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RubyGems (http://rubygems.org/) is a packaging system for Ruby libraries and applications. Each package within the RubyGems universe is called a gem or RubyGem (in this book both terms are used interchangeably). RubyGems makes it easier to distribute, update, install, and remove libraries and applications on your system. Before the advent of RubyGems, Ruby libraries and applications were distributed in a basic fashion in archive files, or even as source code to copy and paste from the Web. RubyGems makes it easier and more centralized, and also takes care of any prerequisites and dependencies required when installing a library. For example, here s how to install the Ruby on Rails system:

This installs the Rails gems along with all their dependencies. The gem application prompts at each step of the way so you know exactly what s being installed (you can override this with command line options). For example, gem install rails y installs Rails and its dependencies without questioning you at all. You can uninstall gems in as simple a fashion:

If you have multiple versions of the same gem(s) installed, gem will ask you which version(s) you want to remove. By default, gems are searched for in the default repository, hosted by RubyForge (http://www.rubyforge.org/) at http://gems.rubyforge.org/. Any gem files uploaded to Ruby projects hosted on the RubyForge site are made available in the default repository, making a RubyForge account a necessity if you want to distribute your libraries to the widest audience possible in an easy fashion. However, you can run your own gems repository on your own Web site or by using the RubyGems server software. This is less common and requires users of your gems to specify your server name at the same time as installing the gem. RubyGems is covered in full in 7 and several RubyGems are documented in 16.

his appendix provides several reference sections that you ll find useful from time to time while developing applications with Ruby. More specifically, what s in this appendix is limited to direct reference information that you might find useful while otherwise using this book. For a list of external resources, such as Web sites and mailing lists you can query for more detailed or up-to-date information, refer to Appendix C. Over time, it s essential you learn to use Ruby s online references, as they ll be updated in line with how the Ruby language develops. They ll also open your mind to new possibilities and advanced techniques not covered in this book.

Note This section is not designed to be an exhaustive reference. Only the most useful methods of several key classes are covered. For a complete, easy-to-search reference of the Ruby core classes, refer to http://www.ruby-doc.org/core/.

See Enumerable, a module that s mixed in with Array, for more methods. The following are the most commonly used methods available on Array objects: &: Intersects the contents of one array with another. For example: [1, 2, 3] & [2, 3, 4] == [2, 3]. *: Repeats the elements of an array a certain number of times if an integer is supplied; otherwise, joins the array elements together if a string is supplied. For example: [1, 2, 3] * 2 == [1, 2, 3, 1, 2, 3] and [1, 2, 3] * " " == "1 2 3".

+: Concatenates two arrays together into a new array. For example: [1, 2, 3] + [2, 3, 4] == [1, 2, 3, 2, 3, 4]. -: Returns a new array with elements removed. For example: [1, 2, 2, 3] [2] == [1, 3]. <<: Pushes or appends objects on to the end of the array. Equivalent to push. compact (and compact!): Returns a copy of the array with any nil elements removed. For compact!, nil elements are removed from the current array in place. delete_if: Invokes the supplied code block for each element of the array and deletes any elements that result in the code block evaluating to true. For example: [1, 11, 20].delete_if { |i| i > 10 } == [1]. each: Invokes the supplied code block for each element of the array, passing in each element as a parameter. each_index: Invokes the supplied code block for each element of the array, passing in the index of each element as a parameter. empty : Returns true if the array contains no elements. first: Returns the first element of the array. If none, then nil. flatten (and flatten!): Returns the array with all subarrays flattened. For example: [[1, 2], [2, 3], [4, 5]].flatten == [1, 2, 2, 3, 4, 5]. include : Returns true if the supplied object is also found within the array. index: Returns the index of the first instance of the supplied object within the array, if any. For example: %w{a b c d e}.index("d") == 3. join: Joins the array elements together using an optionally supplied string as a separator. last: Returns the last element of the array. If none, then nil. length: Returns the total number of elements within the array. pop: Removes the last element of the array and returns it. push: Pushes or appends the supplied object to the array (as with <<). reverse (and reverse!): Reverses the elements of the array and returns a new array. With reverse! the elements are reversed in the current array in place.