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Programming Fundamentals in Ruby

You've already learned the basics of programming with JavaScript, and you've begun to think like a problem-solver. You've had practice reading JS code and predicting execution by evaluating expressions in your mind.

We'll continue to reinforce the basics of programming, but this time with Ruby. This may seem like a lot of material to cover in a short time, but the truth is our task is simpler than it was when we introduced JS. Instead of teaching you how to program, we'll focus on the differences between Ruby and JS with the goal of utilizing the foundation we've already built.

"Polyglot" is a term used to refer to someone who can use two or more programming languages. By learning two languages, we increase your understanding of basic programming concepts, as well as give you an edge in the job market.

You will reference this material again and again over the next few weeks. Focus on noting the differences between Ruby and JS. You should use this material as you would the HyperPolyglot reference: not as reading material, but as a handy place to define and experiment with the basics of Ruby.



By the end of this, developers should be able to:

  • Run Ruby code using Ruby REPL and interpreter.
  • Identify basic language features and types in Ruby.
  • Write a fizzbuzz script in Ruby.
  • List and use common operators in Ruby.
  • Identify operators in an expression and explain what they do.


  1. Fork and clone this repository. FAQ
  2. Create a new branch, training, for your work.
  3. Checkout to the training branch.
  4. Install dependencies with bundle install.


In order to accomplish our learning objectives, we've got quite a number of Ruby features to cover. These include:

  • Running scripts from the command line.
  • Evaluating code interactively in a REPL.
  • Variable declaration and naming conventions.
  • Strings, interpolation, and concatenation.
  • Methods and functions, including predicates.
  • Fixnums, Floats, and Numbers.
  • Falsiness.
  • Flow control.
  • Loops, ranges, and enumerable iteration.
  • Implicit and explicit returns.
  • Expression evaluation and conditional assignment.
  • Type coercion.
  • Logic, shortcut evaluation, and operator precedence.


The depth and breadth of the Ruby Core and Standard Library are so extensive that we'll always want to check them before building something ourselves. The official Ruby docs are our friends, and we should use them liberally. Over time, we'll learn to remember the more common methods, but even then it can be extremely useful to consult the documentation.


gem install bundler
gem install pry

Pry is a console-based REPL for working with Ruby. Simply type in an expression into Pry, and it returns the result (preceded by =>, also known as a 'hash rocket').

[1] pry(main)> 1 + 1
=> 2
[2] pry(main)>

In addition to allowing us to run code one line at a time, Pry can also be used (like the node-debug REPL we saw in Unit 1) to interactively debug our code. Though we won't be focusing on that aspect of Pry today, it'll be a critical tool throughout the rest of this unit.

Pry is packaged within a Ruby gem, meaning it is a library or module that is not a part of the core Ruby library. You have to install the Pry gem to use it. One way is to run the gem install pry command from the command line.

However, since we have pry listed in our project's Gemfile, we only need to do bundle install, which installs each gem in the Gemfile, including Pry. Using Bundler and the Gemfile is the preferred way to manage and install dependencies for your Ruby projects.

Now that Pry is installed, you can start using it by typing pry at the command line:

$ pry
[1] pry(main)>

You can now execute commands. To quit, type quit (or exit or ctrl-d).

Let's use pry to explore some of the fundamentals of the Ruby language.

Ruby :: Core Syntax, Variables, and Operators


Numbers in Javascript compared to Ruby are very similar with some small differences.

In Javscript there was really only one official type of number, but in Ruby there is a different type if a number is a whole integer (no decimals) or a float (contains decimals).

Integers: 1, 23, and -10000 Decimals: 3.14, -808.08 and 12.043e-04

Commas are not allowed in numbers, but underscores are. So if you feel the need to mark your thousands so the numbers are more readable, use an underscore.

population = 12_000_000_000


We have been writing JavaScript without semicolons; however, when searching the internet for help, you most likely have encountered semicolons in solutions. In Ruby, there is a conspicuous lack of semicolons.

[1] pry(main)> 1
=> 1

The end of a line (almost always) marks the end of an expression; semicolons are only required if you have two distinct expressions on one line (e.g. name = "Antony"; height_in_feet = 6). The most likely place where you might spot a semicolon in Ruby is inside a for loop, and those (as you'll soon see) are used very infrequently in Ruby.

Variable Declaration

Ruby handles variables differently than we've previously seen. In Ruby, variables can be simply defined, without previously being declared. This means that with Ruby, we don't need keywords like let and const before variables. We can simply declare the variable and assign it a value variable = value.

[1] pry(main)> a = 1
=> 1

However, this only works if we assign the variable a value. Why? Because otherwise, Ruby will default to trying to evaluate your variable, and because you haven't defined it yet, Ruby will throw an error.

[1] pry(main)> counter
NameError: undefined local variable or method 'counter' for main:Object
from (pry):1:in '__pry__'
[2] pry(main)> counter = 0
=> 0
[3] pry(main)> counter
=> 0
[4] pry(main)>

Ruby has its own set of scoping rules for variables, just like JavaScript does, and they work in (mostly) similar ways.


In Ruby, everything is an expression - a statement composed of a combination of operands (data) and operations. In JavaScript, things like + and - are true operators - keywords built into the language itself, and imbued with fixed, unchangeable meanings. In Ruby, in contrast, most "operators" you encounter are actually method calls on some object; the main exceptions are assignment operators (e.g. =), logical operators (e.g. ||, &&, !), and control flow operators (e.g. and, or, not).

Brief Aside: Syntactic Sugar

Ruby doesn't have an increment operator, either pre (++i) or post (i++). Use += instead.

pry(main)> counter = 0
=> 0

pry(main)> counter += 1
=> 1

pry(main)> counter += 1
=> 2

pry(main)> counter
=> 2

counter += 1 is really just Ruby making you type fewer characters to accomplish counter = counter + 1. This is commonly referred to as 'syntactic sugar' - when a programming language has syntax that's deliberately designed to make code shorter/more semantic/easier to write. Ruby has a ton of syntactic sugar. JavaScript allows us to use this shorthand for assignment too. Just like with JavaScript and other languages, you can combine assignment with many different operators like *, -, and even ||.

[1] pry(main)> counter ||= 0          # counter = counter || 0
=> 0
[2] pry(main)> counter += 1           # counter = counter + 1
=> 1
[3] pry(main)> counter *= 5           # counter = counter * 5
=> 5
[4] pry(main)> counter -= 1           # counter = counter - 1
=> 4

Lab: Pry

In pairs, open up pry and take five minutes trying out the operators we've used in JavaScript on numbers and numeric variables. Does anything surprising or confusing happen? Write those things down to share with the class. Have a look in operator_examples.rb if you need some prompts.

Ruby :: Strings

To see all the methods that strings have in Ruby, open up pry, type a string followed by a '.', and hit tab; alternatively, you can call "some string".methods.sort for a full list. And, of course, the Ruby documentation has a full list as well.

Strings objects come with several conversion methods that all start to_ and then a letter or abbreviation hinting at what conversion they perform. to_i and to_f are used commonly - these convert the string into one of two types of numbers, integers (whole numbers) and floats (decimal numbers).

String Interpolation

Ruby attributes different meanings to single-quoted and double-quoted strings. Single-quoted strings are referred to as 'string literals'; they interpret their contents as a literal sequence of characters, with only two recognized escape sequences - \' and \\. Double-quoted strings, in contrast, support a much wider variety of escape characters, including \n (new line), \t (tab), and \s (space); if \n appeared in a single-quoted string, it would be interpreted as the character \ followed by the character \n, rather than a new line.

One neat thing that comes out of this is the ability to do string interpolation, inserting variables directly into the middle of a string.

Ruby doesn't implicitly convert numbers to strings, so all those string conversions need to be done manually using .to_s.

[1] pry(main)> name = "Lauren"
=> "Lauren"
[2] pry(main)> height_in_feet = 5
=> 5
[3] pry(main)> puts height_in_feet
=> nil
[4] pry(main)> puts name + " is " + height_in_feet + " feet tall."
TypeError: no implicit conversion of Fixnum into String
from (pry):3:in '+'
[5] pry(main)> puts name + " is " + height_in_feet.to_s + " feet tall."
=> "Lauren is 5 feet tall."

Fortunately, Ruby does give us an alternative - at least with double-quoted strings. If the sequence #{ ... } appears inside a double-quoted string, Ruby will replace it with the value of the expression inside the curly braces, converted to a string.

So instead of:

[1] pry(main)> name = "Lauren"
=> "Lauren"
[2] pry(main)> height_in_feet = 5
=> 5
[3] pry(main)> name + " is " + height_in_feet.to_s + " feet tall."
=> "Lauren is 5 feet tall."

We can use:

[1] pry(main)> name = "Lauren"
=> "Lauren"
[2] pry(main)> height_in_feet = 5
=> 5
[4] pry(main)> "#{name} is #{height_in_feet} feet tall."
=> "Lauren is 5 feet tall."

This also works:

[5] pry(main)> "ten + seven == #{10 + 7}"
=> "ten + seven == 17"

Lab: String Methods

In your pairs, go to the Ruby documentation for strings (link above), and look up three of the methods available to Ruby strings. Open up pry and test them out on some sample strings. Try to incorporate string interpolation at least once. Once you're done, we'll reconvene as a class and discuss the methods we've explored.

Ruby :: Flow Control


# Ruby version
if name == "Jason"
  puts "It's Jason"
elsif name == "Lauren"
  puts "It's Lauren"
  puts "Not Jason or Lauren"

A Ruby if looks quite similar to a JavaScript if. Some of the major differences are:

  • In Ruby, we use elsif, not else if.

  • Conditions don't require parentheses (though they can still accept them).

  • No curly braces required. Simply break up your condition from your code with a newline (as above), a semicolon, or the keyword then (e.g. if....then).

  • The end of the if is indicated by the keyword end. end is an extremely common keyword in Ruby, appearing at the end of pretty much any contiguous section of code.

unless can sometimes be used to replace an if with a negated test and no elsif or else.

if !(name == "Jason")
  puts "Not Jason!"


unless name == "Jason"
  puts "Not Jason!"


Similarly to if, a while loop also looks almost the same in Ruby as it does in JavaScript.

i = 3
while i < 10 do
  puts i += 1

The do ... end is a common construction in Ruby because it specifies what's known as a block, a grouping of several lines of code. We'll learn more about blocks soon.

for loops in Ruby exist, but are not commonly used. Instead, we use the upto enumerator. We'll learn more about enumerators soon.

1.upto(max) do |i|
  # code to execute in loop

Code Along: upto

In our pry consoles, let's enter:

1.upto(10) do |i|
  puts i

What do you expect to print? What does print?

Lab: FizzBuzz

This time, rather than using pry, we're going to write a longer program in Atom, and then run it in the terminal using ruby, a command line Ruby environment. Open up the file fizzbuzz.rb; in pairs, you're going to solve "FizzBuzz", a simple programming challenge based on a childrens' game. Essentially, your program should print out all of the numbers from 1 up to max_num, which is a variable to which you can assign an arbitrary (positive, integer) value. However, if a number is divisible by 3, instead of printing the number, your program should print the word "fizz"; for numbers divisible by 5, it should print "buzz"; for numbers divisible by both 3 and five, it should print "fizzbuzz".

For example:

max_num = 16

## what should print to the console


To run your code, simply navigate to the root of this repository and run ruby lib/fizzbuzz.rb

Running a script in this way should seem familiar, since it's exactly what we were doing with node in Unit 1. It's a deliberate similarity - Node was modeled off of other console-based runtime environments, as a way of giving JavaScript a solid platform for running on the server side.

Ruby :: Methods

Ruby draws no distinction between functions that are properties of objects and functions that aren't; in Ruby, all of them are called 'methods'.

To define a method, you use the following syntax:

def square? (num)
  Math.sqrt(num).to_i**2 == num

The question mark is conventional for methods that return a boolean. Another common convention in Ruby is a trailing exclamation point, which indicates that a method is a 'mutator' - this means that the method changes the object that it is called from, rather than returning a new object.

This behavior is also sometimes referred to as operating 'in place'.

Ruby methods use an implicit return - by default, they will return the value of the last expression evaluated (which may or may not be a return expression). However, Ruby does also have a return keyword which, as it does in JavaScript, immediately terminates the function/method and sends back a value. In the case of the method above, square? will return the value of that last expression, Math.sqrt(num).to_i**2 == num.

Lab: Fizzbuzz Method

Take your code from the previous exercise and turn it into a method called fizzbuzz; this method should accept an argument, max_num.

At the end of your program, add the following two lines:


binding.pry inserts a breakpoint into the program; this will cause the program to stop and let us look around. The second line, with the empty string, is only necessary because binding.pry won't work if it's the last line in your program - it needs to stop before something else.

Once you're finished writing your method, run the program with ruby fizzbuzz.rb; the console should take you to pry, allowing you to read from (and even write to) your program. Once there, try calling your fizzbuzz method with the following arguments : 10, 15, 30, 50. Does your code work like you'd expect?

You can also verify your code is working by running

bundle exec rspec spec/fizzbuzz_method_spec.rb

Ruby :: Collections


Arrays in Ruby are almost identical to arrays in JavaScript, down to the square braces.

[1] pry(main)> my_array = ["a","b","c"]
=> ["a","b","c"]
[2] pry(main)> my_array[0]
=> "a"
[4] pry(main)> my_array[2] = "z"
=> "z"
[5] pry(main)> my_array
=> ["a","b","z"]


A Ruby hash acts somewhat like a dictionary (or object) in JavaScript, in that it consists of pairs of keys and values.

[1] pry(main)> dict = {}
=> {}
[2] pry(main)> dict["a"] = 23
=> 23
[3] pry(main)> dict["a"]
=> 23

However, there are a couple of important differences. For instance, Ruby hashes do not allow you to access their keys through a dot notation; you must use square braces.

We can also define an object with keys and values already in it:

nums = {
  "odds"  => [1, 3, 5],
  "evens" => [2, 4, 6]

In the example above, our hash has the keys "odds" and "evens" in quotes, which map to values [1, 3, 5] and [2, 4, 6] respectively. We use the hash rocket symbol here to link our keys to their values.

Lab: FizzBuzz with Hashes

In pairs, use your current fizzbuzz code to help you create a new method called fizzbuzz_hash. In this method, create a hash containing keys "fizz", "buzz", "fizzbuzz", and "other", each with arrays as values. As you iterate through all the numbers from 1 to max_num, add each number to one of the arrays mentioned above; numbers divisible by 3 only should go into the "fizz" array, numbers divisible by 5 only should go into the "buzz" array, numbers divisible by both should go into the "fizzbuzz" array, and numbers divisible by neither should go into the "other" array. Finally, once you're done, return the hash as the result of fizzbuzz.

Run your code from the console using ruby lib/fizzbuzz.rb.

This time, you can verify your code is working by running

bundle exec rspec spec/fizzbuzz_hash_method_spec.rb

Common Gotchas When Learning Ruby After JavaScript

  • == and === mean different things between the two languages. In JavaScript, === is a 'strict equality' comparator, while == is a 'loose equality' comparator; since == has some weird exceptions, the convention is to almost always use ===. In Ruby, however, the reverse is true; you should == to test for equality, and not use === (which does something different).

  • Use .equal? if we need to test for identity (two variables that reference the same object).

  • Ruby has several different numeric types (unlike JavaScript), but most operations "do what we expect".

  • Booleans: Only false and nil are falsy in Ruby. Everything else is truthy.

  • We don't need to use parentheses when invoking a method (as we saw above with the + method - one exception later). But sometimes they add clarity, so it can be beneficial to include them.

  • The Ruby comment character is #. Everything following a # on a line is ignored by the interpreter.

  • p, [$stdout.]puts, [$stdout.]print are not directly analogous to console.log but are often used for a similar purpose when writing scripts run from the terminal.

  • Ruby's convention is to use underscores between words in names (a.k.a. 'snake_case'). Constants start with a capital letter.

  • Use Atom tab completion to avoid the common, and hard to find error of writing def method_name and forgetting the closing end. Good indentation will help with this as well.

  • Ruby doesn't implicitly convert numbers to strings.

Additional Resources


  1. All content is licensed under a CC­BY­NC­SA 4.0 license.
  2. All software code is licensed under GNU GPLv3. For commercial use or alternative licensing, please contact