Ruby idioms : Avoid check class membership

Avoid use class, is_a? and kind_of? to check class membership. Replace these statement with a message to the object that respond to the message. The code below is an anti-pattern.

if mortgage.class==FixRateMortgage
   mortgage.change_rate_not_allowed
else
   mortgage.rate=rate
end

The code below is more idiomatic in Ruby.

class Mortgage
  attr_accessor :rate
end 

class FixedRateMortgage < Mortgage
  def initialize()
    @rate=5
  end
  def rate=(rate)
    change_rate_not_allowed
  end

  def change_rate_not_allowed
     puts "The rate in a fixed rate mortage can not be change"
  end
end

Sending the method rate= to either kind of classes will result in the appropriate behaviour.

Commons bugs in ruby: blocks and local variables

Originally blocks did not have truly local variables. The block
parameters were really local variables in the enclosing method.

x=0
1.upto 100 do |z|
  x = x+z
end       
x  #=> 5050 

Commons bugs in ruby: Changing collection while iterating over it

You should never, never, never iterate over a collection which the iteration loop somehow modifies. Elements of the collection will be moved during the iteration. Elements might be missed or handled twice.

 b=(1..10).collect # => [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
 b.each do |elem|
   b.delete elem
 end #=> [2, 4, 6, 8, 10]