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def list_squared(m, n)
  divs_hash = {}
  (m..n).reduce([]) do |result, i|
    sum = 1
    divs = [1]
    for k in (2..i/2)
      if (i % k).zero?
        current = (i/k)
        if divs_hash.has_key?(current.to_s)
          divs_hash[current.to_s].each do |c|

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"some string".gsub(pattern, replacement)  # every occurrence of pattern is replaced by replacement
"other string".gsub(pattern) { |match| replacement } # every match is replaced by return value (replacement) of block

"hello".gsub(/./) { |c| "#{c.ord} " } # 104 101 108 108 111 

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(?=foo)  # followed by foo
(?<=foo) # preceded by foo
(?!foo)  # is not followed by foo
(?<!foo) # is not preceded by foo

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[1,2,3].zip([5,6,7,9], [7]) #[[1, 5, 7], [2, 6, nil], [3, 7, nil]]

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class Ln
  include Comparable
  attr_reader :value
  
  def initialize(n)
    @current = n
    @value = Math.log2(n)
  end
  
  # Objects of the range have to comparable

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class Person
  include Comparable
  # Comparable adds methods, which allow to compare objects
  attr_reader :age
  
  def initialize(age)
    @age = age
  end

  def <=>(another) # method is obligatory, as Comparable uses it to construct methods

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module SomeModule
  # instance methods:
  def some_instance_method
    # do something here
  end
  
  # module, which contains class methods
  module ClassMethods
    # class methods:
    def some_class_method

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select * from crosstab($$
  select row_name, some_category, some_value --only 3 columns!
  from some_table
$$, $$values(category1), (category2), ..., (categoryn)$$ -- same can be achieved by selecting distinct values
) as g(RowName type, Category1 type, Category2 type, ..., CategoryN type);
-- type of CategoryN should be the same as type of value

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select count(*) over(order by id rows between unbounded preceding and unbounded following); -- all rows in partition
select count(*) over(order by id rows between 2 preceding and 1 following); -- 2 rows before, current and next row
select count(*) over(order by id rows unbounded preceding); -- all rows before current

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select * from t1, lateral (select * from t2 where t2.id = t1.id);