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With load, the code is loaded and reprocessed anew each time you use the load method. require, on the other hand, only processes external code once.

Note Ruby programmers generally use require rather than load. The effects of load are only useful

if the code in the external file has changed or if it contains active code that will be executed immediately. However, a good programmer will avoid the latter situation, and external files will only contain classes and modules that will, generally, rarely change.

Both load and require can take local or absolute filenames. For example, require 'a' first looks for a.rb in the current directory, and then iterates through a multitude of other directories on your hard drive looking for a.rb. By default, these other directories are the various directories where Ruby stores its own files and libraries, although you can override this, if necessary. Ruby stores the list of directories to search for included files in a special variable called $:. You can see what $: contains by default, using irb:

$:.each { |d| puts d }

/usr/local/lib/ruby/site_ruby/1.8 /usr/local/lib/ruby/site_ruby/1.8/i686-darwin8.8.1 /usr/local/lib/ruby/site_ruby /usr/local/lib/ruby/1.8 /usr/local/lib/ruby/1.8/i686-darwin8.8.1 .

Note This result is what appears on my machine, running Mac OS X. The list of directories will probably

differ significantly on your machine, particularly if you re using Windows, where the path layout will be entirely different, with the drive letter at the start and backslashes instead of forward slashes.

If you want to add extra directories to this, it s simple:

$:.push '/your/directory/here' require 'yourfile' $: is an array, so you can push extra items to it, or use unshift to add an element to the start of the list (if you want your directory to be searched before the default Ruby ones useful if you want to override Ruby s standard libraries).

Note Ruby keeps track of the files include has processed by using the name used to access them. If you have two paths pointing to the same file, and include the same file but by using two unique, full filenames, Ruby will duly load the same file twice.

point in your Ruby code and they ll behave as if they were processed at that point. For example:

$debug_mode = 0 require $debug_mode == 0 "normal-classes" : "debug-classes"

It s an obscure example, but what it does is check if the global variable $debug_mode is set to 0. If it is, it requires normal-classes.rb, and if not, debug-classes.rb. This gives you the power to include a different source file dependent on the value of a variable, ideal for situations where your application has regular and debug modes. You could even write an application that works perfectly, but then use a different require to include a whole different set of files that have new or experimental functionality. A commonly used shortcut uses arrays to quickly load a collection of libraries at once. For example:

%w{file1 file2 file3 file4 file5}.each { |l| require l }

This loads five different external files or libraries with just two lines of code. However, some coders are not keen on this style, as it can make the code harder to read, even if it s more efficient.

Code from files that are included into others with require and load has the same freedom as if the code were pasted directly into the original file. This means files that you include can call load and require themselves. For example, assume a.rb contains the following:

require 'b'

And b.rb contains the following:

require 'c'

And c.rb contains the following:

And d.rb contains the following:

Hello!

d.rb includes a.rb with require, a.rb includes b.rb, and b.rb includes c.rb, meaning the example method is available to d.rb. This functionality makes it easy to put together large projects with interdependent parts, as the structure can be as deep as you like.