Converting string from snake_case to CamelCase in Ruby

Question

If you're using Rails, String#camelize is what you're looking for.

  "active_record".camelize                # => "ActiveRecord"
  "active_record".camelize(:lower)        # => "activeRecord"

If you want to get an actual class, you should use String#constantize on top of that.

"app_user".camelize.constantize

How about this one?

"hello_world".split('_').collect(&:capitalize).join #=> "HelloWorld"

Found in the comments here: Classify a Ruby string

See comment by Wayne Conrad

If you use Rails, Use classify. It handles edge cases well.

"app_user".classify # => AppUser
"user_links".classify   # => UserLink

Note:

This answer is specific to the description given in the question(it is not specific to the question title). If one is trying to convert a string to camel-case they should use Sergio's answer. The questioner states that he wants to convert app_user to AppUser (not App_user), hence this answer..

Source: http://rubydoc.info/gems/extlib/0.9.15/String#camel_case-instance_method

For learning purposes:

class String
  def camel_case
    return self if self !~ /_/ && self =~ /[A-Z]+.*/
    split('_').map{|e| e.capitalize}.join
  end
end

"foo_bar".camel_case          #=> "FooBar"

And for the lowerCase variant:

class String
  def camel_case_lower
    self.split('_').inject([]){ |buffer,e| buffer.push(buffer.empty? ? e : e.capitalize) }.join
  end
end

"foo_bar".camel_case_lower          #=> "fooBar"

Benchmark for pure Ruby solutions

I took every possibilities I had in mind to do it with pure ruby code, here they are :

• capitalize and gsub

  'app_user'.capitalize.gsub(/_(\w)/){$1.upcase}
  

• split and map using & shorthand (thanks to user3869936’s answer)

  'app_user'.split('_').map(&:capitalize).join
  

• split and map (thanks to Mr. Black’s answer)

  'app_user'.split('_').map{|e| e.capitalize}.join
  

And here is the Benchmark for all of these, we can see that gsub is quite bad for this. I used 126 080 words.

                              user     system      total        real
capitalize and gsub  :      0.360000   0.000000   0.360000 (  0.357472)
split and map, with &:      0.190000   0.000000   0.190000 (  0.189493)
split and map        :      0.170000   0.000000   0.170000 (  0.171859)