Why You Shouldn’t Use Float for Currency (floating point issues – explained for Ruby and RoR)

This article is a response to https://vladzloteanu.wordpress.com/2010/01/06/ruby-on-rails-interview-questions-update/

It is a VERY BAD IDEEA to use floating point arithmetics to deal with currency. In most of the programming languages. Basically, because you’ll end up loosing money :). And this (on the great majority of cases) is not desirable 🙂 .

I’ll show you some magic (that you may try at home):
~$ ruby --version
ruby 1.8.7 (2009-06-12 patchlevel 174) [i486-linux]
~$ irb
irb(main):003:0> (10.12*100).to_i
=>; 1011

So, what just happened? Continue reading

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Ruby on Rails Interview questions – update

Most of Google searches that hit my blog are about ‘ruby on rails interview questions‘.

Responding to this popular request 😉 , I will post 3 more questions that are related.

1. What type of data would you use to represent currency?

2. Why is it not a good practice to use float for currency?

3. Explain the following behaviour:

~$ ruby --version
ruby 1.8.7 (2009-06-12 patchlevel 174) [i486-linux]
~$ irb
irb(main):003:0> (10.12*100).to_i
=>; 1011

The answer – in my next post .

Rails Internationalization: currency format configuration

One of the nicest things in the built-in Rails internationalization features is the ability to format the currency according to the local culture (the number_to_currency method). For example, many european contries use ‘,’ as a separator and ‘.’ as a delimiter.

However, the examples presented in the help page (http://api.rubyonrails.org/classes/ActionView/Helpers/NumberHelper.html#M001684) are missing something essential.

 number_to_currency(1234567890.50, :unit => "£", :separator => ",", :delimiter => "")
 # => £1234567890,50
 number_to_currency(1234567890.50, :unit => "£", :separator => ",", :delimiter => "", :format => "%n %u")
 # => 1234567890,50 £

Why should i need a helper method where i can set all those parameters by hand?

Digging in the source code, i found how those settings can be put in the locale yml config file. Below is an example from my french localization file.

fr:
  number:
    currency:
      format:
        delimiter: ','
        separator: '.'
        precision: 2
        format: '%n %u'