Expansion Pak No. 1.

The Tiger's Vest (with a Basic Introduction to Irb)

Tiger has vest.  Tiger likes girl robot.  Earth crashing into

Let’s install the very latest Ruby on your computer so you can follow all the examples in the (Poignant) Guide and actually do things right now! (Yes, things!)

  • If you are using Microsoft Windows, begin by downloading the Ruby Installer for Windows. Running this “one-click” installer will setup Ruby for you, as well as a tidy pack of useful software, such as a small text editor and some additional libraries.

  • If you are on Apple’s Mac OS X, then Ruby is already installed.

  • On Debian, use apt-get install ruby.

  • On Gentoo, emerge ruby.

To test if Ruby is installed, open a command shell and run: ruby -v. If Ruby is installed properly, you’ll see a bit of version info.

ruby 2.0.0p648 (2015-12-16 revision 53162) [universal.x86_64-darwin15]
  • To open a command shell in Microsoft Windows, go to the Start Menu and select Run.... Type in: cmd. Press OK. A command shell window will appear.
  • To open a command shell on Mac OS X, run the Terminal application from Spotlight.

Okay, keep that command shell open cause we’ll need it if the Earth gets rescued from its plummet toward the sun.

Tiger saves Earth with Ice Gun.  Girl robot zooms around tuxed

Ruby comes with a very, very, very extremely helpful tool called Irb. Interactive Ruby. In your command shell, type: irb.


You should see the prompt above. This Irb prompt will allow you to enter Ruby code and, upon pressing Enter, the code will run.

So, at the Irb prompt, do a: 3000 + 500.

irb(main):001:0> 3000 + 500
=> 3500

The example 3000 + 500 is legitimate code. We’re just not assigning the answer to a variable. Which is okay in Irb, because Irb prints the answer given back by code you run.

Irb is a great calculator.

irb(main):002:0> ( ( 220.00 + 34.15 ) * 1.08 ) / 12
=> 22.8735
irb(main):003:0> "1011010".to_i( 2 )
=> 90
irb(main):004:0> Time.now - Time.local( 2003, "Jul", 31, 8, 10, 0 )
=> 31119052.510118

The first example demonstrates a bit of math and is read as: 220.00 plus 34.15 times 1.08 divided by 12. The second example takes a binary string and converts it to a decimal number. The third example computes the number of seconds between now and July 31, 2003 at 8:10 AM. The answers to all of these are printed back to us by Irb with a little ASCII arrow pointing.

Reading the Prompt

I know the prompt surely looks bewildering. Well, let’s not delay in dissecting it. It’s very simple. The prompt has three parts, each separated by colons.

The first section, which reads irb(main), shows the name of the program we are running. The second section shows a line number, a count of how many lines of Ruby we’ve typed. The third section is a depth level. Whenever you open a statement which needs closing, the depth level will increase by one. And whenever Irb senses that your code is unfinished, the end of the prompt will become an asterisk.

irb(main):001:0> bell = :pressed
=> :pressed
irb(main):002:0> ice_gun =
irb(main):003:0*   if bell == :pressed
irb(main):004:1>     :on
irb(main):005:1>   else
irb(main):006:1*     :off
irb(main):007:1>   end
=> :on

Notice how the depth level increased to 1 when I opened the if statement. And how the asterisk indicates the continuation of a line.

Tweaking the Prompt

You don’t have to like the prompt’s appearance, though. I’m not forcing you to do anything and, if you want to hack it up, I’m right here beside you.

Irb has a few other included prompts which may better appease your senses. Try irb --prompt simple. This time Irb will treat you to a very basic set of arrows, allowing you to enter your code without the whole status report.

>> %w(my best friend's arm)
=> ["my", "best", "friend's", "arm"]

Ruby comes with a few prompts. The simple prompt seen above. The xmp prompt which has no prompt at all and indents the answer arrow. (It’s supposed to look nice for printing.) Also, the null prompt, which eliminates prompts altogether. Set the prompt by just supplying the name with the --prompt option. (So, irb --prompt null.)

Making your own prompt is okay as well. Irb is completely customizable from inside of itself. The conf object contains all of Irb’s configuration settings. Some of these configuration settings are for controlling the prompt.

>> conf.methods.grep /prompt/
=> ["prompting?", "prompt_s", "prompt_s=", "prompt_c", "prompt_c=", 
    "prompt_i", "prompt_mode", "prompt_i=", "prompt_mode="]

Let’s setup our prompt to display line numbers with just a bit of decor.

>> conf.prompt_i = "%3n :> "           # the normal prompt
>> conf.prompt_s = "%3n .%l "          # the string continuation prompt
>> conf.prompt_c = "%3n .^ "           # the code continuation prompt
>> conf.return_format = "    => %s\n"  # the answer arrow

Above are the four parts to an Irb prompt. The string continuation prompt is displayed when a string is still open when you hit Enter. The %3n describes that Irb should reserve three characters for the line number. The %l saves a place for displaying the type of string being continued. (If you’re continuing a double-quoted string, it shows a double quote. If you’re continuing a regular expression, it shows a slash.)

The rest are little symbols to decorate the prompt. So, in the case of a continuing code line, I show a caret which points up to the line where that line of code started.

You can read more about customizing Irb and saving your configuration to a file in the complete guide to Irb, available in the free-for-your-wandering-Internet-eyes Programming Ruby.

Tab Completion

One feature of Irb which is rarely mentioned is its beneficial tab completion.

irb --readline -r irb/completion

Basically, when you hit Tab, Irb will take a guess at what you’re trying to type. Try typing: [].col and hit Tab. Irb will finish it. [].collect and it leaves your cursor at the end so you can add further.

If there are several matches, just hitting Tab will do nothing. But hit it twice and Ruby will give you a complete list of possible matches.

This is great if you just want to see all the methods for a certain object. Type in any number, a dot, and use Tab.

>> 42.
              42.floor                   42.next                    42.step
42.__id__     42.freeze                  42.nil?                    42.succ
42.__send__   42.frozen?                 42.nonzero?                42.taint
42.abs        42.hash                    42.object_id               42.tainted?
42.between?   42.id                      42.prec                    42.times
42.ceil       42.id2name                 42.prec_f                  42.to_f
42.chr        42.inspect                 42.prec_i                  42.to_i
42.class      42.instance_eval           42.private_methods         42.to_int
42.clone      42.instance_of?            42.protected_methods       42.to_s
42.coerce     42.instance_variable_get   42.public_methods          42.to_sym
42.display    42.instance_variable_set   42.quo                     42.truncate
42.div        42.instance_variables      42.remainder               42.type
42.divmod     42.integer?                42.respond_to?             42.untaint
42.downto     42.is_a?                   42.round                   42.upto
42.dup        42.kind_of?                42.send                    42.zero?
42.eql?       42.method                  42.singleton_method_added
42.equal?     42.methods                 42.singleton_methods
42.extend     42.modulo                  42.size

Now, trying typing Kernel:: and then Tab. All the core methods. Don’t ever forget this and use it all the time.

Except the robot flew away and the ice gun went on and

Okay, one last thing and then I’ll quit bugging you with all this great technology. But I have to say it loud, so take cover! I’m across the world here, folks, but the volume comes down from the sky—a bold, red crescendo of—


(Ruby’s Own 411 or 555-1212 or Yes, Operator, Get Belgrade on the Line—I'll Be Right Here—Just Plain Hammering The Pound Key Until Someone Picks Up…)

And Ri picks up the line. “This is Ri. Class and colon, please.”

You rush in, “This is an instance method, Operator. Enumerable#zip.”

ri Enumerable#zip

Without delay, right up on your teletype display (so swiftly that even the cat perched atop cranes his neck around, gapes and hands it the royal cup Most Blatantly Great Thing Since Michael Dorn):

--------------------------------------------------------- Enumerable#zip
     enum.zip(arg, ...)                   => array
     enum.zip(arg, ...) {|arr| block }    => nil
     Converts any arguments to arrays, then merges elements of _enum_
     with corresponding elements from each argument. This generates a
     sequence of +enum#size+ _n_-element arrays, where _n_ is one more
     that the count of arguments. If the size of any argument is less
     than +enum#size+, +nil+ values are supplied. If a block given, it
     is invoked for each output array, otherwise an array of arrays is

        a = [ 4, 5, 6 ]
        b = [ 7, 8, 9 ]

        (1..3).zip(a, b)      #=> [[1, 4, 7], [2, 5, 8], [3, 6, 9]]
        "cat\ndog".zip([1])   #=> [["cat\n", 1], ["dog", nil]]
        (1..3).zip            #=> [[1], [2], [3]]

It’s an unabridged Ruby dictionary servo—the Power of Just Asking is at your fingertips—don’t tell me you’ve never heard of this no-money-down lifetime-supply-of-proper-explanations!

To get an explanation of any class, along with a complete directory to all of its methods, all in a very soothing voice fit for calming any of you panicking cosmonauts out there fighting the pull of some zero-tolerance tractor beam, just use at your command shell: ri Class.

But for help on class methods, you cuff on: ri Class::method.

Though instance methods use a pound key rather than a dot. (Since the dot can mean class or instance methods.) I mean: ri Class#method.

The full spread of classes, a full list from the Very Top down to the Earth’s core, can be achieved with ri -l.

And beyond text, you can make HTML:

ri -Tf html String#gsub > gsub.html

Or show colory ANSI:

ri -Tf ansi String#gsub

And this is last and best.

Into the Ri Switchboard

Behind Ri sings a chorus of human voices, primarily Dave Thomas, an author of Programming Ruby, and absolutely the American foster parent of Ruby. Many of these elaborate spiels from Ri are straight from the references of Programming Ruby. Don’t forget to thank Dave periodically.

Ri culls its lush information set from the very code that Ruby is built from. In each of the files of code back in the Ruby Headquarter’s file cabinet, detailed comments describe everything in sight.

In Ruby’s date class, here we have such commented methods:

# Get the time of this date as [hours, minutes, seconds,
# fraction_of_a_second]
def time() self.class.day_fraction_to_time(day_fraction) end

# Get the hour of this date.
def hour() time[0] end

# Get the minute of this date.
def min() time[1] end

The comments show up in Ri. We type: ri Date#time.

-------------------------------------------------------------- Date#time
     Get the time of this date as [hours, minutes, seconds,

Ri figures out much of how a method works, although it expects coders to write a brief description in the comments just before a method or class definition. I would suggest that whenever you write a method, add a brief description in the comments before that method. In time, you can generate Ri documentation for that method.

You can also use a few special characters to enhance your description. For example, if you indent a paragraph and use an asterisk * or a dash - just before the letters of the first sentence, the paragraph will be recognized as a list item. Then, if your description needs to be converted to HTML, you’ll see the list item appear as an HTML unordered list.

# Get the time of this date as an Array of:
# * hours
# * minutes
# * seconds
# * fraction_of_a_second
def time() self.class.day_fraction_to_time(day_fraction) end

Other rules as well: Lists which start with digits followed by periods are recognized as numbered lists. Emphasized words are surrounded by underscores, bold words by asterisks, code words by plus signs. Examples are simply blocks of text indented a few spaces. All of these rules together are called RDoc.

Here’s a bit of RDoc from the initialize method in one of my projects called RedCloth. Notice the indented example as well as the Ruby class and method names flanked with plusses.

# Returns a new RedCloth object, based on +String+ and
# enforcing all the included +restrictions+.
#   r = RedCloth.new( "h1. A <b>bold</b> man", [:filter_html] )
#   r.to_html
#     #=>"<h1>A &amp;lt;b&amp;gt;bold&amp;lt;/b&amp;gt; man</h1>"
def initialize( string, restrictions = [] )
    @lite = false
    restrictions.each { |r| method( "#{ r }=" ).call( true ) }
    super( string )

For the full set of RDoc rules see the Markup section of the README.

Pushing Out Your Own Ri

Ri doesn’t automatically read your files for you, though. You’ve got to push it along, show it the way. Change to the directory of the code you’d like to scan. Then, use the RDoc tool to make it happen.

cd ~/cvs/your-code
rdoc --ri-site

Now, try using ri YourClass to ensure all your descriptions are showing up properly. If you want to make HTML documentation, try this:

cd ~/cvs/your-code

The ~/cvs/your-code directory should now also have a fresh doc directory containing HTML documentation for all of your classes. View index.html for the pleasant news.

Well then. Your hands are in it all now. Welcome to Ruby.

The tiger finds a new home and learns to eventually move on.

Turn page.