Ruby | Exception handling

In Ruby, exception handling is a process which describes a way to handle the error raised in a program. Here, error means an unwanted or unexpected event, which occurs during the execution of a program, i.e. at run time, that disrupts the normal flow of the program’s instructions. So these types of errors were handled by the rescue block. Ruby also provides a separate class for an exception that is known as an Exception class which contains different types of methods.
The code in which an exception is raised, is enclosed between the begin/end block, so you can use a rescue clause to handle this type of exception.

Syntax:

begin
    raise
      # block where exception raise

    rescue
      # block where exception rescue
end

Example:

# Ruby program to create the user
# defined exception and handling it

# defining a method
def raise_and_rescue
  begin

    puts 'This is Before Exception Arise!'

    # using raise to create an exception
    raise 'Exception Created!'

    puts 'After Exception'

  # using Rescue method
  rescue
    puts 'Finally Saved!'

end

puts 'Outside from Begin Block!'

end

# calling method
raise_and_rescue

Output:

This is Before Exception Arise!
Finally Saved!
Outside from Begin Block!

Explanation: In the above program, an exception is raised in the begin block (raise block) which interrupt the flow of execution of the program. To overcome this situation, use the rescue block to handle the raised exception. When rescue block is used, it handles the exception and continues the execution of the program.

Note: Multiple rescue clauses can be used in the same program which means if an exception is not handled by the first rescue clause, then another rescue clause will definitely handle the exception. If no rescue clause match, or if an exception occurs outside the begin/end block, then Ruby moves up to the stack and looks for an exception handler in the caller.

The statements used in Exceptions are:

1. retry Statement: This statement is used to execute the rescue block again from the beginning after capturing the exception.

   Syntax:

   begin
       # block where exception raise
   
   rescue
       # block where an exception is captured
   
   retry
   
       # this statement restarts the rescue
       # block from the beginning
   end
   

   Example:

   # Ruby program to illustrate
   # use of retry statement
   
     begin
   
       # using raise to create an exception
       raise 'Exception Created!'
   
       puts 'After Exception'
   
     # using Rescue method
     rescue
       puts 'Finally Saved!'
   
   # using retry statement
   retry
   end

   Output:

   Finally Saved!
   Finally Saved!
   Finally Saved!
   Finally Saved!
   Finally Saved!
   Finally Saved!
   .
   .
   .
   .
   

   Note: Be careful while using retry statement because it may result into infinite loop.

2. raise Statement: This statement is used to raise an exception.

   Syntax:

   raise

   This syntax is used to re-raise the current exception. It is generally used by the exception handler where an exception is interrupted before passing it on.

   raise "Error Message"

   This syntax is used to create a RuntimeError exception and it raises up the call stack.

   raise ExceptionType, "Error Message"

   In this syntax, the first argument is used to create an exception and then set the message in the second argument.

   raise ExceptionType, "Error Message" condition

   In this syntax, the first argument is used to create an exception and then set the message in the second argument. You can also set a condition statement to raise an exception.

   Example:

   # Ruby program to illustrate
   # use of raise statement
   
   begin
   
       puts 'This is Before Exception Arise!'
   
          # using raise to create an exception
          raise 'Exception Created!'
   
       puts 'After Exception'
   end

   Output:

   This is Before Exception Arise!
   Exception Created!
   

3. ensure Statement: This statement ensures that required instructions will execute at the end of the code, whether the exception raise or raised exception is rescued, or the program terminates due to an uncaught exception. This block always gives output. This block is placed over the rescue block.

   Syntax:

   begin
        # exception raise
   
   rescue
       # exception rescue
   
   ensure
       # this block always executes
   end
   

   Example:

   # Ruby program to illustrate
   # use of ensure statement
   
     begin
   
       # using raise to create an exception
       raise 'Exception Created!'
   
       puts 'After Exception'
   
     # using Rescue statement
     rescue
       puts 'Finally Saved!'
   
   # using ensure statement
   ensure
      puts 'ensure block execute'
   end

   Output:

   Finally Saved!
   ensure block execute
   

4. else Statement: This statement is present in between the rescue block and ensure block. This block only executes when no exception is raised.

   Syntax:

   begin
     rescue
       # exception rescue
     
     else
       # this block executes when no exception raise
   
     ensure
       # this block always executes
   end
   

   Example:

   # Ruby program to illustrate
   # use of else statement
   
    begin
   
       # using raise to create an exception
       # raise 'Exception Created!'
   
       puts 'no Exception raise'
   
       # using Rescue method
       rescue
           puts 'Finally Saved!'
   
      #  using else statement
      else
           puts 'Else block execute because of no exception raise'
   
      # using ensure statement
      ensure
         puts 'ensure block execute'
   end

   Output:

   no Exception raise
   Else block execute because of no exception raise
   ensure block execute
   

Catch and Throw in Exception Handling

In Ruby, catch and throw blocks are the lightweight mechanism for error handling and used to jump from the exception when there is no additional work is available in the program.
The catch block is used to jump out from the nested block and the block is labeled with a name. This block works normally until it encounters with the throw block. catch and throw method will work faster than raise and rescue method. When the throw statement is encountered, Ruby will check the call stack for the catch statement with the related symbol. The throw statement never executes and always return nil.

Syntax:

throw :label_name
    # this block will not be executed

catch :label_name do
    # matching catch will be executed when the throw block encounter
end

You can also apply a condition in catch and throw statement as shown below:

throw :label_name condition
     # this block will not be executed

catch :label_name do
     # matching catch will be executed when the throw block encounter
end

Example:

# Ruby program to illustrate
# use of catch and throw statement

# defining a  method
def catch_and_throw(value)

  puts value
  a = readline.chomp

  # using throw statement
  throw :value_e if a == "!"
  return a

end

# using catch statement
catch :value_e do

# enter number
  number = catch_and_throw("Enter Number: ")
end

Input:

Enter Number: 1

Output:

1

Input:

Enter Number: !

Output:

nil

Explanation: In the above example the catch_and_throw method is used to print the value enter in the catch statement. If the user enters a number, then it will print number, but if the user enters !, then it will give you nil. Because the throw statement has effectively never executed.