Ruby program stops executing due to error

If your program stops executing due to an error, you're in luck! These types of errors are much easier to fix than when your program runs with unexpected results.

When your program raises an error, there's a good chance the error is telling you what the issue is and where the offending line is. Though I admit, it can sometimes be difficult to decipher the message.

How To Fix

Step 1. Figure out what line in what file the error is happening

Let's look at a generic message:

/path/to/my_program.rb:30:in `my_method_3': <error message>
   for #<MyProgram:0x007fd229275440> (ErrorClass)
   from my_program.rb:10:in `my_method_2'
	from my_program.rb:3:in `new'
	from my_program.rb:60:in `<main>

This message is telling you:

• the program the error is happening in is located in the folder /path/to/
• the error is happening in the file my_program.rb
• how it got to the error (aka: a stack trace):
  • first it executed line 60
  • line 60 called the new method on line 3
  • line 3 called my_method_2 on line 10
  • line 10 called my_method_3 on line 20
• the error is happening on line 30

Step 2. Decipher the error message

Now that you know where the error is happening, if your error is one of the following, you can find further instructions on how to fix this error.

• unexpected end-of-input, expecting keyword_end
• undefined local variable or method
• undefined method for nil:NilClass

If your error isn't in the above list and you'd like more practice reading error messages, try this bonus-assignment Reading error messages, otherwise, proceed to the next step.

Step 3. Google the error

Put the error into google, and try to remove any context specific information from the error.

For example, if I had the error:

my_simple_class.rb:5:in `<class:MySimpleClass>':
undefined local variable or method `baby_cakes'
for MySimpleClass:Class (NameError)
from my_simple_class.rb:3:in `<main>'

I would remove my filename (my_simple_class.rb), line number (5), class name (MySimpleClass), my method (babycakes).

Thus I would be left with:

undefined local variable or method NameError

and I'd google just that.

It would likely lead me to a stackoverflow post. Skip the Question part of the stackoverflow post and go directly to the answers -- that's where the juice is. Look at the number one and number two answers (as occassionally the number one is marked correct, even though the number two answer is better).

If you understand the answer completely and you fix the your issue, then great, time to move on to your next bug!

However, if you don't understand the answer, but it does end up fixing your issue, the next step is to look up what the answer is saying in the ruby documentation and try to understand what it all means for next time.