Learn TDD with Ruby - Objects, Methods and Integers

Ruby is an Object-Oriented Programming (OOP) language, which means that (almost) everything we interact with is an object. That includes basic data types like numbers, strings and even nil. Every value in Ruby has an underlying object representation and can be manipulated with methods.

Ruby is also known to be "weakly typed", which means that type checking is not strictly enforced. This feature allows variables to change types dynamically at runtime. Example:

x = 10      # x is an Integer
x = "hello" # x is now a String

The object-oriented approach combined with the dynamic type system make Ruby a powerful and flexible language. In this chapter we're going to use these features to code an integer to binary converter.

Integer to Binary Converter

The binary numeral system is a way to write numbers using only two digits: 0 and 1. As it only needs two digits, we can say it's a base two number system.

For computers the binary system is extremely efficient because they need to store information in only two simple different states: "on" or "off" (1 or 0). Sets of binary numbers can be used to represent any information, such as text, audio, or video.

For the code we're going to work on this chapter I'm assuming you at least know what a binary number is and that it's a base two number system (uses only two digits to represent the possible values).

You don't need to know the math needed to convert a decimal number to binary notation (Ruby has convenient ways to do it). But I'm assuming you know that the binary 1001 is not one thousand and one (you don't even need to know that it's nine).

If you need more information on this topic, this Wikipedia page can be a good start.

Now let's start our Integer to Binary Converter™ project following the TDD cycle:

• Write a test
• Run the test, see it fails and check the error message
• Write enough code to make the test pass
• Refactor

Write the test first

We still have no idea about how to implement this converter, but by looking at the Wikipedia page about binary numbers we can see a table with some equivalents. Here are some examples:

Alright, let's choose one of these values and write our first test. I choose 8.

Create a file named int2bin_test.rb:

require "minitest/autorun"
require_relative "int2bin"

class TestInt2Bin < Minitest::Test
  def test_convert_8
    expected = "1000"
    actual = int2bin(8)
    assert_equal expected, actual
  end
end

Let's run this test and see it failing.

Write the minimal amount of code to for the test to run

When we run the tests, check the error and write the minimal amount of code to fix the error, we're letting the tests guide our development. That's a core concept of Test-Driven Development.

So let's start the cycle of checking the test error message and trying to solve what the message says.

In the very first run we see:

$ ruby int2bin_test.rb

int2bin_test.rb:2:in `require_relative': cannot load such file -- /path/to/int2bin (LoadError)
        from int2bin_test.rb:2:in `<main>'

OK, we're requiring a file that doesn't exist. Then let's create the file, run the test again and see the next error:

$ # creating the file
$ touch int2bin.rb

$ # running the test
$ ruby int2bin_test.rb

# Running:

E

Finished in 0.000271s, 3690.0373 runs/s, 0.0000 assertions/s.

  1) Error:
TestInt2Bin#test_convert_8:
NoMethodError: undefined method `int2bin' for #<TestInt2Bin:0x00000001348f40f0>
    int2bin_test.rb:7:in `test_convert_8'

1 runs, 0 assertions, 0 failures, 1 errors, 0 skips

Now the error message says NoMethodError: undefined method 'int2bin' ....

Let's create that method in our int2bin.rb:

def int2bin
end

Run the test, check the message:

1) Error:
TestInt2Bin#test_convert_8:
ArgumentError: wrong number of arguments (given 1, expected 0)
    /path/to/int2bin.rb:1:in `int2bin'
    int2bin_test.rb:7:in `test_convert_8'

Let's fix the wrong number of arguments in our int2bin.rb:

def int2bin(number)
end

Run the test, check the message:

  1) Failure:
TestInt2Bin#test_convert_8 [int2bin_test.rb:8]:
Expected: "1000"
  Actual: nil

Now our test is actually running with no errors, but it's failing. We're almost there!

At this point you may be thinking that you're wasting your time in this tedious loop of running the test, checking the error message and writing the minimal amount of code to fix the error message. I have two points about this practice:

• It is a nice way to prevent over-engineering - your tests are the requirements in form of code, and your software just needs to meet such requirements.
• You'll soon find ways to automatically run tests right after saving your file.

Even if my arguments are not convincing you, please stick with this practice while we're here.

Write enough code to make the test pass

The failure message says that the expected result is "1000" but it received nil. So, let's fix this like a pedantic programmer and "write the minimal amount of code to make the test pass":

def int2bin(number)
  "1000"
end

Ah hah! Foiled again! TDD is a sham, right?

Maybe we should add another test to int2bin_test.rb:

def test_convert_2
  expected = "10"
  actual = int2bin(2)
  assert_equal expected, actual
end

Running the tests:

# Running:

F.

Finished in 0.000265s, 7547.1687 runs/s, 7547.1687 assertions/s.

  1) Failure:
TestInt2Bin#test_convert_2 [int2bin_test.rb:14]:
Expected: "10"
  Actual: "1000"

2 runs, 2 assertions, 1 failures, 0 errors, 0 skips

If our pedantic instincts evolve to the point where we want to be a prick, we could add an if in our code just to answer with "10" when the argument is 2. But that feels like a game of cat and mouse.

Let's stop here and start to work on the code that will actually convert an integer to its binary representation.

Rubyists best friends

"Everything" is an Object

In the beginning of this chapter I said: every value in Ruby has an underlying object representation and can be manipulated with methods.

That includes the integer numbers. They are objects and we can interact with them using their methods.

Another fact about Ruby objects is that all of them have a string representation that can be obtained by the #to_s method (to_s stands for "to string").

As our goal is to convert an integer to its binary representation, and this representation is written in a string, maybe we can get some help from Integer#to_s.

Now is the perfect time to know two great friends of all Rubyists:

• the Ruby documentation
• the Interactive Ruby Shell, aka irb

Ruby documentation

Here's one of our great friends: https://ruby-doc.org/

As we want to work on Integers, we need to check the documentation about the Integer class: https://ruby-doc.org/current/Integer.html

In that page we can see a pretty decent amount of information about Integers.Iincluding what they can do, in other words, which methods they have.

We don't need to read all that page, but use it as a reference when needed. As we are suspecting the Integer#to_s can help us, now is a good momento to use it. So let's take a look at its documentation (below I bring only the part related to our problem):

to_s(base = 10) → string

Returns a string containing the place-value representation of self in radix base (in 2..36).

12345.to_s     # => "12345"
12345.to_s(2)  # => "11000000111001"

Hey! That looks promising! The method accepts an argument that acts as the base for the string representation we want to get from the integer. As the binary system uses base two, let's check if it can be used in our converter.

Before opening our code editor and writing our implementation, let's play a bit with our next great friend...

Interactive Ruby Shell (irb)

Using irb is an awesome way to quickly try things out with no need to put code in a file, save it and call it from the command line. You just need to type the code and check the results.

In order to use the Interactive Ruby Shell you just need to type irb in your terminal. You should see something similar to this:

$ irb

irb(main):001:0> 

If you see this, you're in the irb prompt. Here you can type Ruby code and see the results immediately.

In our case, we want to check if Integer#to_s is able to give us a binary representation of an integer. So, let's try it with 8.to_s(2):

irb(main):001:0> 8.to_s(2)
=> "1000"

Yeah! That seems to be exactly what we want! Let's try different values:

irb(main):002:0> 7.to_s(2)
=> "111"
irb(main):003:0> 2.to_s(2)
=> "10"
irb(main):004:0> 0.to_s(2)
=> "0"
irb(main):005:0> 15.to_s(2)
=> "1111"

Alright! I'm convinced! Let's use this method in our converter.

By the way: you can use exit to go out from irb.

First implementation

Now that we know Integer#to_s can solve our problem, let's use it in our code:

def int2bin(number)
  number.to_s(2)
end

Running the tests:

# Running:

..

Finished in 0.000261s, 7662.8350 runs/s, 7662.8350 assertions/s.

2 runs, 2 assertions, 0 failures, 0 errors, 0 skips

Great! All tests passing means that it's time to refactor. But, uhm... there's no much room for refactoring in single line function.

Let's just use our current code in an application.

i2b CLI

Now that we have working software, backed by tests, we should be safe to use it in a "real" application.

Let's write an extremely simple application that reads a number from user's input and prints the binary representation of the number.

Create a file named exactly like this: i2b. Note that there's no .rb extension in the file.

Here are the contents to be put in the i2b file. Don't worry if you don't understand everything, I'm going to explain right away:

#!/usr/bin/env ruby

require_relative "int2bin"

print "integer: "
my_number = gets

binary = int2bin(my_number)
puts "binary: #{binary}"

Quickly explaining the code above:

In the very first line we're putting a shebang to tell our OS which interpreter we wan to use to execute the commands in this file, in our case we're telling the OS to use the ruby executable found in the user's PATH (it's not necessary to know all the details here, but if you're curious this article can help).

The print method is just like puts, but it doesn't add a trailing newline. This is useful to keep the cursor right in front of the integer: string.

The gets method is used to get user's input. It returns the data submitted by the user, and we store it in the my_number variable.

The rest of the code should be familiar to you and easy to understand.

In order to run this program, we need to give the executable permission to the file.

chmod a+x i2b

Now we're ready to run it:

$ ./i2b
integer: 

Nice. It's waiting for our input. Let's give it a number.

$ ./i2b
integer: 7
/path/to/int2bin.rb:2:in `to_s': wrong number of arguments (given 1, expected 0) (ArgumentError)
        from /path/to/int2bin.rb:2:in `int2bin'
        from ./i2b:8:in `<main>'

😳 How could this happen? We used TDD to code our function and it passed the tests!

That's time to tell you a truth about Test-Driven Development: TDD is not a way to assure your code does not have bugs.

TDD is a way to facilitate and guide development, giving you short feedback loops (as you don't need to test your software manually) and lead your implementation to a better design.

Although TDD can reduce a lot the appearance of bugs, making sure your code doesn't have bugs is not something TDD can promise.

Debugging with irb

After this kinda frustrating news, let's try to understand what's wrong on our code. Check the main part of the error message:

/path/to/int2bin.rb:2:in `to_s': wrong number of arguments (given 1, expected 0) (ArgumentError)

The message says that the error happened in int2bin.rb:2, which means in the 2nd line of the file.

def int2bin(number)
  number.to_s(2) # 👈 ERROR HAPPENED HERE
end

The message also says that we passed a wrong number of arguments to the to_s method. But in the documentation we saw that Integer#to_s accepts an argument. 🤔 Uhm... Is that number really an Integer?

In order to check that we're going to turn again to one of our best friends: irb.

Ruby provides a way to open an irb session from anywhere in your program using binding.irb. This is helpful for debugging and is exactly what we need now.

Add binding.irb right before the buggy line. Your int2bin.rb should look like this:

def int2bin(number)
  binding.irb
  number.to_s(2)
end

Now let's repeat the steps where we faced the error:

$ ./i2b
integer: 7
From: /path/to/int2bin.rb @ line 2 :

    1: def int2bin(number)
 => 2:   binding.irb
    3:   number.to_s(2)
    4: end

irb(main):001:0>

Now we're on the irb prompt, right before the point where the crash happened. How cool is that?! 🙂

Let's check what exactly is in the number variable:

irb(main):001:0> number
=> "7\n"

👀 That's a String composed of a character 7 followed by a newline. That means that our int2bin function was called with a String as an argument!

Let's check our i2b again, adding some notes:

#!/usr/bin/env ruby

require_relative "int2bin"

print "integer: "
my_number = gets            # 👈 my_number IS ASSIGNED HERE

binary = int2bin(my_number) # 👈 int2bin IS CALLED HERE
puts "binary: #{binary}"

We're assigning a value to my_number with gets, which returns the user's input as a String. When we pass this string to #int2bin it calls String#to_s instead of Integer#to_s. And if we check the String#to_s documentation, we'll see that it doesn't accept an argument. That's why our program is crashing!

This is an example of how Ruby's dynamism is a double-edged sword. It can be powerful and allow rapid development, but also requires extra attention. In this case the lack of type checking allowed us to pass an unexpected data type that crashed our application.

Now, before working in a solution for this bug, we'll apply another valuable testing practice: when you find a bug, replicate it in a test case before fixing it.

NOTE: once we found the bug, we can now remove the binding.irb line from our int2bin.rb code.

Replicate bugs in tests

Let's write a test giving the problematic String to the int2bin function:

def test_convert_7_as_string
  expected = "111"
  actual = int2bin("7\n")
  assert_equal expected, actual
end

Run the test and see if the crash was really replicated:

  1) Error:
TestInt2Bin#test_convert_7_as_string:
ArgumentError: wrong number of arguments (given 1, expected 0)
    /path/to/int2bin.rb:2:in `to_s'
    /path/to/int2bin.rb:2:in `int2bin'
    int2bin_test.rb:19:in `test_convert_7_as_string'

Nice! Now we can start working on a solution and quickly check if we're on the right path.

Fixing the bug

Fortunately we can easily solve this issue by converting the string to an Integer using the String#to_i method (documentation). It's also fortunate that this method is also available for Integers (documentation).

This is an example of how the Ruby's dynamism can promote rapid development. If we were coding with a strongly typed language, we would need to code different functions to allow receiving different data types. With Ruby we can code only one function and work on ways to handle the dynamic typing. As you can notice, everything is a trade-off (and if you're reading until here, you probably like Ruby's dynamism).

In order to fix the bug we just need to chain to_i and to_s:

def int2bin(number)
  number.to_i.to_s(2)
end

Run the tests and they should be passing now.

Key Concepts

Let's recap what we learned in this chapter.

Ruby

• OOP: everything in Ruby is an object
• Dynamic typing: variables can change types at runtime
• String representation: all objects have a to_s method.
• It wasn't explicitly said but you probably noticed: the Rubyist way to write method names is using snake_case.
• Ruby documentation: essential resource of information.
• irb: quickly experiment Ruby code
• binding.irb is a useful debugging technique
• gets: read user's input
• method chaining: calling multiple methods in sequence (e.g.: number.to_i.to_s(2))

TDD

• Test-first approach: write the test before implementation code.
• Minimal implementation: write just enough code to make the tests pass (without being pedantic, please).
• TDD guides the development, but does not assure our software is free of bugs.
• Replicating bugs in tests: add test cases for discovered issues before fixing.