Raise for throwing errors in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the raise keyword do in Ruby?

The raise keyword stops the program and throws an error. It can also create a custom error message.

beginner

How do you raise a standard error with a custom message in Ruby?

Use raise "Your message here". This stops the program and shows your message as the error.

intermediate

What happens if you use raise without any arguments?

Ruby re-raises the last error that was caught. If no error was caught, it raises a generic RuntimeError.

intermediate

How can you raise a specific error type in Ruby?

Use raise ErrorType, "message". For example, raise ArgumentError, "Invalid argument".

beginner

Why is using raise useful in programming?

It helps catch problems early by stopping the program when something unexpected happens. This makes bugs easier to find and fix.

What does raise "Error happened" do in Ruby?

A Ignores the message and continues

B Prints 'Error happened' but continues running

C Stops the program and shows 'Error happened' as an error message

D Creates a warning but does not stop the program

Using raise stops the program and shows the message as an error.

Which of these raises a specific error type in Ruby?

A error "Wrong input"

B raise ArgumentError, "Wrong input"

C throw ArgumentError

D raise "Wrong input"

The syntax raise ErrorType, "message" raises a specific error type.

What happens if you call raise with no arguments inside a rescue block?

A It re-raises the last caught error

B It raises a new generic error

C It does nothing

D It prints a warning

Calling raise without arguments inside rescue re-raises the last error.

Why should you use raise in your code?

A To print messages to the user

B To speed up the program

C To create variables

D To stop the program when something goes wrong

raise stops the program when an error occurs, helping find bugs.

Which keyword in Ruby is used to throw an error?

A raise

B throw

C error

D catch

The raise keyword is used to throw errors in Ruby.

Explain how to use raise to create a custom error message in Ruby.

Think about how you tell Ruby to stop and show your own message.

Describe the difference between using raise with and without arguments.

Consider what happens when you call raise alone versus with a message.

Practice

1. What does the raise keyword do in Ruby?

easy

A. It stops the program and throws an error.

B. It prints a message to the screen.

C. It creates a new variable.

D. It repeats a block of code.

Solution

1. Step 1: Understand the purpose of raise

   The raise keyword is used to stop the program and signal that an error has occurred.

2. Step 2: Compare with other options

   Printing messages, creating variables, or repeating code are not what raise does.

3. Final Answer:

   It stops the program and throws an error. -> Option A

4. Quick Check:

   raise stops program = D [OK]

Hint: Remember: raise means stop and show error [OK]

Common Mistakes:

• Thinking raise prints messages
• Confusing raise with loops
• Assuming raise creates variables

2. Which of the following is the correct syntax to raise a runtime error with a message in Ruby?

easy

A. raise "Error happened"

B. throw "Error happened"

C. error.raise("Error happened")

D. raise: "Error happened"

Solution

1. Step 1: Recall Ruby syntax for raising errors

   In Ruby, raise followed by a string message raises a RuntimeError with that message.

2. Step 2: Check each option

   raise "Error happened" uses correct syntax. throw "Error happened" uses throw which is different. error.raise("Error happened") is invalid method call. raise: "Error happened" has wrong colon usage.

3. Final Answer:

   raise "Error happened" -> Option A

4. Quick Check:

   Correct raise syntax = A [OK]

Hint: Use raise followed by message in quotes [OK]

Common Mistakes:

• Using throw instead of raise
• Adding colon after raise
• Calling raise as a method on error

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

def check_age(age)
  raise "Age must be 18 or older" if age < 18
  "Access granted"
end

puts check_age(20)

medium

A. Age must be 18 or older

B. No output

C. RuntimeError: Age must be 18 or older

D. Access granted

Solution

1. Step 1: Analyze the condition in the method

   The method raises an error only if age is less than 18. Here, age is 20, so no error is raised.

2. Step 2: Determine the returned value and output

   Since no error is raised, the method returns "Access granted" which is printed by puts.

3. Final Answer:

   Access granted -> Option D

4. Quick Check:

   Age 20 >= 18, no error = B [OK]

Hint: Raise only triggers if condition is true [OK]

Common Mistakes:

• Assuming error always raises
• Confusing error message with output
• Ignoring the if condition

4. Identify the error in this Ruby code snippet:

def divide(a, b)
  raise "Cannot divide by zero" if b = 0
  a / b
end

puts divide(10, 0)

medium

A. Division operator is incorrect

B. Missing parentheses in raise statement

C. Using assignment (=) instead of comparison (==) in the if condition

D. Method divide is not defined

Solution

1. Step 1: Check the if condition syntax

   The condition uses b = 0 which assigns 0 to b instead of comparing it. This is a common mistake.

2. Step 2: Understand the impact

   Because of assignment, b becomes 0, and since 0 is falsy in Ruby, the condition evaluates to false, so the raise is not triggered. The intended check is b == 0.

3. Final Answer:

   Using assignment (=) instead of comparison (==) in the if condition -> Option C

4. Quick Check:

   Use == for comparison, not = [OK]

Hint: Use == to compare, not = which assigns [OK]

Common Mistakes:

• Confusing = and ==
• Thinking raise needs parentheses
• Assuming divide method is missing

5. You want to raise a custom error type ArgumentError with a message "Invalid input" when a method receives a negative number. Which code correctly does this?

hard

A. ArgumentError.raise("Invalid input") if number < 0

B. raise ArgumentError, "Invalid input" if number < 0

C. raise "ArgumentError: Invalid input" if number < 0

D. raise ArgumentError("Invalid input") if number < 0

Solution

1. Step 1: Understand how to raise custom error types

   In Ruby, you can raise a specific error class by passing it as the first argument to raise, followed by the message.

2. Step 2: Check each option's syntax

   raise ArgumentError, "Invalid input" if number < 0 correctly uses raise ArgumentError, "Invalid input". raise "ArgumentError: Invalid input" if number < 0 treats the error as a string only. ArgumentError.raise("Invalid input") if number < 0 is invalid as raise is not a method on ArgumentError. raise ArgumentError("Invalid input") if number < 0 is invalid syntax.

3. Final Answer:

   raise ArgumentError, "Invalid input" if number < 0 -> Option B

4. Quick Check:

   Raise with error class and message = C [OK]

Hint: Use raise ErrorClass, "message" for custom errors [OK]

Common Mistakes:

• Passing error as string only
• Using wrong syntax with parentheses
• Confusing new method call with raise