SoftwareDevPro/ruby_cheatsheet.md

## ruby_cheatsheet.md

      
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    Ruby Cheatsheet

General Syntax


Comments start with a pound character, and go to the end of the line
For multi-line comments use "=begin" and "=end"
Expressions are finished with a semi-colon, followed by a new line
Including a backslash () at the end of a line does not terminate the expression

Types

Numbers


123 1_234 123.45 1.2e-3
0xffff (hex) 0b01011 (binary) 0377 (octal)
?a = ASCII character
?\C-a = Control-A
?\M-a = Meta-A
?\M-\C-a = Meta-Control-A

Strings


no interpolation
# (interpolation) and backslashes \n
%q (no interpolation
%Q (interpolation and backslashes)
% (interpolation and backslashes)
echo command intrepretation with interpolation and backslashes
%x (echo command intrepretation with interpolation and backslashes)

* Basic types also include ranges, symbols, arrays, and hashes

Range Examples


1..10
1...10
"a".."z"
"a"..."z"
(1..10) === 5   # true
(1..10) === 10  # false
(1...10) === 10 # false
(1..10) === 15  # false

Reserved Words (can't be used for variable names, identifiers)

alias, and, BEGIN, begin, break, case,
class, def, defined?, do, else, elsif,
END, end, ensure, false, for, if,
module, next, nil, not, or, redo,
rescue, retry, return, self, super, then,
true, undef, unless, until, when, while,
yield, __FILE__, __LINE__
Basic Naming Conventions

# Snake case for file names
customer_import.rb

# Snake case for methods, variables and symbols
first_name = 'Mike'
def display_customer
  # ...
  # ...
end
:light_red

# All uppercase for constants
API_KEY = 'ZZQ112'

# Capital-case for classes and modules
class SupervisingManager
  # ...
  # ...
end
module TechnicalSupport
  # ...
  # ...
end

Variables

Variable Types


$global_variable
@@class_variable
@instance_variable
CONSTANT
::TOP_LEVEL_CONSTANT
SomeClass::CONSTANT
local_variable

Global Constants


TRUE- true value
FALSE - false value
NIL - nil value
STDIN - standard input and default value for $stdin
STDOUT - standard output and default value for $stdout
STDERR - standard error output and default value for $stderr
ENV - hash which contains current environment variables
ARGF - alias to $<. ARGV, meta-IO across all files
ARGV - array of all arguments given on run
DATA - file object of the script
RUBY_VERSION- version string
RUBY_ENGINE - ruby implementation running
RUBY_RELEASE_DATE - release date string for version
RUBY_PLATFORM - platform identifier

Variable Declaration Examples

# Strings
full_name = 'Joe Smith'

# Integers
count = 20

# Floating-point Numbers
book_price = 12.10

# Boolean Values
active? = true
root_user? = false

# Array Types
fruits = [ 'Pineapple', 'Banana', 'Kiwi' ]

# Hash
fruit_color = { banana: 'yellow' }

# Array of hashed-types
customers = [
  { id: 2000, name: 'Joe Smith' },
  { id: 3001, name: 'John Q' },
  { id: 4002, name: 'Sir Harrington' }
]

# Struct Types
Person = Struct.new(:name, :age)
person1  = Person.new 'joe', 22
person2 = Person.new 'mike', 12

# Set to 'Default Value' only if nil or false
title = custom_title || 'Default Value' 

# Perform assignment if null
search ||= params[:search]

# Safe navigation operator &. (skip if nil)
name = customer&.first_name

Pseudo Variables


self - receiver of the current method
nil - instance of class NilClass
true - instance of class TrueClass
false - instance of class FalseClass
__FILE__ - current source file name
__LINE__ - current line number in source file

Pre-defined Variables


DEBUG - boolean status of the -d switch
FILENAME - current input file from ARGF
LOAD_PATH - load path for scripts and binary modules
stderr - current standard error output
stdin - current standard input
stdout - current standard output
VERBOSE - verbose flag, as set by the -v switch
$! - exception object passed to #raise
$@ - stack backtrace generated by last exception raised
$& - string matched by last successful match
$; - string to the left of last successful match
$` - string to the right of last successful match
$+ - highest group matched by last successful match
$1 - The nth group of last successful match
$~ - MatchData instance of last match
$= - flag for case insensitive (defaults to NIL)
$/ - input record separator
$\ - output record separator
$, - output field separator for print and array
$; - default separator for string
$. - current line number for last file from input
$> - default output for print, and printf
$0 - name of script being executed
$$ - process number of ruby running the script
$? - status of last executed child process

Example: Print a string to the screen

# Print with line break
puts 'This string will print on screen with a line break'

# Print with no line break
print 'The string will print on the screen with no line break'

Strings

Common String Methods


+, * - Adds two strings together, repeat the string
length - How long the string is
strip - Removes leading and trailing white space
to_i - Changes a string into a number
upcase, downcase - Changes the case of the string
each_char - Loops through the string returning each character
include? - Returns true if a string is in another string
[]  - Returns character or substring
gsub - Substitutes a new string where a pattern is found

String Method Examples

# Get the length of a string
'This is a string'.length  # 16

# Check to see if the string has no length
'Hello World'.empty?   # false
''.empty?   # true

# Convert all characters to uppercase
'you can do it'.upcase  # YOU CAN DO IT

# Convert all characters to lowercase
'HELLO WORLD'.downcase  # hello world

# Convert first character to uppercase return the rest lowercase
'jOHN'.capitalize  # John

# Remove white space from beginning/end of a string
'  This is a string with space  '.strip 

# Return a string left justified and padded with a character
'strin'.ljust(20, '.')  # 'strin...............'

# Check if a string include specified character(s), case sensitive
'hello World'.include? 'hello'  # true 

# Chaining 2 or more methods together
'Hello World'.downcase.include? 'world' # true

# Finding an index position (start at postion 0)
'Welcome to this web site'.index('web') # 16

# Return string character(s) (indices start at position 0)
'This is a string'[3]     # s
'This is a string'[0..5]  # This i
'This is a string'[-2]    # n (last character)

# Replace first sub string
'Hello son my dog'.sub 'son', 'cat'. # Hello cat my dog

# Replace all sub string
'Hello dog my dog'.gsub 'dog', 'spiders'. # Hello spiders my spiders

# Split a string into an array
'Dog Cat Bird'.split ' '  # ["Dog", "Cat", "Bird"]

# Read in input from the console
input = gets

# Read in input, and eat last character (ie. new line)
input = gets.chomp

# Get command-line arguments (ex. ruby main.rb arg1 arg2)
puts ARGV  # ['arg1', 'arg2']

ARGV.each { |option| puts option }

Numbers

Common Numeric Methods


+, -, *, / - Basic arithmetic
**2 - Exponentiation (for example, raise to the second power)
( ) - Use parentheses to adjust your math operation precedence
even? - Returns true if even
odd? - Returns true if odd
round - Rounds to the nearest integer
upto, downto - Loops up or down from number to another number

Number Examples

2.88.round   # 3
2.88.floor   # 2
2.68.ceil    # 3

3.next  # 4

puts 5 / 2    # 2 (integers with integer result integer)
puts 5 / 2.0  # 2.5 (float with integer result float)

puts 3.even?  # false
puts 3.odd?   # true

# Random number
random_num = rand(1..100000)

Looping


# Basic Loop
loop do
  puts "Stop loop by using 'break' statement"
  break

  puts "Skip one occurence by using 'next' statement"
  next
end

# While Loop
while number < 200
  puts number
  number += 1
end

# Looping over a range
(1..100).each { |i| puts i }
(1..100).each do |i|
  puts i
end

# Looping a specific number of times
10.times { puts "Hello Ruby!" }

Ruby Arrays

Array Access Examples

animals = ['Bird', 'Dog', 'Cat']
animals = %w(Bird Dog Cat) 

animals.length # 3

animals.first  # Bird
animals.last   # Cat

animals[0]     # Bird
animals[-2]    # Dog
animals[3]     # nil
animals[1..2]  # ['Dog', 'Cat']

# Iteration
animals.each do { |animal| puts animal } 

animals.each_with_index do |animal, index|
  puts animal  # Bird (first loop)
  puts index   # 0
end

Array Methods


my_array[1] - Accesses the array in my_array at index 1
length - Returns the length of the array
<<, push - Adds an object at the end of the array
pop - Removes an object at the end of the array
insert - Adds an object at the start of the array
shift - Removes an object at the start of the array
sort - Sorts the array
shuffle - Randomizes positions of objects in the array
sample - Picks a random object from the array
each - Loops over the array, returning each element
join - Combines each element into a string

Array Method Examples

animals.include? 'Bird'  # true

[1, 5, 2, 4, 3].sort  # [1, 2, 3, 4, 5]
[1, 2, 3].reverse  # [3, 2, 1]

animals.push 'Lizard' # append at the end
animals <<  'Reptile' # append at the end
animals.unshift 'Arachnid' # Append in front

animals.pop # remove last
animals.delete_at(0) # remove first element
animals.shift  # remove the first element

animals.join ', '  # 'Bird, Dog, Cat'

# Appending two arrays together
array1 = %w(lizard reptile shark)
array2 = %w(fish hamster)
array3 = array1 + array2 # ["lizard", "reptile", "shark", "fish", "hamster"]

# Concat in the same array
array1.concat array2 
puts array1  # ["lizard", "reptile", "shark", "fish", "hamster"]

# Constructing arrays with * splat operator
puts ['dog', *array2, 'bird']  # ['dog', 'fish', 'hamster', bird']

Conditional Statements

# Equal ==   And &&   Or ||   Not !
if action == 1
  puts "action 1"
elsif action < 5
  puts "action not 1 but less than 5"
else
  puts "action greater than 5"
end

# Unless (negated if statement)
puts 'The user is not deleted' unless deleted == true

# Ternary Operator
deleted ? 'The user is deleted' : 'The user is not deleted'

# Truthy or falsy Values
# false and nil equates to false. 
# Every other object like 1, 0, "" all evaluate to true

# Case When Else (similar to switch statement in other languages)
case score
when 0
  "F"
when 1..50
  "D"
when 51..70
  "C""
when 71..99
  "B"
when 100
  "A"
else
  "Score error"

Casting Between Types (Converting Something To A Different Type)

# Convert number to string "456"
456.to_s
# Convert string to integer 456
"456".to_i 
# Convert string to float 456.0
"456".to_f 

# Creating an array from a range
(1..5).to_a  # [1, 2, 3, 4, 5]
('w'..'z').to_a # ['w', 'x', 'y', 'z']

Hash (Dictionaries)

Hash Methods


contact_hash[“dad”] - Accesses the hash in the contact_hash variable using key “dad” and returns its value
contact_hash[“mom”] = 7878 - Adds new value to the hash stored in the contact_hash variable using key “mom“
length - Returns how many key/value pairs are stored in the hash
include? - Checks if a key exists in the hash
keys - Returns an array of all the keys in the hash
values - Returns an array of all the values in the hash
empty? - Returns true if the hash is empty
each - Loops over each key/value pair in the hash, returning each pair
merge - Combines two hashes into one hash, possibly overwriting the first hash’s keys with the second hash’s keys

Hash Examples

product = {}

product['title'] = "MSI Laptop"
product[:price] = 2599.99

product = { 'title' => 'MSI Laptop', 'price' => 2599.99 }
# {"title"=>"MSI Laptop", "price"=>2599.99}
product = { title: 'MSI Laptop', price: 2599.99 }
# {:title=>"MSI Laptop", :price=>2599.99}}

# Return default value 0
puts product.fetch(:cost, 0)  

product.keys   # [:title, :price]
product.values # ['MSI Laptop', 2599.99]

product.each do |key, value| 
  puts key
  puts value
end

Date and time

Time.now.utc    # 2021-01-20 23:15:40 UTC
Time.now.to_i   # 1611184555

christmas = Time.new(2021, 12, 25)
puts christmas.wday # return 6 (Friday)

now = Time.now  # 2021-01-20 15:16:44 -0800
now.year    # 2021
now.month   # 1
now.day     # 20
now.hour    # 15
now.min     # 16
now.sec     # 33
now.sunday? # false

# Return current time minus 20 seconds
past = Time.now - 60  

# 60 secs * 60 mins * 24 hours (1 Day)
past_day = Time.now - 86400 

# Only works in Rails
past_day = Time.now - 1.day

# Format Time
# %d    Day of the month (01..31)
# %m    Month of the year (01..12) Use %-m for (1..12)
# %k    Hour (0..23)
# %M    Minutes
# %S    Seconds (00..60)
# %I    Hour (1..12)
# %p    AM/PM
# %Y    Year
# %A    Day of the week (name)
# %B    Month (name)

time = Time.new
time.strftime("%d of %B, %Y")    # "20 of January, 2021"

Regular Expressions

Modifiers/Brackets/Metacharacter/Quantifiers


'.' - any character except newline
[set] - any single character of a set
[^set] - any single character not part of a set
'*' - 0 or more previous regular expressions
*? - 0 or more previous regular expressions (non-greedy)
+  - 1 or more previous regular expressions
+? - 1 or more previous regular expressions (non-greedy)
? - 0 or 1 previous regular expression
| - alternation
() - grouping of regular expressions
^ - beginning of a line or string
$ - end of a line or string
#{m,n} - at least M but most N regular expressions
#{m,n}? - at least M but most N regular expressions (non-greedy)
\A - Beginning of a onstring
\b - Backspace (0x08, inside [] only)
\B - Non-word boundary
\b - Word Boundary
\d - Digit, same as [0-9]
\D - Non-digit
\S - Non-whitespace character
\s - Whitespace character [ \t \n \r \f]
\W - Non-word character
\w - Word character [0-9, A-Za-z_]
\z - End of a string
\Z - End of a string or before newline at the end
(?#) - Comment
(?:) - Grouping without back references
(?=) - Zero-width positive look-ahead assertion
(?ix-ix) - Turns on/off i/x options, localized in the group, if any
(?ix-ix:) - Turns on/off i/x options, localized in non-capturing group

Regular Expression Examples

# Define a regular expression
zip_code = /\d{5}/

# Attempt to match on "Hello"
"Hello".match zip_code  # nil

# Use the zip_code regex to extract the white house zip code
"White House zip: 20500".match zip_code  # <MatchData "20500">

# Use the zip_code regex to extract all zip codes
"White House: 20500 and Air Force: 20330".scan zip_code # ['20500', '20330'] 

# Split a string into an array using a space regex
"Apple Orange Banana".split(/\s+/) # ['Apple','Orange', 'Banana']

Special Character Classes


[:alnum:] - Alpha-numeric characters
[:alpha:] - Alphabetic characters
[:blank:] - Whitespace
[:cntrl:] - Control characters
[:digit:] - Decimal digits
[:graph:] - Graph characters
[:lower:] - Lower-case characters
[:print:] - Printable characters
[:punct:] - Punctuation characters
[:space:] - Whitespace including tabs, carriage returns, and more
[:upper:] - Upper-case characterscharacters
[:xdigit:] - Hexadecimal digits

Function Declaration Examples

def greeting(name = 'Joe')
  "Hello #{name}"  # implicit return
end
puts greeting('Paul')  # Hello Paul

# Variable number of arguments
def greeting(*names)
  names.each { |name| puts "Hello #{name}" }
end

# Named parameters
def display_product(price, options = {})
  puts price, options[:hidden], options[:rounded], options[:total]
end
display_product 2599, hidden: false, rounded: false, total: 1222

Map, Select, Detect and Reduce

# Map (return a modified array)
names = [ 'joe', 'pete', 'jake' ]
names_capitalized = names.map do |name|
  name.capitalize
end
# ['Joe', 'Pete', 'Jake']

# Short hand version using map
names_capitalized = names.map { |name| name.capitalize }

# Symbol to Ruby proc
names_capitalized = names.map &:capitalize

# Select (return all matches)
products = [
  { name: 'MSI Laptop', active: true, price: 2599.99 },
  { name: 'Samsung Galaxy S', active: false, price: 199.99 },
  { name: 'Kindle Fire', active: true, price: 149.99 },
]
expensive_products = products.select { | product | product[:price] > 1000 }

# Detect (returns first match)
first_inactive_product = products.detect { | product | ! product[:active] }

# Reduce (returns one)
total = products.reduce(0) do |total, product| 
  total = total + product[:price]
end
puts total.round(2)  # 2949.97

Module Examples

# Basic Module
module Display
  def hello_world
    puts 'Hello World'
  end
end

# Mix in
require_relative 'display.rb'
class Customer
  include Display
end
Customer.new.hello_world

# Using module as a namespace
module Person
  class Customer
    def initialize(name)
      @name = name
    end
  end
end
customer = Person::Customer.new('Joe Smith')

# Constants in modules
module Contact
  ACCESS_KEY = 'zzz222'
  class Person
      ACCESS_KEY = 'xyz211'
  end
end
puts Contact::ACCESS_KEY
puts Contact::Person::ACCESS_KEY

# Private methods in modules or classes
module Display
# ...
def initialize
  greeting
end

private
  def greeting
    puts 'hello'
  end
end

OOP (Object Oriented Programming)

# Basic Class Declaration
class Item
# ...
# ...
end

# Object Instantiation
item = Item.new 

# Basi class declaration with constructor and instance variables
class Item
  def initialize(name, cost, active)
    @name = name
    @cost = cost
    @active = active
  end
end
item = Item.new 'MSI Laptop', 2599, false

# Attribute Accessor (get/set)
class Item
  attr_accessor :name, :cost  # read/write
  attr_reader   :name         # read only
  attr_write    :cost         # write only
  ...
end
...
puts item.price  # 2599

# instance method
class Item
  # ...
  def cost_with_tax
    # self keyword is optional (10% tax_percent)
    self.cost + (self.cost * tax_percent / 100)
  end
end
# ...
puts item.price_with_tax # 2858.9

# Static class method and static class variable
def self.calc_tax(amount)
 @@count = 1
end
puts Item::calc_tax(1599.99)

# Class Inheritance
class Customer < Person
  attr_accessor :number

  def initialize(name, number)
    super(name)
    @number = number
  end
end

Errors / Exceptions Handling

# Raise an error/exception and output an error message
raise "This is an error"

# Inspect variable value
raise items.inspect # [{:id=>100, :name=>"MSI Laptop"},{:id=>200, :name=>"Samsung Phone"}]

# Exception Handling
begin
  # Any exceptions happen here ex. 0 / 1, division by zero
  0 / 1 
rescue
  # when the exception is raised, this code will run
  puts "Exception"
  handle_exception()
end

# Receive/Handle Exception object
begin
  0 / 1
rescue ZeroDivisionError => e
  puts e.class.name
  puts e.message
end

Files

File Methods


File.join(p1, p2, ..., pN) => "p1/p2/.../pN" (platform independent paths)
File.new(path, mode_string = "r") => file
File.new(path, mode_num, [, perm_num]) => file
File.open(filename, mode_string = "r") { |file| block } => nil
File.open(filename [,mode_num [, perm_num]]) { |file| block => nil }
IO.foreach(path, sepstring = $/) { |line| block }
IO.readlines(path) => array

File Mode Strings


"r" - R/O, start of file (default)
"r+" - R/W, start of file
"w" - W/O, truncates or creates
"w+" - R/W, truncates or creates
"a" - W/O, end of file or creates
"a+" - R/W, end of file or creates
"b" - binary file (DOS/Windows platform)

File I/O Examples

# Read in a file
text = File.read('test.txt')

# Read a file in by lines
lines = File.readlines("test.txt")
lines.each do |line|
  puts "Line: #{line}"
end

# Write to a file
File.write('test.txt', 'write something...')

File.open(“home.html”, “w”) do | file | 
  file.puts ‘text to write’ 
end

# Read in a CSV file
require 'csv'
table = CSV.parse(File.read("data.csv"), headers: true)
table[0]["id"]   # 1200
table[0]["name"] # "MSI Laptop"

# Write to a CSV file
items = [
  { name: "MSI Laptop", price: 2599 },
  { name: "Android Samsung Phone", price: 999 }
]

CSV.open("data.csv", "w", headers: items.first.keys) do |csv|
  items.each { |item| csv << item.values }
end