The One Ruby Reference

This document should be used as a quick reference for Ruby; for more depth, click through to the “Full Explanation”s.

How to interpret this reference

This reference is (with a few exceptions) organized by Ruby Class, e.g. String, Array, Hash, etc. Click the firstdraft icon in the top-left corner of the page to slide out the table of contents, scan it, and find a section that interests you. Usually, each section describes a Ruby method. For example, here’s what the section String#split looks like:

  • String#split => Array is called the method signature:
    • String since the method is defined for that class.
    • # since it is an instance method (as opposed to a class method, in which case the notation would be .).
    • => Array since the return value of this method is an instance of the class Array.
  • Then a description of the method.
  • Then examples.
  • Then, usually, a link to the longer explanation from the original chapter where the method was first introduced.

Please let us know if you find errors, omissions, or have any other suggestions or comments. Thank you!

ActiveRecord_Relation (array of records)

All Array methods

ActiveRecord_Relations inherit from Array, so all Array methods work:

.where

ActiveRecord_Relation#where(Hash) ⇒ ActiveRecord_Relation

Call .where on an ActiveRecord_Relation to filter the array of records based on some criteria.

where returns an ActiveRecord_Relation based on the given Hash.

  • .where returns an ActiveRecord_Relation regardless of how many results there are. In particular, even if there is only one result, the return value will still be another ActiveRecord_Relation.

  • The argument to .where is usually a Hash:

    • Filtering by an exact match within a column:

      { :id => 42 }

    • Filter by multiple columns at once:

      { :photo_id => 23, :user_id => 42 }

    • Filter using an Array (results will match any of the values in the Array):

      { :owner_id => [4, 8, 15 ] }

    • Filtering within a range of values:

      { :dob => (1.year.ago..Date.today) }

      { :last_name => ("A".."C") }

    • If you really want to, you can omit the curly brackets around the Hash for brevity, and Ruby will figure out what you mean. So, ultimately, you can write something like this:

      Contact.where(:last_name => ("A".."C"))

      instead of this:

      Contact.where({ :last_name => ("A".."C") })

Related Methods: .where.not, .or, .limit, .order

Full explanation

where.not

ActiveRecord_Relation#where.not(Hash) ⇒ ActiveRecord_Relation

Call .where.not on an ActiveRecord_Relation to filter the array of records to exclude records based on the given Hash. .where.not will return an ActiveRecord_Relation.

Contact.where({ :last_name => "Mouse" }).where.not({ :first_name => "Mickey" })
  • Returns an ActiveRecord_Relation.
  • The acceptable arguments to .not are the same as to .where; see that method for a list.

Related methods: .where, .order, .limit

Full explanation

.or

ActiveRecord_Relation#or(ActiveRecord_Relation) ⇒ ActiveRecord_Relation

Call .or on an ActiveRecord_Relation to combine the array of records with another array of records:

  • Returns an ActiveRecord_Relation.
  • The argument to .or must be another ActiveRecord_Relation from the same table.
  • Broadens the result set.
Contact.where({ :first_name => "Mickey" }).or(Contact.where({ :last_name => "Betina" }))

Full explanation

.order

ActiveRecord_Relation#order(Hash) ⇒ ActiveRecord_Relation

or

ActiveRecord_Relation.order(Symbol) ⇒ ActiveRecord_Relation

Call .order on an ActiveRecord_Relation to sort the array based on one or more columns. This method returns an ActiveRecord_Relation.

Contact.all.order({ :last_name => :asc, :first_name => :asc, :date_of_birth => :desc })

  • Returns an ActiveRecord_Relation.

  • The argument to .order is usually a Hash.

    • The keys in the Hash must be Symbols that match names of columns in the table. These are the columns that will be used for sorting. If there are multiple keys, the order in which they are provided will be used to break ties.
    • The value associated to each key must be either :asc (for ascending order) or :desc (for descending order).

  • The argument to .order can also just be one Symbol, a column name.

Contact.all.order(:last_name)

In that case, :asc order is assumed.

Full explanation

.limit

ActiveRecord_Relation#limit(Integer) ⇒ ActiveRecord_Relation

Call .limit on an ActiveRecord_Relation to cap the number of records in the array.

Contact.where({ :last_name => "Mouse" }).limit(10)

Full explanation

.offset

ActiveRecord_Relation#offset(Integer) ⇒ ActiveRecord_Relation

Call .offset on an ActiveRecord_Relation to discard the first few records in the array:

Contact.where({ :last_name => "Mouse" }).offset(10).limit(10)

Full explanation

.map_relation_to_array

ActiveRecord_Relation#map_relation_to_array(Symbol) ⇒ Array

Call .map_relation_to_array on an ActiveRecord_Relation to retrieve the values stored in just one column of the records, and discard all other data.

  • .map_relation_to_array returns a regular Ruby Array of scalar values in the column.

    • Not a single value, even if there was only one record in the ActiveRecord_Relation.
    • Not an ActiveRecord_Relation, so you can no longer use methods like .where, .order, etc. You can use Array methods like .sort, .sample, etc.

  • The argument to .map_relation_to_array must be a Symbol that matches the name of a column in the table.

  • You cannot call .map_relation_to_array on an individual ActiveRecord row. If you want the value in a column for an individual row, simply call the accessor method directly:

Contact.all.map_relation_to_array(:last_name) # => ["Betina", "Mouse", "Woods"]

# for an array of records
people.last_name # undefined method for array; bad
people.map_relation_to_array(:last_name) # => ["Betina", "Woods"]; good

Full explanation

.maximum

ActiveRecord_Relation#maximum(Symbol) ⇒ Object

Call .maximum on an ActiveRecord_Relation to grab and return the biggest value of a particular column.

User.all.maximun(:age)
# => 102
  • Returns a value from an ActiveRecord column.

Full explanation

.minimum

ActiveRecord_Relation#minimum(Symbol) ⇒ Object

Call .minimum on an ActiveRecord_Relation to grab and return the smallest value of a particular column.

Photo.all.minimum(:caption) 
# => "... a mind needs books as a sword needs a whetstone, if it is to keep its edge."
  • Returns a value from an ActiveRecord column.

Full explanation

.sum

ActiveRecord_Relation#sum(Symbol) ⇒ Integer or ⇒ Float or ⇒ String

Call .sum on an ActiveRecord_Relation to find the sum of the values in a single column:

@poem.scores.sum(:points)
  • Returns an Integer or Float (or even a String), depending on datatype of the column that was summed.
  • The argument to .sum must be a Symbol that matches the name of a column in the table.

Full explanation

.average

ActiveRecord_Relation#average(Symbol) ⇒ Integer or ⇒ Float

Call .average on an ActiveRecord_Relation to find the mean of the values in a single column:

@restaurant.reviews.average(:rating)
  • Returns an Integer or Float, depending on datatype of the column that was averaged.
  • The argument to .average must be a Symbol that matches the name of a column in the table.

Full explanation

ActiveRecord (A single record)

All column names

You get a method for every column in the table; for example, if you have retrieved an individual record from the Contact table, you can call .first_name, .last_name, etc, on it.

This means that every ActiveRecord object will have methods .id, .created_at, and .updated_at, since every table has those columns.

Full explanation

Any other instance methods you define in the class

It’s often helpful to define your own instance methods in the model file; for example, you might want to define a method .full_name:

class Contact < ApplicationRecord
  def full_name
    return self.first_name + " " + self.last_name
  end
end

You would then be able to call .full_name anywhere in the application that you wind up with an individual Contact object.

Full explanation

Association Helper Methods

belongs_to

belongs_to(Symbol, Hash) ⇒ ActiveRecord

Let’s say I have a movie in a variable m. It is annoying and error prone to, whenever I want the director associated with a movie, have to type

d = Director.where({ :id => m.director_id }).at(0)

Wouldn’t it be great if I could just type

d = m.director

and it would know how to go look up the corresponding row in the directors table based on the movie’s director_id?

Unfortunately, I can’t, because .director isn’t a method that Movie objects automatically know how to perform — the method would be undefined. (Movie objects know how to perform .director_id because we get a method for every column in the table.)

We could define such an instance method in the model ourselves without too much trouble. But, fortunately, since domain modeling and associations are at the heart of every application’s power, Rails gives us a shortcut. Go into the Movie model and add a line like this:

belongs_to(:director, { :class_name => "Director", :foreign_key => "director_id" })

This line tells Rails:

  • belongs_to: We only one result (not an array of results).
  • :director: Define a method called .director for all movie objects.
  • :class_name => "Director": When someone invokes .director on a movie, go fetch a result from the directors table.
  • :foreign_key => "director_id": Use the value in the director_id column of the movie to query the directors table for a row.

This is exactly what we would do if we defined the instance method by hand:

def director
  return Director.where({ :id => m.director_id }).at(0)
end

Either way, we now can utilize this handy shortcut anywhere in our application when we have a movie m and we want to get the record in the directors table associated with it:

m.director

Even better, if you’ve named your method and foreign key column conventionally (exactly matching the name of the other table), you can use the super-shorthand version:

belongs_to(:director)

  • If you omit specifying the :class_name, Rails assumes that the table that you want to query is named the same thing as the method you are defining.

  • If you omit specifying the :foreign_key, Rails assumes that the foreign key column is named the same thing as the method plus _id.

  • If either of those things happens to not be true, then just include the Hash as the second argument to belongs_to and spell it all out:

    belongs_to(:owner, { :class_name => "User", :foreign_key => "poster_id" })

    This would give us a method called .owner that returns a User, even though the foreign key column is called poster_id. We still have complete control, if we need to depart from conventional method/foreign key names for some reason.

has_many

has_many(Symbol, Hash) ⇒ ActiveRecord_Relation

Let’s say I have a director in a variable d. It is annoying and error prone to, whenever I want the films associated with the director, have to type

f = Movie.where({ :director_id => d.id })

Wouldn’t it be great if I could just type

f = d.movies

and it would know how to go look up the corresponding rows in the movies table based on the director’s id?

Unfortunately, I can’t, because .movies isn’t a method that Director objects automatically know how to perform — the method would be undefined.

We could define such an instance method in the model ourselves without too much trouble. But, fortunately, since domain modeling and associations are at the heart of every application’s power, Rails gives us a shortcut. Go into the Director model and add a line like this:

has_many(:movies, { :class_name => "Movie", :foreign_key => "director_id" })

This line tells Rails:

  • has_many: We want many results in an array.
  • :movies: Define a method called .movies for all director objects.
  • :class_name => "Movie": When someone invokes .movies on a director, go fetch results from the movies table.
  • :foreign_key => "director_id": Use the director_id column of the movies table to filter using the id of the director.

This is exactly what we would do if we defined the instance method by hand:

def movies
  return Movie.where({ :director_id => self.id })
end

Either way, we now can utilize this handy shortcut anywhere in our application when we have a director d and we want to get the records in the movies table associated with it:

d.movies

Even better, if you’ve named your method and foreign key column conventionally (exactly matching the name of the other table), you can use the super-shorthand version:

has_many(:movies)

  • If you omit specifying the :class_name, Rails assumes that the table that you want to query is named the same thing as the method you are defining.

  • If you omit specifying the :foreign_key, Rails assumes that the foreign key column is named the same thing as this model plus _id.

  • If either of those things happens to not be true, then just include the Hash as the second argument to belongs_to and spell it all out:

    has_many(:filmography, { :class_name => "Movie", :foreign_key => "director_id" })

    This would give us a method called .filmography that returns Movies. We still have complete control, if we need to depart from conventional method/foreign key names for some reason.

has_many/through

has_many(Symbol, Hash) ⇒ ActiveRecord_Relation

After you have established all of your one-to-many association helper methods, you can also add many-to-many helper methods with the :through option on has_many:

class Movie < ApplicationRecord
   has_many(:characters)
   has_many(:actors, { :through => :characters, :source => :actor })
end

class Character < ApplicationRecord
  belongs_to(:movie)
  belongs_to(:actor)
end

class Actor < ApplicationRecord
   has_many(:characters)
   has_many(:movies, { :through => :characters, :source => :movie })
end

String

.concat (.+)

String#concat(Integer) ⇒ String

or

String#concat(String) ⇒ String

Appends the given arguments to a string. when given an integer as an argument, it converts the integer into ASCII code.

"hi".concat(33) # => "hi!"

"Rub".concat(121)  # => "Ruby"

When given a string literal as an argument, it adds that string to the original string. .+ or + is similar to the .concat method. + will return a new combined String instead of modifying the original String. Each line of code below will give the same output.

"hi".concat(" there")
"hi".+(" there")
"hi" +(" there")
"hi" + " there"

This method returns a new String

Full explanation

* method

Multiplies the original string by the given integer and returns the new modified String.

"Ya" * 5

Returns "YaYaYaYaYa"

"Ya" * 0

Returns ""

Full explanation

.upcase

String#upcase ⇒ String

Converts all lowercase letters to their uppercase counterparts in the given string and returns the new modified String.

"hello".upcase`

Returns

"HELLO"

Full explanation

.downcase

String#downcase ⇒ String

Converts all the uppercase letters to their lowercase counterparts from the given String. Returns the new modified String.

"HI".downcase`

Returns

"hi"

Full explanation

.swapcase

String#swapcase ⇒ String

Converts all the uppercase letters to their lowercase counterparts and lowercase letters to their uppercase counterparts from the given string. Returns the new modified String.

"Hi There".swapcase

Returns

"hI tHERE"

"hI tHERE".swapcase

Returns

"Hi There"

Full explanation

.reverse (String)

String#reverse ⇒ String

Returns a new String with the characters from the original String in reverse order.

"stressed".reverse

Returns

"desserts"

Full explanation

.length

String#length ⇒ Integer

Returns the Integer number of charactersin the String.

"hippopotamus".length

Returns

12

Full explanation

.chomp

String#chomp ⇒ String

or

String#chomp(String) ⇒ String

When not given any argument, removes the "\n" (newline) character from the end of the string. When given an argument of a charcter or a string, it remove that argument from the end of the orginal string.

Returns a new String with the character removed.

"Hey!\n".chomp  
"Hey!".chomp("!")  
"Hey There".chomp("There")  
"Hey!".chomp("y")

"Hey!"  
"Hey"  
"Hey "  
"Hey!"

p "What is your name?"
name = gets # supposes the user inputs "Clark" and then hits return
p "Hi" + name

Prints

"Hi Clark\n"

p "What is your name?"
name = gets # supposes the user inputs "Clark" and then hits return
p "Hi" + name.chomp

Prints

"Hi Clark"

p "What is your name?"
name = gets.chomp # supposes the user inputs "Clark" and then hits return
p "Hi" + name

"Hi Clark"

Full explanation

.gsub

String#gsub(String, String) ⇒ String

Substitutes the all occurances of the first argument with the second argument in original string and returns a new String with the substitutes made.

"Hello".gsub("ello", "i")

Returns

"Hi"

"Hi.there".gsub(".", " ")

Returns

"Hi there"

Giving an empty string as the second arugment deletes any occurences of the first arugument in the string.

"example @ ruby.com".gsub(" ", "")

Returns

"[email protected]"

Full explanation

.to_i (String)

String#to_i ⇒ Integer

Converts a string literal that contains a number to an integer. The Integer is returned.

"8".to_i

Returns

8

p "What is your lucky number?"
lucky_number = gets.chomp # Suppose the user types is "7" and then hits return
square = lucky_number ** 2 # This will throw an error.

Returns

NoMethodError (undefined method '**' for "7":String)

This is where the .to_s method comes handy.

p "What is your lucky number?"
lucky_number = gets.chomp # Suppose the user types is "7" and then hits return
square = lucky_numbe.to_i ** 2 # This will throw an error.
p square

Returns

"49"

Full explanation

.strip

String#strip ⇒ String

Removes all leading and trailing whitespace in the string.

Returns a new String that has been modified from the original.

"   hi there ".strip

Returns

"hi there"

Full explanation

.capitalize

String#capitalize ⇒ String

Capitalizes the first character of a string.

Returns a new String that has been modified from the original.

"capitalize".capitalize

Returns

"Capitalize"

Full explanation

.split

String#split ⇒ Array

or

String#split(String) ⇒ Array

Splits a string into an substrings and creates an Array of these substrings. when not given argument, .split uses whitespace to divide the string. when given an argument, .split divides the string on that argument.

Returns an Array of the divided String.

"Hello hi byebye".split

Returns

["Hello", "hi", "byebye"]

"one!two!three!".split("!")

Returns

["one", "two", "three"]

Full explanation

.include?

String#include?(String) ⇒ Boolean

Returns a boolean(true or false) based on whether the string argument is inside the string the method is being called on.

"Happy".include?("H")

Returns

true

"Happy".include?("Z")

Returns

false

Full explanation

Integer

whole numbers

Math Operations for the Integer Class

12 + 5 # => 17
12 - 5 # => 7
12 * 5 # => 60
12 / 5 # => 2
The / operator for integers only returns a whole number and omits the remainder.

Full explanation

% (modulus) operator

Integer % Integer ⇒ Integer

Returns the Integer remainder from a divisions.

13 / 5

Returns

3

** operator Integer

Integer ** Integer ⇒ Integer

Raises a number to a power.

Returns an Integer

3 ** 2 # => 9
2 ** 3 # => 8

Full explanation

.odd? and .even? method

Integer#odd? ⇒ Boolean

or

Integer#even? ⇒ Boolean

Returns a boolean(true or false) based on whether the integer is odd or even.

7.odd?

Returns

true

8.odd?

Returns

false

8.even?

Returns

true

7.even?

Returns

false

Full explanation

rand

Integer#rand ⇒ Float

or

Integer#rand(Integer) ⇒ Integer

or

Integer#rand(Range) ⇒ Integer

  • Creates a random Float between 0 to 1
  • Can be given an optional Integer argument that will generate and return an Integer between 0 and the argument.
  • Can be given an optional argument of a Range that will generate a random Integer that between the Range.
rand           # returns => 0.21374618638...
rand(10)       # returns => 7
rand((10..20)) # returns => 19

Full explanation

.to_s

Integer#to_s ⇒ String

Converts an integer to a string literal.

Returns a String

8.to_s

“8”

lucky_number = rand(10) # assigns a random integer between 0 to 9 to the variable lucky_number
p "My lucky number is " + lucky_number + "!"

The above block of code won’t work and will throw an error.

TypeError (no implicit conversion of Integer into String)

This is where the .to_s method comes handy.

lucky_number = rand(10) # assigns a random integer between 0 to 9 to the variable lucky_number
p "My lucky number is " + lucky_number.to_s + "!"

“My lucky number is 7!”

"There are " + 7.to_s + " pineapples."

“There are 7 pineapples”

Full explanation

.to_f

Integer#to_f ⇒ Float

converts an integer to a Float(decimal).
7.to_f

7.0

number = 10       #  
p number / 3      # Returns => 3
p number.to_f / 3 # Returns => 3.3333333333333335

More examples

Full explanation

Float

decimals

Math Operations for the Float Class

Standard operations are similar to those for the Integer class. The only exception is the / operator which returns fractional results.

12 / 5     # => 2 
12.0 / 5.0 # => 2.4 
12 / 5.0   # => 2.4  
12.0 / 5   # => 2.`

** Float operator

Float ** Float ⇒ Float

The **operator for Floats can additionally be used to calculate roots.

9 ** 0.5     # => 3.0, since 9^(1/2) = sqaureroot of 9  
8 ** (1/3.0) # => 2.0, since 8^(1/3) = cuberoot of 8

Full explanation

.round

Float#round ⇒ Integer

or

Float#round(Integer) ⇒ Float

  • Returns the whole part (Integer) of a decimal when not given any argument.
  • When given an argument, returns a Float rounded to the number of decimal places specified by the argument.

3.14159.round # => 3
3.14159.round(3) # => 3.142
3.14139.round(3) # => 3.141

Full explanation

.to_i (Float)

Float#to_i ⇒ Integer

Converts a float to an integer by rounding the float down to closest whole number.

Returns an Integer

"8.9".to_i

8

"8.1".to_i

8

Integer and Float division examples

12 / 5 # => 2
(12 / 5).to_f # => 2.0
12.to_f / 5 # => 2.4
12 / 5.to_f # => 2.4
(12.0 / 5 ).to_i # => 2
(12 / 5.0).to_i # => 2

Date

Creating a Date

To use the Date class in a Ruby program, we need to say:
require "date"
Note: Only Ruby programs need to have a require statement for the Date class. Rails already does this for you.

Date.new

Date.new ⇒ Date

Use the .new method to create a new instance of a Date object. The .new method can be used with or without argument. When given no arguments, the default date is set to Jan 1st, -4712 BCE.

Date.new                  # => #<Date: -4712-01-01 ((0j,0s,0n),+0s,2299161j)>
Date.new(2001)            # => #<Date: 2001-01-01 ...>
Date.new(2001,2,3)        # => #<Date: 2001-02-03 ...>
Date.new(2001,2,-1)       # => #<Date: 2001-02-28 ...>

Full explanation

Date.today

Date.today ⇒ Date

Initializes a Date object to the current date.

Returns a Date

Date.today # => #<Date: 2019-04-16 ((2458590j,0s,0n),+0s,2299161j)>

Full explanation

Date.parse()

Date.parse(String) ⇒ Date

Returns a Date object initialized to a date, interpreted from the given String argument.

Date.parse("2001-02-03")   # => #<Date: 2001-02-03 ...>
Date.parse("20010203")     # => #<Date: 2001-02-03 ...>
Date.parse("3rd Feb 2001") # => #<Date: 2001-02-03 ...>

Full explanation

Subtraction

Two dates can be subtracted from one another. The - operator returns a Rational which can be converted into an Integer to find the days in between the two dates.

number_of_days = Date.today - Date.parse("July 4, 1776") 
# => number_of_days = (88674/1)
number_of_days.to_i # => 88674

Full explanation

Date.mday

Date.mday ⇒ Integer

Returns the day of the month (1-31).

held_on = Date.new(2001,2,3)
held_on.mday # => 3

Full explanation

Date.wday

Date.wday ⇒ Integer

Returns the day of the week as an Integer (0-6, Sunday is 0).

held_on = Date.new(2001,2,3)
held_on.wday # => 6

Full explanation

Days of the Week

Date#moday? ⇒ Boolean

date = Date.new
date.monday?    # => true if date is a Monday.
date.tuesday?   # => true if date is a Tuesday.
date.wednesday? # => true if date is a Wednesday.
date.thursday?  # => true if date is a Thursday.
date.friday?    # => true if date is a Friday.
date.saturday?  # => true if date is a Saturday.
date.sunday?    # => true if date is a Sunday.

Returns a Boolean, true or false, if this given Date is a particular day of the week.

Full explanation

Array

list of objects represented with square brackets, [].

Creating an Array

Array.new ⇒ Array

initializes a new empty Array.

cities = Array.new # => cities = []
or
cities = [] # => cities = []

.push

Array#push(Object) ⇒ Array

Adds elements to the end of an Array. Returns the modified Array.

cities.push("Chicago")
cities.push("Los Angeles")
cities.push("New York City")

or

cities = ["Chicago", "Los Angeles", "New York City"]
# Initializes and adds elements to an Array

.at()

Array#.at(Integer) ⇒ Object

Takes an Integer argument and return the element in that position of an Array. The following lines of code show the various forms of the .at method and return the same output.

Returns an Object

cities = ["Chicago", "Los Angeles", "New York City"]
cities.at(2)  
cities.[](2)  
cities[2]

"New York City"

Note:

  1. Ruby indexes the elements in an array starting at zero, that is, the first element of an array will have the index zero.
  2. Trying to access an element using an index greater than the length of the array will give you nil.
    cities.at(3) # => nil
  3. Using a negative index will retrieve elements from the end of the least.
    cities.at(-1) # => "New York City"
    cities.at(-2) # => "Los Angeles"
    cities.at(-3) # => "Chicago"
    cities.at(-4) # => nil

Full explanation

.first and .last

Array#first ⇒ Object or Array#last ⇒ Object

Retrieves and returns the first or the last element of an array.

Returns an Object

cities.first # => "Chicago"
cities.last) # => "New York City"

.index

Array#index(Object) ⇒ Integer

Returns an Integer that is the index of an element.
cities.index("Los Angeles") # => 1

.count

Array#count ⇒ Integer or Array#count(Object) ⇒ Integer

Returns the number of elements in a list, when give no arguments. If given an argument, returns the number of times that arguments occurs in the array. In both instances, this method returns an Integer

nums = [8, 3, 1, 19, 23, 3]
nums.count # => 6
nums.count(3) # => 2
nums.count(2) # => 0

Full explanation

.reverse (Array)

Array#reverse ⇒ Array

Returns a new ArrayArray with the elements of the original Array but in the reversed order.
nums.reverse # => [3, 23, 19, 1, 3, 8]

Full explanation

.sort

Array#.sort ⇒ Array

Returns a new Array with the elements of the original Array but in the sorted in increasing order.
nums.sort # => [1, 3, 3, 8, 19, 23]

Example: Sorting an Array in decreasing order

nums = [8, 3, 1, 19, 23, 3]
nums.sort # => [1, 3, 3, 8, 19, 23] 
nums.reverse # => [3, 23, 19, 1, 3, 8] 
nums.sort.reverse # => [23, 19, 8, 3, 3, 1], first sorts then reverses the Array.

.shuffle

Array#shuffle ⇒ Array

Returns a new Array with the elements of the original Array but with the order shuffled randomly.
nums.shuffle # => [3, 23, 8, 19, 1, 3]
nums.shuffle # => [19, 3, 1, 8, 3, 23]

Full explanation

.sample

Array#sample ⇒ Array

Returns a random element from the array.
nums.sample # => 23
nums.sample # => 3

Full explanation

.min and .max

Array#min ⇒ Object or Array#max ⇒ Object

Retrieve the elements of minimum and the maximum values in the array.
nums.min # => 1
nums.max # => 23

Full explanation

.sum (Array)

Array#sum

Returns the sum of all the elements in the array.
nums.sum # => 57

Note This method only works in the elements in the Array are not a Hash

Full explanation

Hash

list of objects represented with curly brackets, {}. Unlike Arrays, each cell is not automatically numbered but given a label by us.

Interlude: Symbol

Symbols are a sequence of characters and are used to to label something internally in the code. They are created by starting them off with a colon and follow the same naming conventions as variables, :hello.
:hello.class # => Symbol

Full explanation

Creating a Hash

Hash.new ⇒ Hash

person1 = Hash.new
or
person2 = {}

.store

Hash#store(Object, Object) ⇒ Object

Adds elements to a Hash by taking two arguments, a label (or key) and a piece of data (or value).

This method returns the Object that was stored.

The key can be any type, although is usually a Symbol. The value can also be of any type.

person1.store(:first_name, "Raghu")
person1.store(:last_name, "Betina")
person1.store(:role, "Instructor")
# => person1 = {:first_name=>"Raghu", :last_name=>"Betina", :role=>"Instructor"}

or we can fill up a hash by typing in the hash literal
person2 = { :first_name => "Jocelyn", :last_name => "Williams", :role => "Student" }

Note:

  1. Ruby represents each key/value pair by separating them with a =>, known as a “hash rocket.”
  2. If the value associated with a key already exists when you try to .store something under it, its value will be replaced.

Full explanation

.fetch

Hash#fetch(Object) ⇒ Object

or

Hash#fetch(Object, Object) ⇒ Object

Both .fetch and .[] can be used to retrieves the data held by a key. person1.fetch(:last_name)# => "Betina"
person2.[:last_name] # => "Williams"

If .fetch is given key that is not present in the hash, it will throw an error. But .[] is given key that is not present in the hash, it returns nil.

Fallback: pass in a second default argument that .fetch will return if the key is not present in the hash.
person1.fetch(:middle_name, "None provided") # => "None provided"

Full explanation

Conditionals

Basic Anatomy of multibranch if statements:

if condition1
  # do something if condition1 is true
elsif condition2
  # do something if condition2 is true
else # if both condition1 and condition2 were falsy
  # do something else
end

Example:

p "Type a number less than 10 and greater than 0:"
user_input = gets.chomp.to_i # gets user input, removes newline character, converts the string to integer. 
if user_input == 5 
  p "You win!" # Will print this if the user input is 5
elsif user_input < 10 && user_input > 0 # check if the user input is valid
  p "You lose!" # Will print this if the user input is between 1 and 9
else
  p "You didn't type in a valid number." # Will print this if the user input is not between 1 and 9
end

Don’t forget the end keyword.

Full explanation

Loops

while statements:

while boolean
  # ruby code here
end

⇒ nil

while is similar to if. The difference is everytime the execution of the program reaches the end it jumps back and evaluates the truthiness of the condtion next to the while statement and decides whether or not to execute the code within the while loop.

while condition 
  # do something while condition is true
end # jump back to the while statement

Example:

limit = 5
while limit > 0 
  p limit
  limit = limit - 1
end

5
4
3
2
1

Note:
If the condition next to the while always evaluates to be “truthy,” then the program will be stuck in a neverending loop, infamously known as an infinite loop.

Full explanation

Blocks

.times

Integer#times do
  # ruby code here
end

⇒ Integer

or

Integer#times do |Integer|
  # ruby code here
end

⇒ Integer

The .times method takes a Block as an argument and will execute the code within that block the number of times specified by the integer. A Block of code is the code written in between the keywords do and end. This looping method returns an Integer of the number of times the loop ran.

10.times do
  p "Hi"
end

The above block of code will print “Hi” 10 times all on newlines.

To keep a track of the iteration number, .times can create a block variable that starts of counting the iteration number starting at zero. After each execution of the code within the block, the block variable is incremented by 1.

10.times do |counter|
  p counter
end

The above block of code will print the numbers 0 to 9 all on newlines.

Full explanation

Other methods

.upto

Integer#upto do |Integer|
  # ruby code here
end

⇒ Integer

The upto method takes the first Integer the method is called on and uses it to initialize the value of the block variable. The second Integer becomes the stopping condition to the loop as the block variable’ increases by one after each iteration. The method returns an Integer; the initial value of the block variable.

5.upto(10) do |counter|
  # do something
end

The above block of code starts the block variable counter at 5 and executes the block until counter is 10.

.downto

Integer#downto do |Integer|
  # ruby code here
end

⇒ Integer

The downto method takes the first Integer the method is called on and uses it to initialize the value of the block variable. The second Integer becomes the stopping condition to the loop as the block variable’ decreases by one after each iteration. The method returns an Integer; the initial value of the block variable.

10.downto(5) do |counter|
  # do something
end

The above block of code starts the block variable counter at 10 and executes the block until counter is 5.

.step

Integer#step(Integer, Integer) do |Integer|
  # ruby code here
end

⇒ Integer

The step method initializes the block variable to be the value of the Integer that called the method. The first Integer argument is the the value the block variable is when the loop will stop. The last Integerargument is what value to modify the block variable after each iteration. This method returns the Integer that called the method.

1.step(10, 3) do |counter|
  p counter
end

1
4
7
10

The above block of code starts the block variable counter at 1 and executes the block until counter is 10 but after each iteration the counter will be incremented by 3 instead of 1. .step can also be used to decrement the counter by a certain value.

10.step(1, -4) do |counter|
  p counter
end

10
6
2

Looping through Arrays

.each

Array#each do |Object|
  # ruby code here
end

⇒ Array

Given an array, the .each method will loop through each element of the array starting with the very first one.

Returns the Array the method was called on.

cities = ["Chicago", "LA", "NYC"]
cities.each do |city|
  p city
end

"Chicago"  
"LA"  
"NYC"

The block variable city holds the value of the elements in the array cities. It starts with the first element "Chicago" and then changes with each interation, holding the value of the next element ("LA") in the array and so on.

Full explanation

.each_with_index

Array#each_with_index do |Object, Integer|
  # ruby code here
end

⇒ Array

To keep a track of the iteration number while looping through an array, .each_with_index creates an additional block variable that starts of counting the iteration number starting at zero. After each execution of the code within the block, the block variable is incremented by 1.

cities.each_with_index do |city, count|
  p count.to_s + " " + city
end

"0 Chicago"  
"1 LA"  
"2 NYC"

city holds the value of elements in the array cities. count holds the index of the element that city currently holds.

Note:
Variables created as a block variables can only be used within that block (between do and end). Using that variable outside that block will throw an error.

Full explanation