and end.

The rescue part runs only if an error happens.

Examples

This catches a division by zero error and prints a friendly message.

Ruby

begin
  puts 10 / 0
rescue ZeroDivisionError
  puts "Can't divide by zero!"
end

This tries to open a file and handles the case when the file is missing.

Ruby

begin
  file = File.open('missing.txt')
rescue Errno::ENOENT
  puts "File not found."
end

Sample Program

This program asks for a number, tries to divide 100 by it, and handles two errors: invalid input and division by zero.

Ruby

begin
  puts "Enter a number:"
  num = Integer(gets.chomp)
  puts "100 divided by your number is #{100 / num}"
rescue ArgumentError
  puts "That's not a valid number!"
rescue ZeroDivisionError
  puts "Can't divide by zero!"
end

Important Notes

You can have multiple rescue clauses to handle different errors.

If no error happens, the rescue part is skipped.

Use ensure after rescue if you want code to run no matter what.

Summary

Use begin/rescue/end to catch and handle errors safely.

Put risky code inside begin and handle errors in rescue.

This helps your program keep running even when something goes wrong.

Practice

1. What is the purpose of the begin/rescue/end block in Ruby?

easy

A. To declare variables

B. To create a loop that repeats code

C. To define a new method

D. To handle errors and prevent the program from crashing

Solution

1. Step 1: Understand the role of begin block

   The begin block contains code that might cause an error.

2. Step 2: Understand the role of rescue block

   The rescue block catches errors and handles them so the program doesn't crash.

3. Final Answer:

   To handle errors and prevent the program from crashing -> Option D

4. Quick Check:

   begin/rescue = error handling [OK]

Hint: Remember: begin/rescue is for catching errors safely [OK]

Common Mistakes:

• Thinking begin/rescue creates loops
• Confusing rescue with method definition
• Using begin/rescue to declare variables

2. Which of the following is the correct syntax to handle exceptions using begin/rescue/end in Ruby?

easy

A. begin
risky_code
rescue
handle_error
end

B. try {
risky_code
} catch {
handle_error
}

C. if error then
handle_error
end

D. begin:
risky_code
except:
handle_error

Solution

1. Step 1: Identify Ruby syntax for error handling

   Ruby uses begin, rescue, and end keywords for exceptions.

2. Step 2: Compare options with Ruby syntax

   begin risky_code rescue handle_error end matches Ruby syntax exactly; others use syntax from other languages or incorrect keywords.

3. Final Answer:

   begin\n risky_code\nrescue\n handle_error\nend -> Option A

4. Quick Check:

   Ruby error handling = begin/rescue/end [OK]

Hint: Look for Ruby keywords: begin, rescue, end [OK]

Common Mistakes:

• Using try/catch like other languages
• Using if/else for error handling
• Mixing Python or JavaScript syntax

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

begin
  puts 10 / 0
rescue ZeroDivisionError
  puts "Cannot divide by zero"
end

medium

A. Cannot divide by zero

B. 0

C. 10

D. ZeroDivisionError

Solution

1. Step 1: Identify the error in the code

   The code tries to divide 10 by 0, which raises a ZeroDivisionError.

2. Step 2: Understand the rescue block

   The rescue block catches ZeroDivisionError and prints "Cannot divide by zero" instead of crashing.

3. Final Answer:

   Cannot divide by zero -> Option A

4. Quick Check:

   ZeroDivisionError caught = message printed [OK]

Hint: Divide by zero triggers rescue message [OK]

Common Mistakes:

• Expecting program to crash
• Thinking it prints 0 or 10
• Confusing error name with output

4. Find the error in this Ruby code snippet:

begin
  puts 5 + "3"
rescue
  puts "Error caught"
end

medium

A. The begin block should be replaced with try

B. The rescue block is missing the error type

C. You cannot add a number and a string directly

D. The code will run without errors

Solution

1. Step 1: Analyze the operation inside begin block

   Adding 5 (number) and "3" (string) causes a TypeError in Ruby.

2. Step 2: Check error handling

   The rescue block catches any error and prints "Error caught"; the error is due to incompatible types.

3. Final Answer:

   You cannot add a number and a string directly -> Option C

4. Quick Check:

   Type mismatch causes error caught [OK]

Hint: Check data types before adding values [OK]

Common Mistakes:

• Thinking rescue needs error type always
• Assuming code runs without error
• Confusing begin with try

5. How can you modify this code to handle multiple specific errors separately?

begin
  # risky code
rescue
  puts "An error occurred"
end

hard

A. Use if statements inside begin block

B. Use multiple rescue clauses with specific error classes

C. Replace rescue with else to catch errors

D. Put all code inside rescue block

Solution

1. Step 1: Understand handling multiple errors

   Ruby allows multiple rescue clauses to catch different error types separately.

2. Step 2: Apply multiple rescue clauses

   Write rescue clauses like rescue ZeroDivisionError and rescue IOError to handle errors distinctly.

3. Final Answer:

   Use multiple rescue clauses with specific error classes -> Option B

4. Quick Check:

   Multiple rescue = specific error handling [OK]

Hint: Use separate rescue for each error type [OK]

Common Mistakes:

• Using else instead of rescue
• Trying to catch errors with if statements
• Putting code inside rescue block