gistfile1.rb

# bad
#
def calculate(string_or_array)
  if !string_or_array.is_a?(Array)
     string_or_array = [ string_to_array ]    
  end
  …
end

# good
#
def calculate(string_or_array)
  array = Array string_or_array
  …
end

# nb: Kernel.Array is the new .to_a
# ruby-1.8.7-p302 :018 > Array "123"
#  => ["123"] 
# ruby-1.8.7-p302 :019 > Array %w(auto bahn hubschrauber)
#  => ["auto", "bahn", "hubschrauber"] 
# ruby-1.8.7-p302 :020 > Array "internetheld"
#  => ["internetheld"] 
# ruby-1.8.7-p302 :021 > Array nil
#  => [] 
# ruby-1.8.7-p302 :022 > Array false
#  => [false] 
# ruby-1.8.7-p302 :023 > Array true
#  => [true]