Intro to Ruby

Follow Along
http://j.mp/nicar-13-ruby

Schedule

1. Set up the room
2. Variables, types, conditional, iteration
3. Methods and classes
4. RubyGems, APIs, Scraping

Set up the room.

ruby yourfile.rb
  
$ irb
irb(main):001:0> puts "hello"
hello
=> nil

Intro.

Programming is 3 things

Assignment, Conditional, Iteration

Assignment

name = "Al"

Conditional

if name == "Al"
  puts "Hello #{name}"
end

Iteration

names = ["Al", "Joe", "Sally"]
names.each do |name|
  puts "#{name} is here!"
end

Let's go!

Strings & Integers

address  = 55
street   = "Broadway"
city     = "New York"
state    = "NY"
zip_code = "10003"

city + ", " + state
#=> "New York, NY"

"#{city}, #{state}"
#=> "New York, NY"

"#{address} #{street}"
#=> "55 Broadway"

address + " " + street
#=> TypeError: String can't be coerced into Fixnum
  from (irb):10:in `+'
  from (irb):10

address.to_s + " " + street
#=> "55 Broadway"

Type coercion

5.to_s    #=> "5"
"5".to_i  #=> 5
5.to_f    #=> 5.0

str = ""
str << city
str << ", "
str << state

#=> "New York, NY"

str = ""
str += city
str += ", "
str += state

#=> "New York, NY"

address += address
#=> 110

= vs. ==

a = 3
b = 3

a == b
#=> true

a = 3
b = 3

if a == b
  puts 'they match'
end

a = 1
b = 2

if a != b
  puts "They don't match!"
end

a = 3
b = 3

if a == b
  puts "they match"
else
  puts "they don't match"
end

Arrays & Hashes

[]

students = ["Amy", "Joe", "Sally", "Harry"]

students.pop
#=> "Harry"

students
#=> ["Amy", "Joe", "Sally"]

students.shift
#=> "Amy"

students
#=> ["Joe", "Sally"]

students << "Amy"
#=> ["Joe", "Sally", "Amy"]

students[0]
#=> "Joe"

students[1]
#=> ?

students[1]
#=> "Sally"

person = {}

person = {}
person[:first_name] = "Al"
person[:last_name]  = "Shaw"

person = {
  :first_name => "Al",
  :last_name  => "Shaw"
}

person[:first_name]
#=> "Al"

people = []
people.push(person)
#=> [{:first_name=>"Al", :last_name=>"Shaw"}]

Iteration

students.each do |student|
  puts "#{student} is in class today"
end

#=>
Joe is in class today
Sally is in class today
Amy is in class today

even_numbers = [2, 4, 6, 8, 10]
odd_numbers  = []
# how can we programmatically 
# fill up this array with odd numbers

even_numbers.each do |n|
  odd_numbers << n + 1
end

odd_numbers
#=> [2, 4, 6, 8, 10]

even_numbers.map do |n|
  n + 1
end
#=> [3, 5, 7, 9, 11]

even_numbers.map { |n| n + 1 }
#=> [3, 5, 7, 9, 11]

even_numbers
#=> [2, 4, 6, 8, 10]

even_numbers.map! { |n| n + 1 }
#=> [3, 5, 7, 9, 11]

Methods

def add(a, b)
  a + b
end

add(1, 2)
#=> 3

# Who's in class today?
def is_in_class(student)
  attendance = {
    "Amy"   => true,
    "Joe"   => true,
    "Sally" => false,
    "Harry" => true
  }

  if attendance[student]
    puts "here"
  else
    puts "absent"
  end
end

# Try it
is_in_class("Sally")
#=> Absent
is_in_class("Joe")
#=> Here

Classes are factories for objects

class Student
  attr_reader :name, :grade
  def initialize(name, grade)
    @name  = name
    @grade = grade
  end
end

s = Student.new("Sally", 8)
#=> #<Student:0x1052aca28 @name="Sally", @grade=8>

s.name
#=> "Sally"

Why should I care?

Let's build a system that assigns students to teachers!

class Student
  attr_reader :name, :grade, :teacher
  def initialize(name, grade)
    @name  = name
    @grade = grade
  end

  @@teachers = {
    "Shanahan"  => 5,
    "Kovacek"   => 5,
    "Streich"   => 5,
    "Ankunding" => 5,
    "Marks"     => 5
  }

  def assign_teacher
    @@teachers.each do |t_name, slots|
      if slots > 0
        @teacher = t_name
        @@teachers[t_name] -= 1
        puts "#{@name} is assigned to #{t_name}"
        puts "#{t_name} now has #{slots - 1} slots left"
        return
      end
    end
  end
end

s = Student.new("Sally", 8)
#=> #<Student:0x1052aca28 @name="Sally", @grade=8>

s.assign_teacher
Sally is assigned to Streich
Streich now has 4 slots left

s.teacher
#=> "Streich"

Everything in Ruby is an object

1.+(2)
#=> 3

class Fixnum
  def +(num)
    self - num
  end
end

1 + 2
#=> -1

I just blew your mind

A useful example:

[1,2,3].sum
NoMethodError: undefined method `sum' for [1, 2, 3]:Array
  from (irb):1

class Array
  def sum
    self.reduce(0) {|memo, it| memo += it; memo}
  end
end

[1,2,3].sum
#=> 6

RubyGems are bits of code that add functionality to Ruby

$ gem install json

require 'rubygems'
require 'json'

Let's use the JSON gem to play with the Facebook API

require 'rubygems'
require 'json'
require 'open-uri'

JSON.parse(open("http://graph.facebook.com/ashaw").read)

{"gender"=>"male",
 "id"=>"2902652",
 "locale"=>"en_US",
 "username"=>"ashaw",
 "first_name"=>"Al",
 "last_name"=>"Shaw",
 "name"=>"Al Shaw",
 "link"=>"http://www.facebook.com/ashaw"}

Finally, let's use Nokogiri to do some simple scraping

$ gem install nokogiri

require 'rubygems'
require 'nokogiri'
require 'open-uri'

FEC.gov

url = "http://query.nictusa.com/cgi-bin/dcdev/forms/ \
C00490045/766953/sa/11AI"

doc = Nokogiri::HTML(open(url).read)

names = []
donors = doc.css("table tr td[align=LEFT]")
donors.each do |donor|
  name = donor.css("b")[0].content
  names << name
end

names

#=>
["Marc Andreessen",
 "Michael Armstrong",
 "Alan Ashton",
 "Lynda Balfour",
 "Frank Baxter",
 "Glen Beck",
 "James Berardinelli",
...
]

Thank you. Questions?

Al Shaw    NICAR 2013    @A_L