Ruby Programming Concepts

What is a Programming Language?

A programming language gives instructions to a machine. It does more than a markup language, like RST or HTML, because it has logic that it can use to manipulate data. Programming in a markup language, you can create a website with static content, wherein you must change your source in order to change what is displayed. But using a programming language you can create a dynamic website, which can take input from a user or another program and use it to change what is displayed.

Install Ruby

Ruby is installed using RVM, the Ruby Version Manager. Cloud9 provides a pre-installed emulation of RVM, which we will use.

First, log in to Cloud9 and on your Dashboard in the left sidebar click Create New Workspace.

Open a terminal window in the rails sample app’s Cloud9 project workspace. In the Terminal window, run the following command to check that RVM is working, and also choose a ruby version to use in this project:

rvm use 1.9.3

Executing Ruby

There are multiple ruby execution modes: script or program files (program.rb), interactive ruby commands (irb), and embedded ruby (script.erb).

Ruby Files

Example Ruby files

Create an edit a file, hello.rb. Enter the text and save the file:

#!/usr/bin/ruby
puts 'Hello world'

Then click the Run button in the menu bar.

Ruby program files use the extension .rb. Any ruby file may be executed with the command:

ruby filename

To avoid calling the ruby interpreter explicitly, a “shebang” statement may be added in the first line of the file:

#! /usr/bin/ruby

IRB

Example interactive Ruby

Open a Cloud9 terminal window, and type:

irb

IRB, or interactive Ruby, is an interactive environment for Ruby scripting. IRB is a Ruby command line, run inside the system console. Exit IRB back to the shell command prompt by typing exit.

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Variables

A variable is a named space in memory that stores some data.

A variable is identified by its name.

Variable names can include underscores but not spaces.

A variable exists within a scope, and is deleted by a garbage collector when it is no longer needed.

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Assigning variables

Look for a variable named x:

x

Assign a value to x:

x = 1

Assignment

To create a variable in ruby, you assign it an initial value. The = in ruby is known as the assignment operator. It is used like: variable_name = variable_value

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Data Types

Using data types

Assign a boolean:

x = true

Assign a string:

x = "This is a string"

Assign an integer:

x = 1

Assign a float:

x = 0.1

Assign an array:

x = ["Item 1", "Item 2", "Item 3"]

Assign a hash:

x = {:name => "variable x", :value => 1}

Type a variable by assignment:

x.is_a? Hash
x = 1
x.is_a? Hash

Ruby uses the following basic types for variables:

• Boolean
• String
• Integer
• Decimal / Float
• Array
• Hash

The type of a variable is determined by syntax of the assigned data value. Ruby is a dynamically typed language. That means that changing the value of a variable implicitly changes its type, since a variable inherits its data type from its value.

Datatype	variable syntax
Boolean	unquoted true or false
String	“Text enclosed in quotes”
Integer	A number without a decimal point
Float	A number with a decimal point
Array	A comma-separated list of values enclosed by square brackets
Hash	A comma-separated key-value pair enclosed by curly braces

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Arithmetic

Arithmetic operations

Enter the statements:

x = 1
x - 0 * 2

Result: 1

Enter the statement:

(x - 0) * 2

Result: 2

Ruby does all of the normal arithmetic operations on integer and decimal/float variables.

Operation	Operand	Sample Syntax
Addition	+	x + 2
Subtraction	-	x - 2
Multiplication	*	x * 2
Division	/	x / 2
Exponentiation	**	x ** 2
Modulus	%	x % 2

The order of operations is the same as in regular math. Parentheses, which group operations, have the highest order of precedence.

Assignment Operators

Arithmetic assignment

Enter the statements:

x = 2
x -= 1

Result: 1

Enter the statement:

x *= 2

Result: 2

Assignment may be combined with a single arithmetic operator which performs the given arithmetic operation on the variable, and assigns the result of that operation to the new value of the variable, replacing the old value.

For example:

Operand	Initial x	Sample Use	Result x
+=	1	x += 2	3
-=	3	x -= 2	1
*=	1	x *= 2	2
/=	2	x /= 2	1
%=	1	x %= 1	0

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String Concatenation

String concatenation

In IRB, create a string variable called “formatted_x” that uses the value of the integer variable “x” in it:

formatted_x = "$" + x + ".00"

Note that the string literals to be concatenated are surrounded by quotes, and anything not surrounded by quotes represents a variable or method’s value.

Changing the value of x will not update the value of formatted_x, which is a separate variable. A method is required to auto-update formatted_x every time that x is changed. Read on to find out how later.

Note

The + operator is overloaded in Ruby to provide two operations: addition of numbers, and concatenation with strings.

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Comparison Operators

Sample comparisons

Create some variables describing a fast food order:

sandwich = "cheeseburger"
condiments = ["pickles", "mustard", "ketchup"]
combo = true
size = "medium"
drink_upsize = true

Now place an order:

order = {
  :sandwich => sandwich,
  :condiments => condiments,
  :combo => combo,
  :size => size,
  :drink_upsize => drink_upsize
}

Does this order include a combo and an upsized drink?:

combo && drink_upsize

Now change the variable values, then compare with and:

drink_upsize = false
combo && drink_upsize

Now compare using or:

combo || drink_upsize

Results: true, false, true

Comparison operators compare two values, and return a boolean result of true or false. Ruby syntax for standard comparisons are listed following:

Operation	Operand	Sample Syntax
Equal To	==	x == y
Not equal To	!=	x != y
Greater than	>	x > y
Less than	<	x < y
Equal or Greater	>=	x >= y
Equal or Lesser	<=	x <= y
Boolean and	&&	x && y
Boolean or	||	x || y

Is equal to

Comparisons with Is equal to

Compare a boolean variable’s value to a boolean:

combo == true

Compare two boolean variables:

combo == drink_upsize

Comparing a boolean to a string:

combo == "true"

Compare two strings:

sandwich == "burger"

The Is Equal To comparison can be used on two values of any type to determine if they equal each other.

Warning

Do not confuse == with =!

Note

Ruby has additional comparison operators for particular programming uses:

Operation	Operand	Result
Math comparison	<=>	1 if greater than 0 if equal to -1 if less than
threequal method	===	when clause equality
.eql? method	.eql?	equal type and values
equal? method	equal?	equal object ids

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Conditionals

If...Else

For example:

if order[:sandwich] == "cheeseburger"
  puts "You ordered a cheeseburger"
end

if order[:sandwich] == "cheeseburger"
  puts "You ordered a cheeseburger!"
else
  puts "You didn't order a cheeseburger."
end

If...Else

Conditional statements are the simplest form of branching in coding. The first condtional is the if...else. It’s used to say: if this codition is true, do one thing, if it’s not, do another thing.

First, the interpreter checks to see if the conditional statement after the if is true. If it is, the code in the if section is run, and if it is false, the interpreter moves on to the code in the else section.

The structure of the if...else statement is:

if conditional statement
  code to run if conditional returns true
else
  code to run if conditional returns false
end

Notice you always need an end after any conditional statement, to let the interpreter know where the code that belongs to your branch ends. If you don’t have it, you will get an error.

Elsif

Check if either a burger or cheeseburger was ordered:

if order[:sandwich] == "cheeseburger"
  puts "You ordered a cheeseburger!"
elsif order[:sandwich] == "burger"
  puts "You ordered a burger!"
end

Add a branch for the case that something else was ordered:

if order[:sandwich] == "cheeseburger"
  puts "You ordered a cheeseburger!"
elsif order[:sandwich] == "burger"
  puts "You ordered a burger!"
else
  puts "You didn't order a burger at all."
end

If...Elsif

Using elsif (short for “else if”, and used only in Ruby) allows you to add more than one conditional to the same statement as your if or if...else.

The conditions are checked in order, and when a condition is met, the code in that branch is executed, then the block is exited, so no other conditions are checked.

Unless

Use unless with a comparison statement:

unless order[:combo] == false
  puts "You get fries and a drink with that sammy!"
end

Use unless with a boolean variable:

unless order[:drink_upsize]
  puts "You get a" + order[:size] + " fries and drink with that sammy!"
end

Unless

Unless is just the opposite of if. Use it when you want some code to run unless this one particular condition is met.

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Ternary Operator

The ternary operator `` ? : `` is shorthand syntax for an if...else statement.:

(order[:sandwich] == "burger") ? puts "Burger" : puts "Not a burger!"

First, write a statement that will return a boolean, such as a conditional. Follow it with a question mark. The first thing after the question mark is the code to execute if the statement returns true. Then there is a colon, and then the code to be executed if the statement in the parentheses returns false.

Methods & Blocks

Example Method

Write a method that decides what drink comes with a combo. Then call the method.:

def pick_drink(combo, combo_size)
  if combo == true
    "What kind of drink do you want with that combo?"
    drink_input = gets
    drink = size + drink_input
    puts "Your drink is a #{drink}."
  end
end
pick_drink(combo, size)

Methods are used to run multiple lines of code without having to repeat them.

Variables can be passed in to methods using parentheses. In the method definition, you give the variable a name to be used within the method. Then, when the method is called, any value passed in that place will be used as the value of the variable. If a method is defined to take variables in Ruby, then you must pass values in to that method for those variables, or you will get an argument error.

Methods are assigned in Ruby like so:

def method_name
  ...
end

Methods can be called on objects in Ruby using object notation: object.method.

Method Scope and Return

If a variable is defined inside of a method, it is limited to the scope of that method, meaning that the variable cannot be accessed anywhere outside of that method.

A method must be defined within the current scope before it can be called.

To pass data from inside of a method to outside of it, use return. If no return is specified in a method, the method will return the value of the last statement by default.

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Nil and Blank

Nil represents something that has not yet been given a value. When used in a conditional, nil is interpreted like false. However nil is not equal to false or 0. To check whether a variable has a value or not, use object.nil?.

Blank represents nil or an empty string. The object.blank? method can be used to check whether a variable is either an empty string or nil.

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Loops

Loops allow you to repeat a line or block of code.

Do loop

Do something 3 times:

3.times do
  x += 1
  puts "x is " + x
end

Do something for each object in an array:

condiments.each do |condiment|
  puts condiment
end

Do Loop

The do loop is used to repeat code a specified number of times.

Do can be used in combinatio with the each method to iterate over an array or collection of objects.

While Loop

The while loop repeats code as long as a given condition is true.:

x = 1
while x < 5 do
  puts x
  x += 1
end

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Get User input

Use gets to take user input from the console in Ruby.

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Coding Best-Practices: KISS, DRY

These acronyms are handy for remembering two important coding concepts:

Keep It Simple Stupid

and

Don’t Repeat Yourself

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Put it all together

Take an Order

Combine variables, loops or methods, conditionals, comparison, and user input to make a program that takes an order and responds to the user:

puts "What kind of sandwich would you like?"
    sandwich = gets.strip!
    puts "Ok. Would you like a combo? (yes or no)"
    combo_answer = gets.strip!

    def assign_combo(combo_answer)
      if combo_answer.downcase == "yes"
            combo = true
      elsif combo_answer.downcase == "no"
            combo = false
      else
            puts "Please type yes or no:"
            combo_answer = gets.strip!
            assign_combo(combo_answer)
      end
    end
    combo = assign_combo(combo_answer)

    if combo == true
      puts "Ok, what size combo would you like?"
      size = gets.strip!
    end
    puts "Ok, you ordered a #{combo ? size + ' ' + sandwich + ' combo' : sandwich}. Thank you!"

See if you can write a ruby script in a .rb file that will take an order when it is run.

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Footnotes

1. Wiki books guide to Ruby Programming