Get a string that is the decimal representation of that number grouped by commas after every 3 digits. Like 1000 #=> '1,000'. --See the issues with some edge cases for floats.

thousands.rb

# General case, English notation:
def pretty_number(num)
  num.to_s.tap do |s|
    :anything while s.gsub!(/^([^.]*)(\d)(?=(\d{3})+)/, "\\1\\2,")
  end
end

# Puts the commas in the thousands, allows to use a character other than ',':
def pretty_number(number, comma = ',')
 number.to_s.tap do |s|
   :recursive_horror while s.gsub!(/^([^.]*)(\d)(?=(\d{3})+)/, "\\1\\2#{comma}")
 end
end

# Allows to use characters other than comma and dots:
def pretty_number(number, comma = ',', dot = '.')
 number.to_s.gsub('.', dot).tap do |s|
   false while s.gsub!(/^([^#{dot}]*)(\d)(?=(\d{3})+)/, "\\1\\2#{comma}")
 end
end

# For example, in the US:
pretty_number 1000_000.55
#=> "100,000.5"

# In Spain:
pretty_number 1000_000.55, '.', ','
=> "1.000.000,55"


# Known issues

#  1. Float#to_s could be represented with an exponential value, which would derail the
#     whole conversion. So proceed with care:

number = 23232423423442300000.334   #=> 2.32324234234423e+19
pretty_number number                #=> "2.32324234234423e+19"

#  2. Floats with many decimals would get an approximate value. Using BigDecimal would
#     require a different approach (check the class of then parameter), but that could be
#     a good way to go

number = 23232423423442300000.334   #=> 2.32324234234423e+19
number.to_i                         #=> 23232423423442300928