Collections

We've used Strings, Integers, Floats, and Booleans.

What if you want a list or collection of data?

Ruby has two basic list types, Array and Hash.

Arrays

Arrays store ordered, non-unique lists of any kind of data.

>> list = []
=> []

>> first_three = [1, 2, 3]
=> [1, 2, 3]

>> vector = [18.4, 12.8, 9.5]
=> [18.4, 12.8, 9.5]

Arrays can hold any types

# Strings
>> grocery_list = ["Juice", "Biltong", "Bunny chow", "Ostrich steaks"]
=> ["Juice", "Biltong", "Bunny chow", "Ostrich steaks"]

# Integers and Strings
>> zombies = [28, "days", "later"]
=> [28, "days", "later"]

# even nil, or other arrays
>> weird_array = [nil, [1, 2], nil]
=> [nil, [1, 2], nil]

Arrays use an index to determine the position.

The index starts at 0!

#   index 0, index 1,  index 2
>> ["Apple", "Banana", "Lemons"]
=> ["Apple", "Banana", "Lemons"]

The items in an array are called elements.

Access elements by using [] with the index.

>> fruits = ["Apple", "Banana", "Pear"]
=> ["Apple", "Banana", "Pear"]

>> fruits[0] # 0 is the index of the first element
=> "Apple"

>> fruits[1]
=> "Banana"

>> fruits[2]
=> "Pear"

car = "Toyota"
parking_lot = [car]

Remember, index 0 is the first element!

numbers = [1, 2, 3, 4]
letters = ['a', 's', 'd', 'f']
colors = ['green', 'red', 14, 12]

What is the value of...

1. element zero of numbers?
2. element two of letters?
3. element three of colors?

Q: What happens if you access an index that doesn't exist?

>> fruits = ["Apple", "Banana", "Pear"]

>> fruits[3]
=> nil

>> fruits[100]
=> nil

Q: What about negative indeces?

>> fruits = ['Apple', 'Banana', 'Pear']

>> fruits[-1]
=> ?

>> fruits[-2]
=> ?

A: Negative indeces access from the end!

>> fruits = ['Apple', 'Banana', 'Pear']

>> fruits[-1]
=> "Pear"

>> fruits[-2]
=> "Banana"

# Out of bounds negative index is also nil.
>> fruits[-5]
=> nil

Exercise: Creating Arrays

Hang with your group. Try making some arrays!

Here's a few for inspiration:

bananas = ['bananas', 'bananas', 'bananas', 'bananas']

weird = [nil, nil, nil, 1000000, nil]

primes = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41]

Methods on Arrays

Adding & Removing Items

# Create an array
>> todo_list = ["laundry", "pay rent"]
=> ["laundry", "pay rent"]

# Add an element to an array
>> todo_list.push("clean room")
=> ["laundry", "pay rent", "clean room"]

# Remove an element from an array
>> todo_list.delete("laundry")
=> ["pay rent", "clean room"]

The Append operator

<< is called the append operator. It appends the argument to the object and returns the new object.

Ruby always prefers the << operator over the push method.

# It works for Arrays
>> grocery_list = ['apples', 'oranges']
>> grocery_list << 'potatoes'
=> ["apples", "oranges", "potatoes"]

# And Strings too!
>> greeting = "My name is "
>> greeting << "Brian"
=> "My name is Brian"

>> greeting
=> "My name is Brian"

Working with multiple arrays

We can also combine and find the difference of difference of multiple arrays using the + and - and * operators.

>> first = [1, 2, 3]
>> second = [2, 3, 4]

# combining arrays
>> first + second
=> [1, 2, 3, 2, 3, 4]

# difference of arrays
>> first - second
=> [1]

# multiplying arrays
>> broken_doorbell = ['ding', 'dong'] * 4
=> ["ding", "dong", "ding", "dong", "ding", "dong", "ding", "dong"]

Unique elements

Sometimes we only want unique elements from an array. We can use uniq for this:

>> [1,2,3,4,4,5,5,6].uniq
=> [1,2,3,4,5,6]

>> ([1,2,3,4,5] + [2,3,4,5,6]).uniq
=> [1,2,3,4,5,6]

Splitting a string into an array

>> "bring me juice".split
=> ["bring", "me", "juice"] # the .split method returns an array!

>> "first,second,third".split(",")
=> ["first", "second", "third"]

>> "www.google.com".split(".")
=> ["www", "google", "com"]

Exercise: Array methods

Here are some example arrays to get you started.

fruits = ['Apples', 'Oranges', 'Bananas']
numbers = [1, 3, 4, 2, 7, 4, 9, 10, 10, 10]
no_data = []

Hang out with your group and play around with these common ruby Array methods:

Loops, Iteration, Enumeration

What if we want to display all of the items from the list?

We could do this:

puts fruits[0]
puts fruits[1]
puts fruits[2]
puts fruits[3]
puts fruits[4]
puts fruits[5]
puts fruits[6]
puts fruits[7]

Shame! Repeating yourself isn't lekker, boet!

What's the Ruby Way look like?

To display all of the items, we use an iterative loop to enumerate through all of the elements of the array.

fruits.each do |fruit|
  puts fruit
end

Let's see what this means!

Loop

Loop: A set of code that will repeat indefinitely

# Don't run this in pry
loop do
  puts "I'm looping forever!"
end

Iteration

Iteration: Running code in a loop until a particular condition is met.

i = 0

while i < 10
  puts i
  i = i + 1
end

Enumeration

Enumeration: An iteration where the condition is simply doing something to each item of a collection.

>> fruits = ["Apple", "Banana", "Pear"]

>> fruits.each do |fruit|
>>  puts fruit
>> end
Apple
Banana
Pear
=> ["Apple", "Banana", "Pear"]

Blocks Again

array.each do |item|
  puts item
end

• Some methods can take a block as an argument.
• each takes a block and runs that code on each element.

Common Enumeration Methods

Each

.each runs a block of code on each element in the collection, and returns the original array

numbers = [1, 2, 3]
numbers.each do |element|
  puts element
end
1
2
3
=> [1, 2, 3]

Each_With_index

.each_with_index is just like .each except that it also gives you the index of the element along with the element.

# Same as above, but simpler
fruits.each_with_index do |fruit, index|
  puts "Fruit #{index}: #{fruit}"
end
Fruit 0: Apple
Fruit 1: Banana
Fruit 2: Pear
=> ["Apple", "Banana", "Pear"]

Map

.map runs a block of code on each item in the list, and stores the return values of each block. This transforms the original array into a new one.

transformed_array = [1, 2, 3].map do |element|
 element * 3
end
=> [3, 6, 9]

RECAP

• Create ordered lists of elements of any type
• Access elements of an array using their numeric index
• Modify arrays with .push, <<, and .delete methods
• Enumerate over items in an array with .each, .each_with_index, .map
• Combine, subtract, and multiply arrays with +, -, and *
• Many different array methods

Hashes

What's a Hash?

• Stores a list of data like an array, but...
• It uses an object key instead of a numeric index

Hashes are also often called Dictionaries, because they work the same way.

dictionary = {
  "win" => "be successful or victorious in (a contest or conflict).",
  "lose" => "be deprived of or cease to have or retain (something)."
}

Creating a Hash Phone Book

>> numbers = {'Erica' => 123456789, 'Aaron' => 567890934}
=> {'Erica' => 123456789, 'Aaron' => 567890934}

>> numbers['Erica']
=> 123456789

>> numbers['Aaron']
=> 567890934

>> numbers['Nobody']
=> nil

A hash maps a set of unique keys to values.

You can never have two values for the same key.

The key/value pair is also referred to as an element.

METHODS ON HASHES

>> numbers = {'Erica' => 123456789, 'Aaron' => 567890934}
=> {'Erica' => 123456789, 'Aaron' => 567890934}

# this returns an Array
>> numbers.keys
=> ['Erica', 'Aaron']

# this also returns an Array
>> numbers.values
=> [123456789, 567890934]

>> numbers.has_key?('Erica')
=> true

>> numbers.has_key?('Nobody')
=> false

Modifying a Hash

>> numbers = {"Erica" => 123456789, "Aaron" => 567890934}
=> {"Erica" => 123456789, "Aaron" => 567890934}

# Adding a new element
>> numbers["Ian"] = 892381232

# Removing an element (aka key/value pair)
>> numbers.delete("Aaron")

# Final state with Aaron removed and Ian added
>> numbers
=> {"Erica" => 123456789, "Ian" => 892381232}

The order of the elements in a Hash is not reliable.

Treat them like random grab bags.

Enumerating

Over Elements in a Hash

# Enumerating with an array
numbers = [1,2,3]
numbers.each do |number|
  puts number
end
# 1
# 2
# 3

# Enumerating with a hash
phone_numbers = {'Erica' => 123456789, 'Aaron' => 567890934}
phone_numbers.each do |key, value|
  puts value
end
# 123456789
# 567890934

Exercise: Hashes

Create some hashes, iterate over them, and add/remove elements from them.

Here's an example:

hash = { 'one' => 1, 'two' => [1, 2], 'three' => 'three' }

Hash ideas: - family members and their ages - classmates and their phone numbers - recipes (bonus, ingredients are an array)

Symbols

Symbols are a more computer-friendly way of using strings in code.

"name" # inefficient
:name  # efficient

Declaring Symbols

Symbols are written with a colon followed by a snake_case string. No capitals or numbers in symbols!

This is good style:

:name
:next_page

This is legal but not good style.

:HelloThere # capitalizing words
:summerof69 # using numbers

Symbols as Hash Keys

Two ways of writing hashes when using symbols

>> health_info = {:age => 30, :weight => 150}
=> {:age=>30, :weight=>150}

>> health_info = { age: 30, weight: 150 }
=> {:age=>30, :weight=>150}

The second way is strongly preferred in modern Ruby, because it's much easier to type.

RECAP

• Creating lists (arrays) and dictionaries (hashes)
• These are called collections
• Modifying and enumerating them
• Using symbols and how they're used as keys for hashes

EXERCISES

http://iexperience.github.io/year02/s1/exercises/4A.html