Test Driven Development (TDD) is defined as the ”software development process that relies on the repetition of a very short development cycle: first the developer writes an (initially failing) automated test case that defines a desired improvement or new function, then produces the minimum amount of code to pass that test, and finally refactors the new code to acceptable standards” [Wikipedia].

Test Driven Development Cycle

Fig.1 - Test Driven Development Cycle

TDD is a great way to create simple, elegant and powerful code. I’m a big fan of simplicity  in coding. Contrary to what one may think creating simple code is very hard. Researching simplicity it’s a very time consuming and arduous task. It’s much easier to create complicate code that nobody can understand . One way you can easily use TDD in ruby is by installing rspec. The script I’m using to show how rspec works is a program that takes a list of words in an array and extract the first and last letter of each word and put them in a new array. The method returns the new array. This is the behavior we want to test.

  • Install rspec: I assume you have installed  Ruby 1.9 with gems.
    Just type what is shown below and rspec should install with no issues. Run rspec -v on the command line and it should show the version installed. That’s the proof rspec was installed.
$ gem update
$ gem install rspec
$ rspec -v

  • Cut/paste the code below, save it as rspecTest.rb (change the name if you like) and then run it as shown:

#Test method called in rspec
def test

#Method we want to test with rspec
def get_first_last_letter(word_list)

  first_last_letter = [] #array initialization

  #extract the first and last letter from each word
  word_list.each { |x| first_last_letter << "#{x[0]}#{x[-1]}" }

  return first_last_letter

#Executing the script without rspec testing
  if (File.basename($0)) == File.basename(__FILE__)

    word_list = ["cocoa", "animal", "felt", "fertile"]
    puts get_first_last_letter(word_list)
    exit 0

#testing with rspec
describe "testDev" do
  it "testing" do

  word_list = ["cocoa", "animal", "felt", "fertile"]

  #testing expected outcome
  get_first_last_letter(word_list).should eql (["ca","al","ft", "fe"])


When you run the code by typing rspec rspecTest.rb you should see the following outcome:

rspec rspecTest.rb

Finished in 0.00103 seconds
1 example, 0 failures

Now let’s make the test fail! For this we are going to change line #32 in the program to expect a different outcome. In the array after the “eql” write “cb” instead of “ca”:

#Original line
get_first_last_letter(word_list).should eql (["ca","al","ft", "fe"])

#Changed line
get_first_last_letter(word_list).should eql (["cb","al","ft", "fe"])

Now run the test again and this is what you should see:


  1) testDev testing
     Failure/Error: get_first_last_letter(word_list).should eql (["cb","al","ft", "fe"])

       expected: ["cb", "al", "ft", "fe"]
            got: ["ca", "al", "ft", "fe"]

       (compared using eql?)
     # ./rspecTest.rb:28:in `block (2 levels) in '

Finished in 0.00119 seconds
1 example, 1 failure

Failed examples:

rspec ./rspecTest.rb:24 # testDev testing

shell returned 1

We can run the code normally by typing ruby rspecTest.rb the way we would do with any ruby code. This is what you should see:

$ ruby rspecTest.rb

There you go! You are now using rspec to test your code (or method to be precise).

There sare many resources online if you want to know more about rspec. A good book to start  is the following: The RSpec Book: Behaviour Driven Development with Rspec, Cucumber, and Friends (The Facets of Ruby Series)

Happy TDD!


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