Ruby Cheatsheet

Basics

$ irb: to write ruby in the terminal
don't use ' in ruby, use " instead
you can replace most {} with do end and vice versa –– not true for hashes or #{} escapings
Best Practice: end names that produce booleans with question mark
CRUD: create, read, update, delete
[1,2].map(&:to_i)
integer: number without decimal
float: number with decimal
tag your variables:
- $: global variable
- @: instance variable
- @@: class variable
1_000_000: 1000000 –– just easier to read*

Vars, Contants, Arrays, Hashes & Symbols

Variables

my_variable = "Hello"
my_variable.capitalize! # ! changes the value in place
my_variable ||= "Hi" # ||= is a conditional assignment only set the variable if it was not set before.

Constants

MY_CONSTANT = # Throws warning, not error if changed

Arrays

# Create an Array
letters_arr = ("a".."f").to_a  # ["a","b","c","d","e"]
names_arr = %w[seph laura carl]  #=> ["seph", "laura", "carl"]
nums_arr = (1..10).to_a   # [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

# Access and Info
letters_arr.first # "a" see also .last
letters_arr[1]  # "b"
letters_arr[2..-1]  # ["c", "d", "e"] called a slice, takes a range or a comma seperated starting poing and num of elements to take [1, 2]
multi_d = [[0,1],[0,1]]
[1, 2, 3] << 4  # [1, 2, 3, 4] -> same as [1, 2, 3].push(4)
letters_arr.size = #=> 6

# Tail end
letters_arr.pop()  # ["a", "b", "c", "d"] removes last -> "e"
letters_arr.push("e")  # ["a", "b", "c", "d", "e"]

# Beginning
letters_arr.shift()  # ["b", "c", "d", "e"] removes first
letters_arr.unshift("a")  # ["a", "b", "c", "d", "e"]

# Remove/Keep selection
letter = letters_arr.delete("b")  # returns and deletes first instance, see also .keep_if
scores_arr = [97, 42, 75]
scores_arr.delete_if { |score| score < 80 }  # [97] others deleted in place
scores_arr.select{ |score| score < 80 }  # [42, 75]

# Randomize
nums_arr.sample  #=> 7
nums_arr.sample(4)  #=> [6, 4, 2, 5]
num = nums_arr.delete(nums_arr.sample)  # returns and deletes a random element

# Iteration
names_arr.each { |n| puts e.capitalize! }  # iterates through each element and applies the block
names_arr.each_with_index{|n, i| puts "#{n} placed number #{i+1}"}
(1..4).map { |i| i*i }  #=> [1, 4, 9, 16] returns new array


TODO
.any?
.bsearch
.flatten
.join
.size
.uniq

Hashes

# Create a hash
empty_hash = {}
also_empty_hash = Hash.new
default_hash = Hash.new(0)  # Any value accessed but not set will defualt to 1 (great for histograms). Can also retrieve w/ h.default
string_hash = { "key1" => "value1", "key2" => "value2" }  # Using strings and rockets
symbol_hash = { key1: "value3", key2: "value4" }  # Using symbols and colons

number_hash = {}
number_hash["one"] = 1
number_hash["two"] = 2
number_hash["three"] = 3
number_hash => {"one": 1, "two": 2, "three": 3}

# Accessing Values
string_hash["key1"]  #=> value1
symbol_hash[:key1]  #=> value3
default_hash[:non_existent_key]  #=> 1

# Adding / Removing Values
also_empty_hash[:red_bull] = "Gives you wings"

# Iteration
number_hash.each_key {|k| print k, " "} #=> key1 key2
number_hash.each_value {|v| print v, " "}  #=> 1 2 3
number_hash.select {|k,v| v > 1} # selects all keys in hash that have a value greater than 1  #=> {"two"=>2, "three"=>3}
number_hash.each {|k,v| print v ** 2, " "} #=> 1 4 9 See also: .each_pair

#*** IRL Use ***
number_hash.each do |k,v|
  number_hash[k] += 1 if number_hash.has_key? k
end
#=> {"one"=>2, "two"=>3, "three"=>4}

num_arr = []
100.times {num_arr << rand(1..10)}

num_arr.each do |v|
  empty_hash[v] ? empty_hash[v] += 1 : empty_hash[v] = 1
end
#=> {4=>13, 1=>9, 5=>10, 10=>10, 9=>9, 3=>13, 8=>9, 6=>17, 2=>5, 7=>5}

# GOOD UNTIL HERE

TODO
.has_value?
.member?
.merge
.delete
.delete_if
.keep_if
.key?
.value?

Symbols

:symbol # symbol is like an ID in html. :Symbols != "Strings"
# Symbols are often used as Hash keys or referencing method names.
# They can not be changed once created. They save memory (only one copy at a given time). Faster.
:test.to_s # converts to "test"
"test".to_sym # converts to :test
"test".intern # :test
# Symbols can be used like this as well:
my_hash = { key: "value", key2: "value" } # is equal to { :key => "value", :key2 => "value" }

Functions to create Arrays

"a b c d".split(" ") # takes string and returns an array ["a", "b", "c", "d"] great for CSV

Methods

Methods

def greeting(hello, *names) # *name is a splat argument, takes several parameters passed in an array
  return "#{hello}, #{names}"
end

start = greeting("Hi", "Justin", "Maria", "Herbert") # call a method by name

def name(variable=default)
  ### The last line in here get's returned by default
end

Classes

custom objects

class ClassName # class names are rather written in camelcase
  @@count = 0
  attr_reader :name # make it readable
  attr_writer :name # make it writable
  attr_accessor :name # makes it readable and writable

  def Methodname(parameter)
    @classVariable = parameter
    @@count += 1
  end

  def self.show_classVariable
    @classVariable
  end

  def Person.get_counts # is a class method
    return @@count
  end

  private

  def private_method; end # Private methods go here
end

matz = Person.new("Yukihiro")
matz.show_name # Yukihiro

inheritance

class DerivedClass < BaseClass; end # if you want to end a Ruby statement without going to a new line, you can just type a semicolon.

class DerivedClass < Base
  def some_method
    super(optional args) # When you call super from inside a method, that tells Ruby to look in the superclass of the current class and find a method with the same name as the one from which super is called. If it finds it, Ruby will use the superclass' version of the method.
      # Some stuff
    end
  end
end

# Any given Ruby class can have only one superclass. Use mixins if you want to incorporate data or behavior from several classes into a single class.

Modules

module ModuleName # module names are rather written in camelcase
  # variables in modules doesn't make much sence since modules do not change. Use constants.
end

Math::PI # using PI constant from Math module. Double colon = scope resolution operator = tells Ruby where you're looking for a specific bit of code.

require 'date' # to use external modules.
puts Date.today # 2016-03-18

module Action
  def jump
    @distance = rand(4) + 2
    puts "I jumped forward #{@distance} feet!"
  end
end

class Rabbit
  include Action # Any class that includes a certain module can use those module's methods! This now is called a Mixin.
  extend Action # extend keyword mixes a module's methods at the class level. This means that class itself can use the methods, as opposed to instances of the class.
  attr_reader :name
  def initialize(name)
    @name = name
  end
end

peter = Rabbit.new("Peter")
peter.jump # include
Rabbit.jump # extend

Blocks & Procs

Code Blocks

Blocks are not objects A block is just a bit of code between do..end or {}. It's not an object on its own, but it can be passed to methods like .each or .select.

def yield_name(name)
  yield("Kim") # print "My name is Kim. "
  yield(name) # print "My name is Eric. "
end

yield_name("Eric") { |n| print "My name is #{n}. " } # My name is Kim. My name is Eric.
yield_name("Peter") { |n| print "My name is #{n}. " } # My name is Kim. My name is Eric. My name is Kim. My name is Peter.

Proc

saves blocks and are objects A proc is a saved block we can use over and over.

cube = Proc.new { |x| x ** 3 }
[1, 2, 3].collect!(&cube) # [1, 8, 27] # the & is needed to transform the Proc to a block.
cube.call # call procs directly

Lambdas

lambda { |param| block }
multiply = lambda { |x| x * 3 }
y = [1, 2].collect(&multiply) # 3 , 6

Diff between blocs and lambdas:

a lambda checks the number of arguments passed to it, while a proc does not (This means that a lambda will throw an error if you pass it the wrong number of arguments, whereas a proc will ignore unexpected arguments and assign nil to any that are missing.)
when a lambda returns, it passes control back to the calling method; when a proc returns, it does so immediately, without going back to the calling method. So: A lambda is just like a proc, only it cares about the number of arguments it gets and it returns to its calling method rather than returning immediately.

Calculation

Addition (+)
Subtraction (-)
Multiplication (*)
Division (/)
Exponentiation (**)
Modulo (%)
The concatenation operator (<<)
you can do 1 += 1 –– which gives you 2 but 1++ and 1-- does not exist in ruby
"A " << "B" => "A B" but "A " + "B" would work as well but "A " + 4 + " B" not. So rather use string interpolation (#{4})
"A #{4} B" => "A 4 B"

Comment

=begin
Bla
Multyline comment
=end

# single line comment

Conditions

If Statement

if 1 < 2
  puts “one smaller than two”
elsif 1 > 2 # *careful not to mistake with else if. In ruby you write elsif*
  puts “elsif”
else
  puts “false”
end
# or
puts "be printed" if true
puts 3 > 4 ? "if true" : "else" # else will be putted

Unless Statement

unless false # unless checks if the statement is false (opposite to if).
  puts “I’m here”
else
  puts “not here”
end
# or
puts "not printed" unless true

Case (switch)

case my_var
  when "some value"
    ###
  when "some other value"
    ###
  else
    ###
end
# or
case my_var
  when "some value" then ###
  when "some other value" then ###
  else ###
end

&&: and
||: or
!: not
(x && (y || w)) && z: use parenthesis to combine arguments
problem = false
print "Good to go!" unless problem –– prints out because problem != true

Printing & Putting

print "bla"
puts "test" # puts the text in a separate line

String Methods

"Hello".length # 5
"Hello".reverse # “olleH”
"Hello".upcase # “HELLO”
"Hello".downcase # “hello”
"hello".capitalize # “Hello”
"Hello".include? "i" # equals to false because there is no i in Hello
"Hello".gsub!(/e/, "o") # Hollo
"1".to_i # transform string to integer –– 1
"test".to_sym # converts to :test
"test".intern # :test
:test.to_s # converts to "test"

User Imput

gets # is the Ruby equivalent to prompt in javascript (method that gets input from the user)
gets.chomp # removes extra line created after gets (usually used like this)

Loops

While loop:

i = 1
while i < 11
  puts i
  i = i + 1
end

Until loop:

i = 0
until i == 6
  puts i
  i += 1
end

For loop

for i in 1...10 # ... tells ruby to exclude the last number (here 10 if we .. only then it includes the last num)
  puts i
end

Loop iterator

i = 0
loop do
  i += 1
  print "I'm currently number #{i}"= # a way to have ruby code in a string
  break if i > 5
end

.each

things.each do |item| # for each things in things do something while storing that things in the variable item
  print “#{item}"
end

on hashes like so:

hashes.each do |x,y|
  print "#{x}: #{y}"
end

.times

10.times do
  print “this text will appear 10 times”
end

.upto / .downto

10.upto(15) { |x| print x, " " } # 10 11 12 13 14 15
"a".upto("c") { |x| print x, " " } # a b c

Sorting & Comparing

array = [5,4,1,3,2]
array.sort! # = [1,2,3,4,5] – works with text and other as well.
"b" <=> "a" # = 1 – combined comparison operator. Returns 0 if first = second, 1 if first > second, -1 if first < second
array.sort! { |a, b| b <=> a } # to sort from Z to A instead of A to Z

Usefull Methods

1.is_a? Integer # returns true
:1.is_a? Symbol # returns true
"1".is_a? String # returns true
[1,2,3].collect!() # does something to every element (overwrites original with ! mark)
.map() # is the same as .collect
1.2.floor # 1 # rounds a float (a number with a decimal) down to the nearest integer.
cube.call # implying that cube is a proc, call calls procs directly
Time.now # displays the actual time

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