Question

I noticed that in the i18n rails guide, they have the following code:

en:
  activerecord:
    errors:
      template:
        header:
          one:   "1 error prohibited this %{model} from being saved"
          other: "%{count} errors prohibited this %{model} from being saved"
        body:    "There were problems with the following fields:"

What do the %{...} mean? How is this different from #{...}?

Answer 1

If you read the guide a little more, you’ll come across the Passing variables to translations section:

You can use variables in the translation messages and pass their values from the view.

# app/views/home/index.html.erb
<%=t 'greet_username', :user => "Bill", :message => "Goodbye" %>

# config/locales/en.yml
en:
  greet_username: "%{message}, %{user}!"

and the Interpolation section:

In many cases you want to abstract your translations so that variables can be interpolated into the translation. For this reason the I18n API provides an interpolation feature.

All options besides :default and :scope that are passed to #translate will be interpolated to the translation:

I18n.backend.store_translations :en, :thanks => 'Thanks %{name}!'
I18n.translate :thanks, :name => 'Jeremy'
# => 'Thanks Jeremy!'

So the %{...} stuff doesn’t have anything to do with YAML, that’s I18n’s way of handling variable interpolation in the messages.