Ruby Lecture Reading Error Messages


In this lab, you'll be reading error messages from tests. This lab is designed so that both running the files and running the test suite via the learn command will show the error messages for you to decode. Moving forward though, you'll be reading error messages mainly through running the test suite.

Click the Fork button to make a copy of this lesson, then clone and download it so you can code along in your local environment.


  1. Read the three different parts of an error message.
  2. Identify four error types — name errors, syntax errors, type errors, and division errors — and fix them
  3. Describe a test suite, where it's found in a lab, and its purpose
  4. Use the learn command in terminal to run the tests for a lab.


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Reading Error Messages

Error messages have 3 parts:

lib/a_name_error.rb:3:in `<main>': undefined local variable or method `hello_world' for main:Object (NameError)

1: The location of the error, the "where".

lib/a_name_error.rb:3:in `<main>':
  • lib/a_name_error.rb is the file the error occurred in.
  • 3 is the line of code with the error.
  • <main> is the scope of the error.

2: The description, the "why".

undefined local variable or method `hello_world' for main:Object

The interpreter does the best job it can to tell you what it thinks went wrong.

3: The type of error, the "who".


This is a Ruby Error Type.

You've solved games of Clue with less information. This is one of the best parts of programming: debugging and fixing errors. It's like you're a detective solving a crime. The only bad thing is that more often than not, you're also the criminal that caused the error in the first place.

Errors are clues, and reading them is the interpreter telling you what to do to fix the program and move on.

Four Common Error Types

Name Errors

NameErrors are caused when a given name is invalid or undefined. Whenever the Ruby interpreter encounters a word it doesn't recognize, it assumes that word is the name of a variable or a method. If that word was never defined as either a variable or a method, it will result in a name error.

Syntax Errors

Syntax errors are pretty self-explanatory: they're the result of incorrect syntax. Thankfully, they're usually followed by a guess about the location of the error. For instance:

2.times do
  puts "hi"

Will result in:

2: syntax error, unexpected end-of-input, expecting keyword_end

Here, Ruby is saying that on line 2, there is a missing end (every do keyword must be followed by some code and then an end keyword). Always read the full details of syntax errors and look for line numbers, which usually appear at the beginning of the error message.

Type Errors

When you try and do a mathematical operation on two objects of a different type, you will receive a TypeError. For example if you try and add a string to an integer, Ruby will complain.

1 + "1"

Will produce the following error:

TypeError: String can't be coerced into Fixnum

Division Errors

DivisionErrors are caused when a given number is divided by 0.

What is a Test Suite?

The tests for each lab will be found inside the spec directory of that lab. Tests are programs, written using the RSpec testing library, that are written to make sure your program is running properly. Generally, tests will call on the methods you define in your programs and check to see if they are working the way they are expected to.

In the future, you will learn how to read tests more thoroughly and even how to write your own tests. For now, all you need to understand is that the code in the spec directory is there to test the code in your program. When you run the learn or the rspec command in your terminal in the directory of the lab you are working on, that runs the code in your spec file and tests your program. The output that appears in your terminal is the result of running those tests. If you pass a test, the output will generally appear green, otherwise, it will appear red and be accompanied by the types of error messages that we're discussing in this README. Paying attention to those error messages will help you to pass the test.


STOP. If you haven't watched the above video, you are making life much harder for yourself! Watch the video lecture above before attempting this lab. : )

Did you watch it? Okay, great. Let's proceed. The point of this lab is to get you comfortable reading error messages and fixing simple programs.

  1. Open this lab.

  2. Run learn from the root of the lab's directory. You should see a lot of red — this is okay! Tests have failed, and now we can fix them.

  • Read the errors. Scroll through the entire output to get a sense of what the failures are trying to tell you. What does expected no Exception, got #<NameError: undefined local variable or method `hello_world' for main:Object> with backtrace mean? How can we fix it?

  • Each error prints out a stack trace, which points to where the code failed and attempts to follow it up the stack — that is, through the bits of code that ran leading up to the failure. You can use these stack traces to pinpoint which line(s) of code need your attention.

  • These stack traces can also point you to which files you should run to get a better sense of the errors. Which leads us to step 3:

  1. Fix the errors in each of the files in lib/. Then confirm the fix by running learn again.

  2. Once your code is passing with the learn command, submit the lab with learn submit

Feel proud. Being able to read an error message and fix it, no matter how basic, is a huge step in being a programmer. Get comfortable with broken code. It's totally normal in programming.

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