Learn Ruby from Scratch

These notes are taken from this Educative online course, which I audited for free.

1. Why should you learn Ruby?

Programming is Creation

Motivate yourself to bring your code to life!

when you run a program, a universe is created where things come to life and interact with each other
as the creator, you define the rules
e.g. creating an application like Twitter
- create users
- users can create tweets
- users can follow each other
- each time new users are created in your application, users have the ability to tweet and follow each other

2. Variables

What are Variables?

This lesson gives an insight into variables and variable assignment.

Naming Things

variables: a way to assign names to objects that our program deals with

Variable Assignment

in Ruby, you can assign a name to an object by using the assignment operator =
```
puts number = 1
```

Output

this assigns the name number to the object that is the number 1
- after, we can refer to this object by using the name number

number = 1
puts number

Output

Variable is not a "thing"

a variable is not an object by itself
- it's just a name for an object
in the example, the number 1 is an object, while number is a name for it because we've assigned it

a = 1
puts a

large_number = 1
puts large_number

apples = 1
puts apples

Output:

1
1
1

try and pick names that reveal your intention
from the example, using a, large_number, and apples as names would be frowned upon because they aren't meaningful names and don't match the object
a variable name is like a post-it note with the name number written on it and stuck on the object, the number 1

Reusing Variable Names

This lesson explains how variable names can be reused in Ruby

a name is unique
- the same name can only be assigned to one object at a time
assigning different values to the same variable results in later assignments overwriting previous ones

variable names can be re-used and re-assigned

number = 4
number = number * 3
puts number + 2

Output

continuing the post-it metaphor, this would stick a post-it with the name number on an object, and then later take it off and stick it on another object

Explanation

Ruby creates the number 4 on the first line and assigns the name number to it
on the second line, Ruby evaluates the expression number * 3, resulting in a new number 12 that is assigned to the number variable
on the third line Ruby adds 2 to the number object, resulting in 14
Ruby passes 14 to puts, which outputs it to the screen
you can write the exact code with puts 4 * 3 + 2
however, using variable names can be useful to break up long lines and make code more expressive and readable
this is a local variable and is the one that's used most often

Things on the Right go First

This lesson describes how Ruby figures out the expression on the right first.

Example

number = 2 + 3 * 4
puts number

Ruby needs to know what the object on the right is before it can assign the name number to it
Ruby first evaluates the expression, which results in the number 14
Ruby then assigns the name number to the object
the code temporarily looks like number = 14 before the assignment = is evaluated

Does this make sense?

on the second line, Ruby passes the number variable (which is 14) to puts, which outputs to the screen

Types of Variables and their Usage

Get to know various kinds of variables in Ruby. Moreover, learn how and when to use them.

Definition and Usage

Naming

Scope

Initialization

Global Variables

Global variables start with a $ sign.

Their scope is global which means that they can be accessed from anywhere in a program.

There’s no need to initilize. Uninitialized global variables have the value nil.

Local Variables

Local variables begin with a lowercase letter or underscore(_).

The scope of a local variable ranges from class, module, def, or from a block’s opening brace to its close brace {}, i.e within its block of initialization.

No need to initialize. An uninitialized local variable is interpreted as methods with no arguments.

Instance Variables

They start with an @ sign.

Their scope is limited to one instance of a class.

There’s no need to initilize. Uninitialized instance variables have the value nil.

Class Variables

Class variables start with an @@ sign.

Their scope is limited to the class in which they are created.

They need to be initialized beforehand, otherwise they’ll result in error.

Examples

Global Variables

$global_var = "GLOBAL"  
non_global_var = "NON-GLOBAL"

def method1 
  puts "Global variable is #$global_var"   
end 

def method2 
  puts "Non-global variable is ", non_global_var    
end 

method1
method2

$global_var is a global variable since it is preceded by a $ sign
- it can be accessed from anywhere in the program
non_global_var is a simple variable
- cannot be accessed from inside the method2 so it gives an "undefined local variable or method" error

Output

Global variable is GLOBAL

main.rb:9:in `method2': undefined local variable or method `non_global_var' for main:Object (NameError)
    from main.rb:13:in `<main>'

Local Variables

var = "Hello World"
def foo 
  var = 1.5
  puts "Value of var in foo : ", var
end 

foo
puts "Value of var outside 'foo' method:", var

there are two variables named var with different scopes
the var inside the method has the local scope

Output

Value of var in foo : 
1.5
Value of var outside 'foo' method:
Hello World

Instance Variables

class Employee   
   def initialize(name)   
      @employee_name = name     
   end   

   def print()   
     puts "Employee name: #@employee_name"   
   end   
end   

# Create Objects   
e1 = Employee.new("Emma")   
e2 = Employee.new("David") 
e3 = Employee.new("Harris") 

# Call Methods   
e1.print()   
e2.print()   
e3.print()

an instance variable starts with an @ sign
it belongs to only one instance of the class
an uninitialized instance variable will have a nil value

Output

Employee name: Emma
Employee name: David
Employee name: Harris

Class Variables

class Employee   
   @@no_of_employees = 0   
   def initialize(name)   
      @employee_name = name   
      @@no_of_employees += 1   
   end   

   def total_no_of_employees()   
     puts "Total number of employees: #@@no_of_employees"   
   end   
end   

# Create Objects   
e1 = Employee.new("Emma")   
e2 = Employee.new("David") 
e3 = Employee.new("Harris") 

# Call Methods   
e1.total_no_of_employees()   
e2.total_no_of_employees()   
e3.total_no_of_employees()

a class variable starts with a @@ sign
it should be initialized before use
it belongs to the whole class and can be accessed from anywhere in the class
a class variable is shared by all the descendants of the class
- its value will change for every instance of the class

Output

Total number of employees: 3
Total number of employees: 3
Total number of employees: 3

Quick Quiz on Variables!

The quiz is located here.

3. Built-in Classes: Numbers

Working with Numbers

This lesson goes through the types of number evaluations in Ruby.

🔢 Numbers are Simply Numbers

you can create a number by writing it e.g. 123
negative numbers are created by prepending a minus -
you can create decimal numbers by writing them e.g. 12.34
you can use an underscore - to separate thousands places e.g. 1_234.56 which is equal to 1234.56
a number is defined by a series of digits, using a dot as a decimal mark, and optionally an underscore as a thousands separator

Kinds of Numbers

there are different kinds of numbers
- integer numbers aka "integers"
- floating point numbers aka "floats"
if you do a calculation with integer numbers you'll always get an integer back

> 1 + 2
=> 3

if there is a float, you'll get a float back

> 1.0 + 2
=> 3.0
> 1 + 2.0
=> 3.0

mathematical operations result in a floating point number except if all numbers used are integer numbers
this is important when you do a division (/ means "divide by")

> 3 / 2
=> 1

decimal places will be cut off
- use floating point (decimal) numbers when doing divisions

> 3.0 / 2
=> 1.5
> 3 / 2.0
=> 1.5

Exercise 1: Playing with Numbers

Problem Statement

In the exercise below, calculate:

how many hours are in a year?
how many minutes are in a decade?
how to convert an age given in years seconds

Try it yourself

def playing_with_numbers(hours_in_year, minutes_in_decade, age)
  hours_in_year = 24 * 365
  minutes_in_decade = 60 * hours_in_year * 10
  age_in_seconds = age * hours_in_year * 60 * 60


  return hours_in_year, minutes_in_decade, age_in_seconds 
end

3 of 3 Tests Passed

Exercise 2: Guess the Type?

The quiz is located here.

Exercise 3: Finding Modulo

Problem Statement

Find out what "modulo" means by asking Google and then calculate the modulo of two numbers given as an input.

Try it yourself

def calculate_mod(num1, num2)
  result = num1 % num2
  
  return result
end

5 of 5 Tests Passed

Exercise 4: Even or Odd?

Check whether a number is even or odd.

Problem Statement

Use a method from the documentation to find out if given numbers are odd or even.

def even_or_odd(num)
  result = num.even? ? "even" : "odd"
  
  return result
end

5 of 5 Tests Passed

4. Built-in Classes: Strings

Working with Strings

This lesson will teach you how you can operate on strings in Ruby.

🔤 A String, in programming languages, is text.

a String is an object that represents a specific
the most simple and common way to create Strings is to enclose some characters in quotes
- you can use single or double qoutes

"This is a string"
'This is another one'

Things you can do with Strings

you can stick them together by using +
- this is called concatenation

> "snow" + "ball"
=> "snowball"

> "hi" + "hi" + "hi"
=> "hihihi"

you can replicate the last operation using *

> "hi" * 3
=> "hihihi"

multiplying a String by a number in Ruby means repeating the String as many times

> "1" + "1" + "1"
=> "111"

> "1" * 3
=> "111"

Ruby behaves the same for Strings that contain nothing but numbers
some other String methods:

> "hello".upcase
=> "HELLO"

> "hello".capitalize
=> "Hello"

> "hello".length
=> 5

> "hello".reverse
=> "olleh"

the last

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