.chars, &:, delete_at, swap, scan

- Did duplicate_count.rb
- Learnt .chars actually splits a string into an array of separate characters. But this is case sensitive.
- To transform an array of uppercase and lowercase letters to lowercase
.select{|item| item.respond_to? :downcase}.map(&:downcase)

: before downcase is to see if downcase method exist.

- respond_to? is to see if a method exists within a class.
https://stackoverflow.com/questions/17893977/confused-about-respond-to-method
https://stackoverflow.com/questions/6849722/confused-about-respond-to-vs-respond-to

- .downcase works on a string but not array.
- .map(&:downcase) is the same as .map { |keyword| keyword.downcase }
The &: represents a block.
- .group_by groups the hashes in further array depending on condition

(1..6).group_by {|i| i%3}   #=> {0=>[3, 6], 1=>[1, 4], 2=>[2, 5]}

b = ['a','a','b','b']
b.group_by(&:itself) => {"a"=>["a", "a"], "b"=>["b", "b"]}
b.group_by{|x| x} => {"a"=>["a", "a"], "b"=>["b", "b"]}

- ('a'..'z').count('a') => 1
Basically this counts a from an array of a to z.

- .split(String), .split(Regex), .split(limit)
"sdfgs".split(//)
 => ["s", "d", "f", "g", "s"]
"sdfgs".split('')
 => ["s", "d", "f", "g", "s"]

split btwn each , and the last object is 4 characters.
"1,2,,3,4,,".split(',' ,  4)      #=> ["1", "2", "", "3,4,,"]

- Remember to note that delete_at(index) returns the value that is deleted.
array.insert(2, array.delete_at(7))

This basically says  delete the obj at index 7 and bring it to 2.

- Learnt how to swap posoitions in an array
array = [4, 5, 6, 7]

array[0], array[3] = array[3], array[0]

array # => [7, 5, 6, 4]

- Learnt that .scan seperates a string by regex or a string

a = "cruel world"
a.scan(/\w+/)        #=> ["cruel", "world"]
a.scan(/.../)        #=> ["cru", "el ", "wor"]
a.scan(/(...)/)      #=> [["cru"], ["el "], ["wor"]]
a.scan(/(..)(..)/)   #=> [["cr", "ue"], ["l ", "wo"]]

a.scan(/\w+/) {|w| print "<<#{w}>> " }
print "\n" #=> <<cruel>> <<world>>
a.scan(/(.)(.)/) {|x,y| print y, x }
print "\n" #=> rceu lowlr