Collections

COLLECTIONS
STARTED 07/20/2021
COMPLETED 07/20/2021
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Arrays
An array is a list of numbered items. Comma separated, square brackets.
test_array = ["Item 1", "Item 2", "Item 3"]
Each item has an index, starting at 0.
test_array[0] = "Item 1"
Overwrite assignments easily using the index. test_array[0] = "Item 0"
A negative index started from the end of the array.
test_array[-1] = "Item 3"

Adding Elements
A single array can have different types of elements (integer, string, float)
Add an element using <<
arr << something will add "something" to the end of the array

Methods
arr.push(something) will add something to the end of the array
arr.insert(position, something) will add something to that index of the array

arr = [1,3,4,5]
arr.insert(1,2)
print arr
# returns [1,2,3,4,5]

Removing Elements
Methods
arr.pop                 will remove the last value in the array
arr.delete_at(index)    will remove the value at the index in the array

print arr will print the actual array with brackets and commas
puts arr will print the elements of the array on a newline without brackets or commas

You can use a range in specifying an array index. arr[1..2]
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Array Manipulations

Addition: add the elements of two arrays together with arr1 + arr2
Subtraction: removes the elements that are present in the second array from the first array arr1 - arr2
Duplicate arrays using * integer.

& operator: results in the values that are present in both arrays with no duplicates
| operator: joins two arrays and removes duplicate values

Moving Elements
arr.reverse: returns an array with the same elements in reverse order (does not change original array)
arr.reverse!: reverses the array in place, changing the original array
arr = [1,2,3]
print arr.reverse   #[3,2,1]
print arr           #[1,2,3]
print arr.reverse!  #[3,2,1]
print arr           #[3,2,1]

Common Methods
arr.length          returns number of elements in an array
arr.size            returns number of elements in an array
arr.sort            returns a new array with elements sorted
arr.uniq            returns a new array with duplicate elements removed
arr.uniq!           removes duplicate elements in place
arr.freeze          prevents array from being modified
arr.include?(obj)   returns true if obj is in the array, false otherwise
arr.min             returns the element of lowest value
arr.max             returns the element of greatest value
arr.index(item)     returns the index number of the item in array
Note: Most of these methods will also work for strings, which are like arrays of letters

A for loop is a basic way to loop over all the elements in an array.
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Hashes & Symbols
A hash is like an array, except the index can be anything. In an array, the index must be an integer. Other words for a hash is a dictionary, map, or associative array.
Hashes use key-value pairs enclosed in curly braces {}.
Hashes can use any object as an index, even an array.
sample_hash = {"key" => value, "key2" => value2}
sample_hash["key"] = value

Symbols are similar to strings, but they are immutable. Created by using a colon and name.
:symbol
The two sample hashes below are equal.
sample_hash2 = { :"Kevin"=>"BBQ",
                 :"Kathy"=>"Ketchup",
                 :"Joanne"=>"Sweet and Sour"
}
sample_hash2[:"Kevin"] = "BBQ"

sample_hash3 = { "Kevin":"BBQ",
                 "Kathy":"Ketchup",
                 "Joanne":"Sweet and Sour"
}
sample_hash3[:"Kevin"] = "BBQ"

Methods
hash.delete(key)    removes the key-value pair from hash by key.
hash.key(value)     returns the key for the given value in hash, nil if no matching value is found.
hash.invert         creates a new hash, reversing keys and values from hash; that is, in the new hash, the keys from hash become values and values become keys.
hash.keys           returns a new array with keys from hash. 
hash.values         returns a new array containing all the values of hash.
hash.length         returns the length of hash as an integer.
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Nested Arrays & Hashes
Nested arrays are arrays that contain arrays.
arr = [ [1,2,3] , [4,5,6] ]
arr[1][2] = 6
arr[outside index][inner index]

Hashes can also be nested.
Hashes and arrays with more than 3 dimensions can be difficult to manage.
hash1 = { bmw: {color: black, year: 2010},
          toyota: {color: red, year: 2019},
          ford: {color: blue, year: 2015}}
puts hash1[:bmw][:color] # returns black
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Iterators
.each iterator is one of the most common
x is the block parameter that takes the assigned value of each of the elements in the loop.

With an array:
arr.each do |x|
    puts x
end
# Prints each value of the array in a new line.

With a hash:
sizes.each do |key, value|
    puts "#{key}=>#{value}"
end
# Prints out each key-value pair of the hash in a new line.

do and end specify code blocks in Ruby. Can be replaced with curly braces.
sizes.each { |key, value| puts "#{key}=>#{value}" }

With a number:
10.times do
    puts "Hi"
end
# Puts "Hi" 10 times.
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Letter Frequency Counter
string.downcase! converts letters in string to lowercase

freqs = {} #Create an empty hash
freqs.default = 0
.default sets the default value for a hash, so when a key is assigned with no value, the default value is used.

.each_char iterates over a string

# Solution Code
text = "I am learning Ruby and it is fun!"
text.downcase!
freqs = {}
freqs.default = 0

text.each_char { |char| freqs[char] += 1}

("a".."z").each {|x| puts "#{x} : #{freqs[x]}" }