Your First Bug

A software bug is an error or flaw in a computer program that causes an incorrect or unintended result. Bugs provide the best learning opportunities in software. A faulty understanding produces faulty code, and debugging that code debugs one's coding knowledge.

Try running the code from the following file in the shell (the dark blue bottom section):

The shell should print something like:

undefined local variable or method 'hello' for main:Object
(repl):1:in '<main>'

Behold your first error! An error is a type of bug that stops your code from running. It's a violation of the rules of the programming language. When the Ruby interpreter--the program that parses and executes Ruby code--encounters an error, it stops the code from running. The interpreter instead throws an error and prints the relevant error message.

Error Messages

Error messages offer clues for debugging. When confronting an error, always start by examining the error message. The above error message has two parts. The first line is the message itself: undefined local variable or method 'hello' for main:Object. The second is the stack trace: (repl):1:in '<main>'. The stack trace essentially lists locations from which the error started. The stack trace is especially useful for debugging long, complicated code spread across multiple files. For now, it's important to note that the stack trace shows you the line number (1 in this case) where the error began.

Consider what the error message tells us about the source of the error. The Ruby interpreter had trouble parsing hello. For the interpreter, this situation is similar to when we used puts before learning what puts is. Because hello doesn't belong to any of the Ruby data types (numbers, strings, booleans, nil), the Ruby interpreter checks if hello is a defined method or variable (we'll teach you about variables in the next chapter). It's neither a method nor a variable, so the Ruby interpreter can't understand the code and throws an error. We can debug our code by making hello a valid string. Type the code from the file into your shell and press enter:

Though this bug may have been trivial to fix, it helped illuminate how the Ruby interpreter parses our code.