ruby-intro.md

!SLIDE subsection

Introduction to Ruby for Programmers

This section is intended as a brief, lightweight overview of the Ruby language; following sections will cover all these topics in much more detail. Students are encouraged to ask questions, but instructors are encouraged to answer, "We'll cover that later."

Ruby vs. Rails

Ruby is a Language

Rails is a Framework

Rails is written in Ruby

Ruby Philosophy

Q: Did you have a guiding philosophy when designing Ruby?

A: Yes, it's called the "principle of least surprise."

I believe people want to express themselves when they program.

They don't want to fight with the language.

Programming languages must feel natural to programmers.

I tried to make people enjoy programming and concentrate on the fun and creative part of programming when they use Ruby.

  - Matz (Yukihiro Matsumoto), Ruby creator

Ruby Philosophy, Applied

• Ruby has a humane interface
  • many ways to do things
• Ruby favors readability and variety over concision and perfection
• sometimes makes code hard to understand (but usually makes it easier)
• contrast to minimal interface
  • one (or a few) "right" ways to do things
  • Python has a minimal philosophy

Many Rubies

• Ruby 1.0 released in 1996
  • Fully Open Source

Versions common today

• MRI 1.8.7
• MRI 1.9.2 or 1.9.3
• JRuby

Ruby Language Overview

• Interpreted
• Dynamically typed
• Object oriented
• Blocks / lambdas / closures
• Perl-like regular expressions
• Closely tied to shell & OS

IRB: Interactive RuBy

@@@ ruby
$ irb
>> 4
=> 4
>> 4+4
=> 8

Please fire up irb on your computer and try this out right now!

Every statement has a value

@@@ ruby
>> 2 + 2
=> 4

>> (2+2).zero?
=> false

>> "foo" if true
=> "foo"

>> "foo" if false
=> nil

>> puts "foo"
foo
=> nil

Optional semicolons, parens, and return

These are equivalent:

@@@ ruby
def inc(x)
  return x + 1;
end

def inc x
  x + 1
end


def inc(x); x + 1; end

def inc(x) x + 1; end

Line Break Gotcha

@@@ ruby
x = 1 + 2
x #=> 3

x = 1
  + 2
x #=> 1

Solution: always put operators on top line

x = 1 +
    2
x #=> 3

Use parens when you need them

@@@ ruby
>> "Hello".gsub "H", "h"
=> "hello"

>> "Hello".gsub "H", "h".reverse
=> "hello"

>> "Hello".gsub("H", "h").reverse
=> "olleh"

Ruby has hash comments, like perl

@@@ ruby
# is a comment
2 + 2 # is a comment

Ruby has a syntax for multiline comments too, but it's silly and nobody uses it.

!SLIDE

Variables are declared implicitly

@@@ ruby
first_name = "Santa"
last_name = "Claus"
full_name = first_name + last_name
#=> "SantaClaus"

!SLIDE custom

Built-in Types

• Numbers
  • 42 (Fixnum)
  • 3.14159 (Float)
• Booleans
  • true
  • false
• Strings
  • "apple"
  • 'banana'
• Symbols
  • :apple

Built-in Types (cont.)

• Regular Expressions
  • /fo*/i
• Arrays
  • ["apple", "banana"]
• Ranges
  • (1..10)
• Hashes
  • {:apple => 'red', :banana => 'yellow'}
  • {apple: 'red', banana: 'yellow'}

String interpolation

@@@ ruby
"boyz #{1 + 1} men"
=> "boyz 2 men"

• Any Ruby code can go inside the braces
• It gets evaluated and stuck inside the string

equal, double-equal, and threequal

• x = 1 means "put the value 1 in the variable x"
• x == 2 means "true if x is 2, otherwise false"
• x === 3 means the same as == but sometimes more
  • threequal is rarely used

Ruby syntax cheatsheet

(The Well-Grounded Rubyist, p. 5, section 1.1.2)

Functions

@@@ ruby
def add a, b
  a + b
end

add 2, 2
#=> 4

• Note: no 'return' required
• def add(a, b) is also legal

Interlude

Are you sick of hearing me speak?

If so, do a lab: 01_temperature is right up your alley.

Classes and methods

@@@ ruby
class Calculator
  def add(a,b)
    a + b
  end
end

calc = Calculator.new
calc.add(2, 2)
#=> 4

• a function inside a class is called a method

Classes

• A class defines a group of behaviors (methods)
• Every object has a class, Object if nothing else

Messages and Methods

• an object is referenced by a variable or a literal
• the dot operator (.) sends a message to an object
• an object receives a message and invokes a method
• with no dot, the default object (self) is the receiver

bang and question mark methods

• method names can end with ! or ?
  • ? means "boolean"
  • ! means "watch out!"

Ruby Naming Conventions

methods and variables are in snake_case

classes and modules are in CamelCase

constants are in ALL_CAPS

Standard is better than better.

-- Anon.

Ruby Identifiers

• local_variable - start with letter or underscore, contain letters, numbers, underscored
• @instance_variable - start with @
• @@class_variable - start with @@
• $global_variable - start with $
• Constant or CONSTANT - must start with uppercase letter
• ClassName - capitalized camel case
• method_name? - like a local variable, but can end with ? or ! or =
• keywords - about 40 reserved words (def) and weirdos (__FILE__)
• literals - "hi" for strings, [1,2] for arrays, {:a=>1, :b=>2} for hashes, etc.

Variable Scopes

@@@ ruby
var   # local variable (or method call)
@var  # instance variable
@@var # class variable
$var  # global variable
VAR   # constant

load and require

• load inserts a file's contents into the current file
• require makes a feature available to the current file
  • skips already-loaded files
  • omits the trailing .rb
  • can also be used for extensions written in C (.so, .dll, etc.)

Credits

• "Ruby Intro" slides based on Ruby Quickstart for Refugees by Jacob Rothstein
• Improved by Alex Chaffee, Sarah Allen, Wolfram Arnold