nnovak13/wdi_2_ruby_lab_exceptions

Ruby Exceptions

Ways to handle errors in Ruby.

• Represents some kind of exceptional or error condition

  It indicates that something has gone wrong.

• Exceptions are raised.

• An exception is a special kind of object, an instance of the class Exception. Or it may be a descendant of that class.

• Ruby has some predefined classes - Exception and its children.

• By default, Ruby programs terminate when an exception occurs.

• It is possible to declare exception handlers.

  An exception handler is a block of code that is executed if an exception occurs during the execution of some other block of code.

• Raising an exception means:

  1. Stopping normal execution of the program.
  2. Tansferring the flow-of-control to the exception handling code.

Cases where an Exception will be raised.

• Divide a number by Zero. See demo
• Open a file that doesn't exist.
• User enters the wrong type of input. See demo
• A method determines that an argument passed to is not correct. See demo
• Calling a method on an object that it does have. See demo
• Typing Control-C on the terminal while a Ruby program is Running. See demo
• RubyOnRails has many methods that can raise and exception. See demo
  • Invalid input from a form.
  • Saving an object to the Database that is not valid.

Demo

Divide by Zero

Look at div_by_zero.rb. Notice how it raises a "Divide by Zero" exception.

Run it.

ruby div_by_zero.rb

Rescue the exception and show the exception attributes.

ruby div_by_zero_exception.rb

Retry (Handling Exceptions)

Do the above for argument_error.rb and argument_error_retry

Lab

Create a ruby app to guess a number. Give the user 3 tries. Use retry

Demo

Creating your own Exceptions.

Look at Person class with a PersonValid exception class.

Lab

Use this PersonValid exception class to validate people.

Validate name and age.

1. Create a Person class with a name and an age.
2. Add a Person instance method to check if the name is has from 2 to 9 characters.
3. Add a Person instance method to check if the person has a valid age. Greater than 0 and less than 110.

Lab (Advanced)

Populate a set of people from data.

[ {john: 44}, {mary:, 33}, {bill: 22}, {harry: 44, 'male'}, {sue: 26} ]

1. Given this above list of hash literals, create people by passing each hash to the Person.new method.
2. Ignore bad data, (hint: use rescue). Another words, don't let bad data break your program.