layout: page

title: "Offical Ruby FAQ"

lang: en

header: |

10. Extension library

10.1 How can I use Ruby interactively?

You can try using irb. The following is paraphrased from Goto Kentaro (Gotoken), and originally appeared in ruby-talk:444.

Get the latest tarball of irb from the contrib directory in the Ruby archive.

Extract the irb directory tree.

Add the location of the irb/ directory to the $RUBYLIB environment variable

Make a symbolic link from $RUBYLIB/irb/irb.rb to a file called irb somewhere in your path.

chmod +x $RUBYLIB/irb/irb.rb

Possibly use rehash to tell your login shell about the new command.

Type irb

If the readline extension module works with your interpreter, it makes irb a lot more fun to use.

There is also a simple program, eval, in the samples/ directory of the Ruby distribution. It lets you enter expressions and view their values. You can copy eval into the site_ruby directory in the Ruby tree, and then invoke it using:

ruby -r eval -e0

10.2 Is there a debugger for Ruby?

There is a gdb-like debugger for Ruby.

ruby -r debug your_program

10.3 How can I use a library written in C from Ruby?

Of all the scripting languages, Ruby is probably the easiest to extend. There are no problems with reference counting and variable types, and very few interfaces to learn. In fact, C code used to extend Ruby often ends up looking surprisingly like Ruby code itself.

First, get the Ruby source distribution and read README.EXT. This is a good document, not only if you're writing an extension library, but also if you want to understand Ruby more deeply.

Next, have a look at the source of the interpreter itself, and at the various supplied extensions in the ext/ directory. You'll also find good examples under contrib/ on the Ruby ftp sites.

10.4 Can I use Tcl/Tk interface in Ruby?

There are two interfaces to Tcl/Tk included in the standard distribution. One is under ext/tcltk/ and loaded with require "tcltk". The syntax is very close to that Tcl, which is passed to Tcl interpreter. Unfortunately, the description for this library is written in Japanese.

The other is under ext/tk/ and loaded with require "tk". Its syntax closer to the style of the Tk interface provided by the Perl and Python interfaces.

10.5 Tk won't work.

Your Tk version may be old, try a newer version.

10.6 Can I use gtk+ or xforms interfaces in Ruby?

You'll find ruby-gtk-x.xx.tar.gz and ruby-forms-x.x.tar.gz under contrib/ in ftp sites.

10.7 How can I do date arithmetic?

A Time object can express only the dates between Jan 1, 1970 and Jan 19, 2038.

Two standard extension library modules are provided: require "date", which is simple and uses the English calendar, and require "date2", which is more general purpose.

Also see sample/cal.rb.

layout: page

title: "Offical Ruby FAQ"

lang: en

header: |

11. Other Features

11.1 What does a ? b : c mean?

It's the same as saying if a then b else c end.

11.2 How can I count the number of lines in a file?

Assuming that the file ends in a linefeed, the following code may give the fastest result.

open("example").read.count("\n") # -> 3

11.3 What do begin and end of MatchingData return?

They act with $ , and return the start index and the end index of the matched data ($0) in the original string. See an example in tab expansion.

11.4 How can I sum the elements in an array?

Rather than solve the specific problem, let's solve the general case. The first thing we'll do is produce a method that will iterate over an Enumerable object and collect a single result. Smalltalk calls that method inject, so we will too:

module Enumerable

def inject(n)

each { |i|

n = yield(n, i)

}

n

end

end

Notice how we've added the method to Enumerable. This means that anything that includes Enumerable can now use inject. But how do we use it? It takes a single argument `n' and a block. For each element in the thing being enumerated, it calls the block, passing in `n' and the element itself. The result of the block is assigned back to `n'. So, to define sum, we could write:

module Enumerable

def sum

inject(0) {|n, i| n + i }

end

end

[1,3,5,7,9].sum # -> 25

(1..100).sum # -> 5050

11.5 How can I use continuations?

Ruby's continuations allow you to create an object representing a place in a Ruby program, and then return to that place at any time (even if it has apparently gone out of scope). Continuations can be used to implement complex control structures, but are typically more useful as ways of confusing people.

In [ruby-talk:4482], Jim Weirich posted the following examples of continuations:

def count_task(count, consumer)

(1..count).each do

|i|

callcc {|cc| consumer.call cc, i }

end

nil

end

def print_task()

producer, i = callcc { |cc| return cc }

print "#{i} "

callcc { |cc| producer.call }

end

def count(limit)

count_task(limit, print_task())

print "\n"

end

def filter_task(factor, consumer)

producer, i = callcc { |cc| return cc }

if (i%factor) != 0 then

callcc { |cc| consumer.call cc, i }

end

producer.call

end

def omit(factor, limit)

printer = print_task()

filter = filter_task(factor, printer)

count_task(limit, filter)

print "\n"

end

def prime_task(consumer)

producer, i = callcc { |cc| return cc }

if i >= 2 then

callcc { |cc| consumer.call cc, i }

consumer = filter_task(i, consumer)

end

producer.call

end

def primes(limit)

printer = print_task()

primes = prime_task(printer)

count_task(limit, primes)

print "\n"

end