Ruby is a simple and powerful object-oriented programming language, created by Yukihiro Matsumoto (who goes by the handle "matz" in this document and on the mailing lists).
Like Perl, Ruby is good at text processing. Like Smalltalk, everything in Ruby is an object, and Ruby has blocks, iterators, meta-classes and other good stuff.
You can use Ruby to write servers, experiment with prototypes, and for everyday programming tasks. As a fully-integrated object-oriented language, Ruby scales well.
Ruby features:
Let's define a class called Person, with a name and an age. We'll test our code by creating a few people and examining them.
class Person
attr_accessor :name, :age
def initialize(name, age)
@name = name
@age = age.to_i
end
def inspect
"#@name (#@age)"
end
end
p1 = Person.new('elmo', 4)
p2 = Person.new('zoe', 7)
p1 # -> elmo (4)
p2 # -> zoe (7)
|
Now let's populate an array of people by reading their names and ages from a file containing lines like:
bert: 8 cookie: 11 elmo: 4 ernie: 8 zoe: 7 |
The code uses regular expressions to parse successive lines from the
input file, creating a new Person object for each match and
pushing it on to the end of the array people.
people = Array.new
File.foreach("ages") { |l|
people << Person.new($1, $2) if l =~ /(.*):\s+(\d+)/
}
people # -> [bert (8), cookie (11), elmo (4), ernie (8), zoe (7)]
|
Now, let's sort the result based on the person's age. There are many ways to do this. We can define a sort block, which tells Ruby how to do the comparison of two people:
sorted = people.sort do |a,b| a.age <=> b.age end sorted # -> [elmo (4), zoe (7), bert (8), ernie (8), cookie (11)] |
Another way would be to change the comparison method for class Person:
class Person
def <=>(other)
@age <=> other.age
end
end
people.sort # -> [elmo (4), zoe (7), bert (8), ernie (8), cookie (11)]
|
Influenced by Perl, Matz wanted to use a jewel name for his new language, so he named Ruby after a colleague's birthstone.
Later, he realized that Ruby comes right after Perl in several situations. In birthstones, pearl is June, ruby is July. When measuring font sizes, pearl is 5pt, ruby is 5.5pt. He thought Ruby was a good name for a programming language newer (and hopefully better) than Perl.
(Based on an explanation from matz in [ruby-talk:00394] on June 11, 1999.)
The following a summary of a posting made by Matz in [ruby-talk:00382] on June 4, 1999. (The birthday of Ruby is corrected in [ruby-list:15977]).
Well, Ruby was born on February 24 1993. I was talking with my colleague about the possibility of an object-oriented scripting language. I knew Perl (Perl4, not Perl5), but I didn't like it really, because it had smell of toy language (it still has). The object-oriented scripting language seemed very promising.
I knew Python then. But I didn't like it, because I didn't think it was a true object-oriented language---OO features appeared to be add-on to the language. As a language manic and OO fan for 15 years, I really wanted a genuine object-oriented, easy-to-use scripting language. I looked for, but couldn't find one.
So, I decided to make it. It took several months to make the interpreter run. I put it the features I love to have in my language, such as iterators, exception handling, garbage collection.
Then, I reorganized the features of Perl into a class library, and implemented them. I posted Ruby 0.95 to the Japanese domestic newsgroups in Dec. 1995.
Since then, highly active mailing lists have been established and web pages formed.
The official Ruby Home Page is
http://www.ruby-lang.org
(in English) and
http://www.ruby-lang.org/ja/
(in Japanese).
You can also find Ruby information at
http://www.rubycentral.com. In particular, there is a
complete
online reference
to Ruby's built-in classes and methods.
comp.lang.ruby was established in May, 2000 (thanks to the
efforts of
Conrad Schneiker).
There are five mailing lists now talking about Ruby. The first is in English, the last four in Japanese:
ruby-talk: English language discussion of Ruby.ruby-list: Japanese language discussion of
Ruby.ruby-dev: List for Ruby developers.ruby-ext: List for people writing extensions for or with Ruby.ruby-math: Ruby in mathematics.You can search the mailing list archives using http://blade.nagaokaut.ac.jp/ruby/ruby-talk/index.shtml. (This is the url for the ruby-talk list: munge as required for the others).
The Ruby mailing list software adds a prefix to the subject lines, for example [ruby-talk:1234]. This can confuse the threading in some mail user agents.
In mutt, you can get threading to work using the following variable setting.
# reply regexp, to support MLs like ruby-talk. set reply_regexp="^(\[[a-z0-9:-]+\][[:space:]]*)?(re([\[0-9\]+])*|aw):[[:space:]]*" |
Officially, the language is called ``Ruby''. On most systems, it is
invoked using the command ``ruby''. It's OK to use ruby
instead of Ruby.
Please don't use RUBY as the language name.
Originally, or historically, it was called ``ruby''.
Programming Ruby: The Pragmatic Programmer's Guide, (the Pickaxe Book) by David Thomas and Andrew Hunt: ISBN 0-20171-089-7, Addison-Wesley, October 2000.
A Japanese language Ruby reference book by matz, et al and published by ASCII is available in Japan (ISBN 4-7561-3254-5). An English translation, `` The Ruby Programming Language,'' is in the works from Addison-Wesley (ISBN 020171096X).
>
A Japanese language ``Ruby Pocket Reference'' is published by O'Reilly Japan (ISBN 4-87311-023-8). Let O'Reilly in the US know if you'd like to see a translation.
>
In addition, `` Mastering Regular Expressions,'' by Jeffrey Friedl, (the Hip Owl Book): ISBN 1-56592-257-3 from O'Reilly & Associates, is a reference work that covers the art and implementation of regular expressions in various programming languages. Most of it is highly relevant to Ruby regular expressions.
People commonly annotate Ruby code by showing the results of executing each statement as a comment attached to that statement. For example, in the following code, we show that the assignment generates the string "Billy Bob", and then result of extracting some substrings.
str = "Billy" + " Bob" # -> "Billy Bob" str[0,1] + str[2,1] + str[-2,2] # -> "Blob" |
Gotoken's xmp package, available from
http://www.ruby-lang.org/en/raa-list.rhtml?name=xmp
is a utility that annotates Ruby source code this way.
Emacs and vim users can integrate this with their editing environments, which is useful if you want to send people e-mail with annotated Ruby code. Having installed xmp, Emacs users can add the following to their .emacs file:
(defun ruby-xmp-region (reg-start reg-end)
"Pipe the region through Ruby's xmp utility and replace the region with
the result."
(interactive "r")
(shell-command-on-region reg-start reg-end
"ruby -r xmp -n -e 'xmp($_, \"%l\t\t# %r\n\")'"
t))
(global-set-key [(meta f10)] 'ruby-xmp-region)
|
Vim users can use the mapping (thanks to hipster):
map <M-F10> :!ruby -r xmp -n -e 'xmp($_, "\%l\t\t\# \%r\n")'<CR> |
In both cases, highlight a region of code and hit Meta-F10 to annotate it.
The syntax of Ruby has been fairly stable since Ruby 1.0, but new features are added every now and then. So, the books and the online documentation can get behind.
If you have a problem, feel free to ask in the mailing list (see ruby-talk mailing list). Generally you'll get timely answers from matz himself, the author of the language, from other gurus, and from those who've solved problems similar to your own.
Please include the output of ruby -v along with any
problematic source code.
If you have a problem using
irb, be
aware that it has some limitations. Try the script using irb
--single-irb, or directly using the ruby command.
There might be similar questions in the mailing list, and it is good netiquette to read through recent mails (RFC1855:3.1.1, 3.1.2) before asking. But do ask on the list, and a correct answer will be forthcoming.
Python and Ruby are both object oriented languages that provide a smooth transition from procedural to OO programming styles. Smalltalk, by contrast, is object only - you can't do anything until you understand objects, inheritance and the sizable Smalltalk class hierarchy. By providing procedural training wheels, Python and Ruby ``fix'' one of the features that may have kept Smalltalk out of the mainstream. The two languages differ by approaching this solution from opposite directions.
Python is a hybrid language. It has functions for procedural programming and objects for OO programming. Python bridges the two worlds by allowing functions and methods to interconvert using the explicit ``self'' parameter of every method def. When a function is inserted into an object, the first argument automagically becomes a reference to the receiver.
Ruby is a pure OO language that can masquerade as a procedural one. It has no functions, only method calls. In a Ruby method the receiver, also called self, is a hidden argument like ``this'' in C++. A ``def'' statement outside of a class definition, which is a function in Python, is actually a method call in Ruby. These ersatz functions become private methods of class Object, the root of the Ruby class hierarchy. Procedural programming is neatly solved from the other direction - everything is an object. If the user doesn't grok objects yet, they can just pretend that ``def'' is a function definition and still get useful work done.
Ruby's OO purity provides a number features that Python lacks or is
still working toward: a unified type/class hierarchy, metaclasses, the
ability to subclass everything, and uniform method invocation
(none of this len() is a function but items() is a
method rubbish). Ruby, like Smalltalk, only supports single
inheritance, but it does have a very powerful mixin concept: a class
definition may include a module, which inserts that module's methods,
constants, etc. into the class.
Ruby, again like Smalltalk, provides closures and code blocks and uses them to the same good effect. The Ruby collection classes and iterators are outstanding, much more powerful and elegant than the ad hoc solutions that Python is sprouting (lambdas and list comprehensions).
Ruby's syntax and design philosophy are heavily influenced by Perl. It has a lot of syntactic variability. Statement modifiers (if, unless, while, until, etc.) may appear at the end of any statement. Some key words are optional (the ``then'' in an ``if'' statement for example). Parentheses may sometimes be elided in method calls. The receiver of a method may usually be elided. Many, many things are lifted directly from Perl. Built in regular expressions, $_ and friends, here documents, the single-quoted / double-quoted string distinction, $ and @ prefixes to distinguish different kinds of names and so forth.
If you like Perl, you will like Ruby and be right at home with its syntax. If you like Smalltalk, you will like Ruby and be right at home with its semantics. If you like Python, you may or may not be put off by the huge difference in design philosophy between Python and Ruby/Perl.
Ruby is much more complex than Python but its features, for the most part, hang together well. Ruby is well designed and full of neat ideas that might be mined for P3K. I'm not sure how many Python programmers will be attracted to it though - it hasn't won me over (yet). But it is worthy of serious study and could be a real threat to Perl.
Posted by
John Dell'Aquila in comp.lang.python,
11/17/2000. Reproduced with permission.
Ruby is developed under Linux, and is written in fairly straightforward C. It runs under UNIX, DOS, Windows 95/98/NT/2000, Mac OSX, BeOS, Amiga, Acorn Risc OS, and OS/2.
August 2002:
MacOSX 10.2 now comes with ruby installed by default. (ruby 1.6.7 (2002-03-01) [powerpc-darwin6.0]) See http://developer.apple.com/technotes/tn2002/tn2053.html.
H Morita notes:
There's a MacOS (not X) port of Ruby, by Hisakuni FUJIMOTO at http://www.imasy.or.jp/~hisa/ruby/macruby.html. However it's based on Ruby 1.1b7, and hasn't been updated since December 1999. It's highly experimental. It may crash and sometimes freeze the OS, even with the sample scripts included in the Ruby distribution. (Sounds like fun ;-).
>
Rob tells us that there's Ruby 1.6.4 for OS/2 at http://www.aminet.org/systems/os2/dev/misc/. However, that link seems broken: can anyone give us something that works?
The latest version of Ruby can be downloaded from:
http://www.ruby-lang.org/en/download.html
Mirror sites are also listed on this page.
Also on this page is a link to a nightly snapshot of the development tree.
If you have a CVS client, you can check out the current source tree using:
% cvs -d :pserver:[email protected]:/home/cvs login (Logging in to [email protected]) CVS password: guest % cvs -d :pserver:[email protected]:/home/cvs co ruby |
If you do not have CVS you can get a nightly snapshot of the development source from ftp://ftp.netlab.co.jp/pub/lang/ruby/snapshot.tar.gz.
Under Unix, Ruby uses the autoconf system to configure the build environment. You don't need the autoconf command on your box to build Ruby from a distribution; just use the commands:
% ./configure [configure options] % make % make test % make install |
You may need superuser privileges to install Ruby if you don't
override the default installation location
(/usr/local). You can get a
full list of configure options using:
% ./configure --help |
If you are working from the CVS archive, you may need to run
autoconf before running configure.
On some systems, the build process may fail to find libraries used by
extension modules (for example the dbm libraries).
You can tell Ruby where to find libraries using options to
configure. From [ruby-talk:5041]:
./configure --with-xxx-yyy=DIR |
where xxx is either
opt extra software path in general dbm path for dbm library gdbm path for gdbm library x11 ...for X11.. tk ...for Tk... tcl ...for Tcl... |
and yyy is either
dir specifies -I DIR/include -L DIR/lib include specifies -I DIR lib specifies -L DIR |
On HP-UX, there may be problems building with gcc. Try using the native compiler instead. WATANABE Tetsuya recommends:
CC="cc -Ae" CFLAGS=-O ./configure --prefix=/opt/gnu |
There may also be problems with HP's native sed. He recommends installing the GNU equivalent.
A single download that contains everything you need to run Ruby under various Windows operating systems. is available from RubyCentral's One-click Windows installer. This installation uses cygwin, and includes Tk support.
If you want other installation options, precompiled binaries for
Windows are also available from
http://www.os.rim.or.jp/~eban/.
If you download the ruby-1.x.y-yyyymmdd-i386-cygwin.tar.gz
package (which is a good choice), you'll also need to download the
cygwin DLL, available from the same page.
Reuben Thomas writes:You could mention that there's a port to Acorn RISC OS, currently of v1.4.3. I made the port, and have no plans to maintain it, but I did send the patches to matz, so newer versions may well compile too.
I do provide a binary distribution of 1.4.3 for the Acorn at http://www.cl.cam.ac.uk/users/rrt1001/ruby.zip.
Ruby is written to take advantage of the rich feature set of a Unix environment. Unfortunately, Windows is missing some of the functions, and implements others differently. As a result, some kind of mapping layer is needed to run Ruby (and other Unix-based programs) under windows.
You may come across different versions of the Ruby executable that use different wrapper mapping layers.
The rbdj version is a stand-alone version of the Windows binary of Ruby. It uses the DJ Delorie tools ( http://www.delorie.com).
The rbcw version is a Windows binary of Ruby that requires the cygwin library, available at http://www.cygwin.com or from the Ruby download pages. Cygwin is a both an emulation layer and a set of utilities initially produced by Cygnus Solutions (now part of Redhat). The Cygwin version of Ruby probably has the fullest set of features under Windows, so most programmers will want to use it.
To use the rbcw version, you will need to install the cygwin .dll
separately. Once you have installed cygwin on your computer, copy
cygwin1.dll (which is found in the bin subdirectory of the
cygwin distribution) to your Windows\System32 folder (or somewhere else on your
path).
Thanks to Anders Schneiderman for the basis of this description
TCL_LIBRARY and
TK_LIBRARY pointing to the directories containing tcl and tk?
All variables and constants reference (point at) some object. (The
exception is uninitialized local variables, which reference
nothing. These raise a NameError exception if used). When
you assign to a variable, or initialize a constant, you set the object
that the variable or constant references.
Assignment on its own therefore never creates a new copy of an object.
There's a slightly deeper explanation in certain special cases.
Instances of Fixnum, NilClass, TrueClass, and FalseClass are contained
directly in variables or constants--there is no reference involved. A
variable holding the number 42 or the constant true, actually
holds the value, and not a reference to it. Assignment therefore
physically produces a copy of objects of these types. We discuss this
more in
Immediate and Reference Objects.
A new scope for a local variable is introduced in the (1) toplevel (main), (2) a class (or module) definition, or (3) a method definition.
i = 1 # (1)
class Demo
i = 2 # (2)
def meth
i = 3 # (3)
print "In method, i = ", i, "\n"
end
print "In class, i = ", i, "\n"
end
print "At top level, i = ", i, "\n"
Demo.new.meth
Produces:
In class, i = 2
At top level, i = 1
In method, i = 3
|
(Note that the class definition is executable code: the trace message it contains is written as the class is defined).
A block (``{'' ... ``}'' or do ... end) almost introduces a
new scope ;-) Locals created within a block are not accessible
outside the block. However, if a local within the block has the same
name as an existing local variable in the caller's scope, then no new
local is created, and you can subsequently access that variable
outside the block.
a = 0 1.upto(3) do |i| a += i b = i*i end a # -> 6 # b is not defined here |
This becomes significant when you use threading--each thread receives its own copy of the variables local to the thread's block:
threads = []
for name in ['one', 'two' ] do
threads << Thread.new {
localName = name
a = 0
3.times do |i|
Thread.pass
a += i
print localName, ": ", a, "\n"
end
}
end
threads.each {|t| t.join }
Produces:
onetwo: : 00
onetwo: : 11
onetwo: : 33
|
while, until, and for are control
structures, not blocks, so local variables within them will be
accessible in the enclosing environment.
loop, however, is a method and the associated block
introduces a new scope.
Actually, the question may be better asked as: "at what point does Ruby work out that something is a variable?" The problem arises because the simple expression ``a'' could be either a variable or a call to a method with no parameters. To decide which is the case, Ruby looks for assignment statements. If at some point in the source prior to the use of ``a'' it sees it being assigned to, it decides to parse ``a'' as a variable, otherwise it treats it as a method. As a somewhat pathological case of this, consider this code fragment, submitted by Clemens Hintze:
def a
print "Function 'a' called\n"
99
end
for i in 1..2
if i == 2
print "a=", a, "\n"
else
a = 1
print "a=", a, "\n"
end
end
Produces:
a=1
Function 'a' called
a=99
|
During the parse, Ruby sees the use of ``a'' in the first print statement and, as it hasn't yet seen any assignment to ``a'', assumes that it is a method call. By the time it gets to the second print statement, though, it has seen an assignment, and so treats ``a'' as a variable.
Note that the assignment does not have to be executed---Ruby just has to have seen it. This program does not raise an error.
a = 1 if false; a # -> nil |
This issue with variables is not normally a problem. If you do bump
into it, try putting an assignment such as a = nil before the
first access to the variable. This has the additional benefit of
speeding up the access time to local variables that subsequently
appear in loops.
A constant defined in a class/module definition can be accessed directly within that class or module's definition.
You can directly access the constants in outer classes and modules from within nested classes and constants.
You can also directly access constants in superclasses and included modules.
Apart from these cases, you can access class and module constants
using the :: operator--ModuleName::CONST1 or
ClassName::CONST2.
The actual argument is assigned to the formal argument when the method is invoked. (See assignment for more on the semantics of assignment.)
def addOne(n)
n += 1
end
a = 1
addOne(a) # -> 2
a # -> 1
|
As you are passing object references, it is possible that a method may modify the contents of a mutable object passed in to it.
def downer(string)
string.downcase!
end
a = "HELLO" # -> "HELLO"
downer(a) # -> "hello"
a # -> "hello"
|
There is no equivalent of other language's pass-by-reference semantics.
A formal argument is a local variable. Within a method, assigning to a formal argument simply changes the argument to reference another object.
All Ruby variables (including method arguments) act as references to objects. You can invoke methods in these objects to get or change the object's state and to make the object do something. You can do this with objects passed to methods. You need to be careful when doing this, as these kinds of side effects can make programs hard to follow.
*'' prepended to an argument mean?
When used as part of a formal parameter list, the asterisk allows arbitrary numbers of arguments to be passed to a method by collecting them into an array, and assigning that array to the starred parameter.
def foo(prefix, *all)
for e in all
print prefix, e, " "
end
end
foo("val=", 1, 2, 3)
Produces:
val=1 val=2 val=3
|
When used in a method call, * expands an array, passing
its individual elements as arguments.
a = [1, 2, 3] foo(*a) |
You can prepend * to the last argument of
when clause of case structure.For example:
x, *y = [7, 8, 9] x # -> 7 y # -> [8, 9] x, = [7, 8, 9] x # -> 7 x = [7, 8, 9] x # -> [7, 8, 9] |
&'' prepended to an argument mean?
If the last formal argument of a method is preceeded with an
ampersand, a block following the method call will be converted into a
Proc object and assigned to the formal parameter.
If the last actual argument in a method invocation is a Proc
object, you can preceed its name with an ampersand to convert in into
a block. The method may then use yield to call it.
square = proc { |i| i*i }
def meth1(&b)
print b.call(9), "\n"
end
meth1 { |i| i+i }
def meth2
print yield(8), "\n"
end
meth2 { |i| i+i }
meth2 &square
Produces:
18
16
64
|
def greet(p1='hello', p2='world')
print "#{p1} #{p2}\n"
end
greet
greet "hi"
greet "morning", "mom"
Produces:
hello world
hi world
morning mom
|
The default value (which can be an arbitrary expression) is evaluated when the method is invoked. It is evaluated using the scope of the method.
The formal parameters of a block appear between vertical bars at the start of the block:
proc { |a, b| a <=> b }
|
These parameters are actually local variables. If an existing local of the same name exists when the block executes, that variable will be modified by the call to the block. This may or may not be a good thing.
Typically, arguments are passed to a block using yield (or an
iterator that calls yield), or by using the
Proc.call method.
A = a = b = "abc"
b.concat("d") # -> "abcd"
a # -> "abcd"
A # -> "abcd"
|
Variables hold references to objects. The assignment A =
a = b = "abc" put a reference to the string ``abc''
into A, a, and b.
When you called b.concat("d"), you invoked the
concat method on that object, changing it from ``abc'' to
``abcd''. Because a and A also reference that same
object, their apparent value changes too.
This is less of a problem in practice than it might appear.
In addition, as of Ruby 1.5.2, all objects may be frozen, protecting them from change.
A constant is a variable whose name starts with an upper case letter. In older Ruby implementations, when a constant was assigned a new value, a warning was issued. In newer Rubies, constants may not be reassigned from within instance methods, but can otherwise be changed at will.
An iterator is a method which accepts a block or a Proc
object. In the source file, the block is placed immediately after the
invocation of the method. Iterators are used to produce user-defined
control structures--especially loops.
Let's look at an example to see how this works. Iterators are often used to repeat the same action on each element of a collection, like this:
data = [1, 2, 3] data.each do |i| print i, "\n" end Produces: 1 2 3 |
The each method of the array data is passed the
do...end block, and executes it repeatedly. On each call, the
block is passed successive elements of the array.
You can define blocks with {...} in place of
do...end.
data = [1, 2, 3]
data.each { |i|
print i, "\n"
}
Produces:
1
2
3
|
This code has the same meaning as the last example. However, in some
cases, precedence issues cause do...end and
{...} to act differently.
foobar a, b do .. end # foobar is the iterator.
foobar a, b { .. } # b is the iterator.
|
{...} binds more tightly to the preceding expression
than does a do block. The first example is equivalent to
foobar(a, b) do..., while the second is foobar(a, b
{...}).
You simply place the block after the iterator call. You can also pass
a Proc object by prepending & to the variable or constant
name that refers to the Proc.
There are three ways to execute a block from an iterator method: (1)
the yield control structure; (2) calling a Proc
argument (made from a block) with call; and (3) using
Proc.new followed by a call.
The yield statement calls the block, optionally passing it
one or more arguments.
def myIterator
yield 1,2
end
myIterator { |a,b| puts a, b }
Produces:
1
2
|
If a method definition has a block argument (the last formal parameter
has an ampersand (&) prepended), it will receive the attached
block, converted to a Proc object. This may be called using
method.call(args...).
def myIterator(&b)
b.call(2,3)
end
myIterator { |a,b| puts a, b }
Produces:
2
3
|
Proc.new (or the equivalent proc or lambda
calls), when used in an iterator definition, takes the
block which is given to the method as its argument, generates a
procedure object from it. (proc and lambda are
effectively synonyms.)
def myIterator
Proc.new.call(3,4)
proc.call(4,5)
lambda.call(5,6)
end
myIterator { |a,b| puts a, b }
Produces:
3
4
4
5
5
6
|
Perhaps surprisingly, Proc.new and friends do not in any
sense consume the block attached to the method--each call to
Proc.new generates a new procedure object out of the same
block.
You can tell if there is a block associated with a method by calling
block_given?.
Proc.new without a block do?
Proc.new without a block cannot generate a procedure object
and an error occurs. In a method definition, however,
Proc.new without a block implies the existence of a block
at the time the method is called, and so no error will occur.
Matz's solution, in [ruby-talk:5252], uses threads:
require 'thread'
def combine(*args)
queues = []
threads = []
for it in args
queue = SizedQueue.new(1)
th = Thread.start(it, queue) do |i,q|
self.send(it) do |x|
q.push x
end
end
queues.push queue
threads.push th
end
loop do
ary = []
for q in queues
ary.push q.pop
end
yield ary
for th in threads
return unless th.status
end
end
end
public :combine
def it1 ()
yield 1; yield 2; yield 3
end
def it2 ()
yield 4; yield 5; yield 6
end
combine('it1','it2') do |x|
# x is (1, 4), then (2, 5), then (3, 6)
end
|
:var mean?
A colon followed by a name generates an integer(Fixnum) called a
symbol which corresponds one to one with the identifier.
"var".intern gives the same integer as
:var, but the ``:'' form will create a local symbol if it
doesn't already exist.
The routines "catch", "throw", "autoload", and so on, require a string or a symbol as an argument.
"method_missing", "method_added" and "singleton_method_added" (and others) require a symbol.
The fact that a symbol springs into existence the first time it is referenced is sometimes used to assign unique values to constants:
NORTH = :NORTH SOUTH = :SOUTH EAST = :EAST WEST = :WEST |
To get the value of the variable corresponding to a symbol, you can
use id2name to get the name of the variable, and then
eval that to get that variable's contents.
In the scope of "symbol", do eval(:symbol.id2name).
a = 'This is the content of "a"' b = eval(:a.id2name) a.id == b.id # b now references the same object as a |
If your symbol corresponds to the name of a method, you can use the
Method.method function to return a corresponding
Method object, which you may then call.
class Demo
def meth
"Hello, world"
end
end
d = Demo.new # -> #<Demo:0x401b4400>
m = d.method(:meth) # -> #<Method: Demo(Demo)#meth>
m.call # -> "Hello, world"
|
loop a control structure?
Although loop looks like a control structure, it is actually
a method defined in Kernel. The block which follows
introduces a new scope for local variables.
Q: Ruby does not have a do { ... } while construct,
so how can I implement loops that test the condition at the end.
Clemens Hintze says: You can use a combination of Ruby's begin
... end and the while or until statement
modifiers to achieve the same effect:
i = 0
begin
puts "i = #{i}"
i += 1
end until i > 4
Produces:
i = 0
i = 1
i = 2
i = 3
i = 4
|
a +b gives an error!
Ruby works hard to distinguish method calls from operators, and
variable names from method names. Unfortunately, there's no way it can
get it right all the time. In this case, ``a +b'' is parsed
as ``a(+b)''. Remove the space to the left of ``+'' or add
a space to the right of ``+,'' and it will be parsed as an
addition.
s = "x"; puts s *10 gives an error.
Again, Ruby sees the asymmetrical space and parses it as
puts(s(*10)) (which isn't too smart, really). Use
``s*10'' or ``s * 10'' to get the desired
result.
p {}?
The {} is parsed as a block, not a Hash constructor.
You can force the {} to be treated as an expression by making
the fact that it's a parameter explicit: p({}).
def pos=(val) to work.
I have the following code, but I cannot use the method
pos = 1.
def pos=(val) print @pos, "\n" @pos = val end |
Methods with = appended must be called with a receiver
(without the receiver, you're just assigning to a local variable).
Invoke it as self.pos = 1.
'\1' and '\\1'?
They have the same meaning. In a single quote string, only \' and
\\ are transformed and other combinations remain unchanged.
However, in a doubled quoted string, "\1" is the byte
\001, while "\\1" is the two character string
containing a backslash and the character "1".
or'' and ``||''?
Q: ``p(nil || "Hello")'' prints "Hello",
while ``p(nil or "Hello")'' gives a parse error.
A: || combines terms within an expression. Because
the first term in this case is nil, the second term is
evaluated.
or is used to combine expressions in conditionals. Ruby is
not expecting a conditional statement in an argument list.
Ruby binds all messages to methods dynamically. It searches first for
singleton methods in the receiver, then for methods defined in the
receiver's own class, and finally for methods defined in the
receiver's superclasses (including any modules which may have been
mixed in). You can see the order of searching by displaying
Classname.ancestors, which shows the ancestor classes and
modules of ClassName.
If after searching the alternatives a matching method could not
be found, Ruby tries to invoke a method called method_missing,
repeating the same search procedure to find it. This allows you to
handle messages to unknown methods, and is often used to provide
dynamic interfaces to classes.
module Indexed
def [](n)
to_a[n]
end
end
class String
include Indexed
end
String.ancestors # -> [String, Indexed, Enumerable, Comparable, Object, Kernel]
"abcde".gsub!(/./, "\\&\n")[1] # -> 10
|
This program does not return "b\n" as one expects, but
returns 10. When the method [] is searched for, it is found
in class String, before searching Indexed. You
should directly redefine [] in class String.
+, -, * ... operators?
+, -, and the like are not operators but method
calls. They can, therefore, be overloaded by new definitions.
class MyString < String
def -(other) # New method
self[0...other.size] # self truncated to other's size
end
end
|
However, the following are built-in control structures, not methods, which cannot be overridden.
=, .., ..., !, not, ||, &&, and, or, :: |
To overload or to define unary operators, you can use
+@ and -@ as the method names.
= is used to define a method to set an attribute of the object:
class Test
def attribute=(val)
@attribute = val
end
end
t = Test.new
t.attribute = 1
|
If operators such as + and - are defined, Ruby
automatically handles the self assignment forms (+=,
-= and so on).
++ and - -?
Ruby does not have the autoincrement and autodecrement operators. You
can use += 1 and -= 1 instead.
Yes and no. Ruby has methods that look like functions in languages such as C or Perl:
def writeln(str)
print(str, "\n")
end
writeln("Hello, World!")
Produces:
Hello, World!
|
However, they're actually method calls with the receiver omitted.
In this case, Ruby assumes the receiver is self.
So, writeln resembles a function but it's actually a method
belonging to class Object and sent as a message to the
hidden receiver self. Ruby is a pure object-oriented language..
Of course you can use such methods as if they were functions.
All classes in Ruby are derived from class Object. The
definition of class Object mixes-in the methods defined in
the Kernel module. These methods are therefore available
within every object in the system.
Even if you're writing a simple Ruby program without classes, you're
actually working inside class Object.
An object's instance variables (those variables starting with
@) are not directly accessible outside the object. This
promotes good encapsulation. However, Ruby makes it easy for you to
define accessors to these instance variables in such a way that users
of your class can treat instance variables just like attributes.
Just use one or more of
Module.attr, attr_reader, attr_writer, or
attr_accessor.
class Person
attr :name # read only
attr_accessor :wearing_a_hat # read/write
def initialize(name)
@name = name
end
end
p = Person.new("Dave")
p.name # -> "Dave"
p.wearing_a_hat # -> nil
p.wearing_a_hat = true
p.wearing_a_hat # -> true
|
You can also define your own accessor functions (perhaps to perform
validation, or to handle derived attributes). The read accessor is
simply a method that takes no parameters, and the assignment accessor
is a method name ending in = that takes a single parameter.
Although there can be no space between the method name and the
= in the method definition, you can insert spaces there when
you call the method, making it look like any other
assignment. You can also utilize self assignments such as +=
and -=, as long as the corresponding + or -
methods are defined.
private and protected?
The visibility keyword private makes a method callable only
in a function form, and so it can only have self as a
receiver. A private method is callable only within the class in which
the method was defined or in its subclasses.
class Test
def func
return 99
end
def test(other)
p func
p other.func
end
end
t1 = Test.new
t2 = Test.new
t1.test(t2)
# Now make 'func' private
class Test
private :func
end
t1.test(t2)
Produces:
99
99
99
prog.rb:7:in `test': private method `func' called for #<Test:0x401b4284> (NameError)
from prog.rb:21
|
Protected methods are also callable only from within their own class or its subclasses, but they can be called both as functions form and using a receiver. For example,
def <=>(other) age <=> other.age end |
Will compile if age is a protected method, but not if it is
private.
These features help you control access to your class's internals.
You change the visibility of methods using private,
protected and public. When used without parameters
during a class definition, they affect the visibility of subsequent
methods. When used with parameters, they change the visibility of the
named methods.
class Foo
def test
print "hello\n"
end
private :test
end
foo = Foo.new
foo.test
Produces:
prog.rb:9: private method `test' called for #<Foo:0x401b4694> (NameError)
|
You can make a class method private using private_class_method.
class Foo
def Foo.test
print "hello\n"
end
private_class_method :test
end
Foo.test
Produces:
prog.rb:8: private method `test' called for Foo:Class (NameError)
|
The default visibility for the methods defined in a class is
public. The exception is the instance initializing method,
initialize.
Methods defined at the toplevel are also public by default.
Yes, you can, but we don't do it lightly! If Ruby sees a capitalized name followed by a space, it will probably (depending on the context) assume it's a constant, not a method name. So, if you use capitalized method names, always remember to put parameter lists in parentheses, and always put the parentheses next to the method name with no intervening spaces. (This last suggestion is a good idea anyway!)
super gives an ArgumentError.
Invoking super with no parameters in a method passes all the
arguments of that method to a method of the same name in a superclass.
If the number of arguments to the original method disagrees with that
of the higher-level method,
an ArgumentError is raised. To get around this, simply call
super and pass a suitable number of arguments.
super invokes the same named method one level up. If you're
overloading a method in a more distant ancestor, use alias to
give it an new name before masking it with your method definition. You
can then call it using that aliased name.
Within the method definition, you can use super. You can
also use alias to give it an alternative name. Finally, you
can call the original method as a singleton method of Kernel.
A destructive method is one which alters the state of an
object. String, Array, and Hash, and others have such methods.
Often there are two versions of a method, one with a plain name, the
other with the same, but followed by !. The plain version
takes a copy of the receiver, makes its change to it, and returns the
copy. The version with the ! modifies the receiver in place.
Beware, however, that there are a fair number of destructive methods
that don't have an !, including assignment operators
(name=), array assignment ([]=), and methods such as
Array.delete.
Remember that assignment in most cases just copies object references, and that parameter passing is equivalent to assignment. This means you can end up with multiple variables referencing the same object. If one of those variables is used to invoke a destructive method, the object referenced by all of them will be changed.
def foo(str) str.sub!(/foo/, "baz") end obj = "foo" foo(obj) # -> "baz" obj # -> "baz" |
In this case the actual argument is altered.
Yes and no.
def m1 return 1, 2, 3 end def m2 return [1, 2, 3] end m1 # -> [1, 2, 3] m2 # -> [1, 2, 3] |
So, only one thing is returned, but that thing can be an arbitrarily complex object. In the case of arrays, you can use multiple assignment to get the effect of multiple return values. For example:
def foo return 20, 4, 17 end a, b, c = foo a # -> 20 b # -> 4 c # -> 17 |
A class can be defined repeatedly. Each definition is added to the last definition. If a method is redefined, the former one is overridden and lost.
As of Ruby 1.5.3, there are. A variable prefixed with two at signs is a class variable, accessible within both instance and class methods of the class.
class CountEm
@@children = 0
def initialize
@@children += 1
@myNumber = @@children
end
def whoAmI
"I'm child number #@myNumber (out of #@@children)"
end
def CountEm.totalChildren
@@children
end
end
c1 = CountEm.new
c2 = CountEm.new
c3 = CountEm.new
c1.whoAmI # -> "I'm child number 1 (out of 3)"
c3.whoAmI # -> "I'm child number 3 (out of 3)"
CountEm.totalChildren # -> 3
|
Earlier versions of Ruby do not have class variables. However,
container classes (Array, Hash, etc) assigned to a
class constant can be used to give the same effect. This example uses
an array. Some folks feel hashes are better.
class Foo
F = [ 0 ] # pseudo class variable - Array 'F'
def foo
F[0] += 1
puts F[0]
end
end
|
This reports on the number of times foo is called across
all instances of class Foo.
class Foo
@a = 123 # (1)
def foo
p @a # (2) ... nil not 123
end
end
|
(1) is a class instance variable, and (2) is an ordinary
instance variable (which, not having been initialized, has a value of
nil). (2) belongs to an instance of class Foo,
and (1) belongs to the class object Foo, which is an
instance of Class class. (phew!)
There is no way to access class instance variables from instance methods.
A singleton method is an instance method associated with one specific object.
You create a singleton method by including the object in the definition:
class Foo end foo = Foo.new bar = Foo.new def foo.hello puts "Hello" end foo.hello bar.hello Produces: Hello prog.rb:10: undefined method `hello' for #<Foo:0x401b45f4> (NameError) |
Singleton methods are useful when you want to add a method to an object and creating a new subclass is not appropriate.
A singleton method of a class object is called a class method. (Actually, the class method is defined in the metaclass, but that is pretty much transparent). Another way of looking at it is to say that a class method is a method whose receiver is a class.
It all comes down to the fact that you can call class methods without having to have instances of that class (objects) as the receiver.
Let's create a singleton method of class Foo:
class Foo
def Foo.test
"this is foo"
end
end
#It is invoked this way.
Foo.test # -> "this is foo"
|
In this example, Foo.test is a class method.
Methods which are defined in class Class can
be used as class methods for every class(!)
A Singleton class is an anonymous class that is created by subclassing the class associated with a particular object. They are another way of extending the functionality associated with just one object.
Take the lowly Foo:
class Foo # -> hello<<7>>nil
def hello
print "hello"
end
end
foo = Foo.new
foo.hello
|
Now let's say we need to add class-level functionality to just this one instance:
class << foo
attr :name, TRUE
def hello
"hello. I'm " + @name + "\n"
end
end
foo.name = "Tom"
foo.hello # -> "hello. I'm Tom\n"
|
We've customized foo without changing the characteristics of Foo,
A module function is a private, singleton method defined in
a module. In effect, it is similar to a
class method, in that it can be called using the
Module.method notation:
Math.sqrt(2) # -> 1.414213562 |
However, because modules can be mixed in to classes, module functions
can also be used without the prefix (that's how all those
Kernel functions are made available to objects):
include Math sqrt(2) # -> 1.414213562 |
Use module_function to make a method a module function.
module Test
def thing
# ...
end
module_function :thing
end
|
Modules are collections of methods and constants. They cannot generate instances. Classes may generate instances (objects), and have per-instance state (instance variables).
Modules may be mixed in to classes and other modules. The mixed-in module's constants and methods blend into that class's own, augmenting the class's functionality. Classes, however, cannot be mixed in to anything.
A class may inherit from another class, but not from a module.
A module may not inherit from anything.
No. However, a module may be included in a class or another module to mimic multiple inheritance (the mixin facility).
This does not generate a subclass (which would require inheritance), but
does generate an is_a? relationship between the class and the
module.
The module Comparable provides a variety of comparison
operators (<, <=, >, between? and so on). It defines
these in terms of calls to the general comparison method,
<=>. However, it does not itself define
<=>.
Say you want to create a class where comparisons are based on the number of legs an animal has:
class MyClass
include Comparable
attr :legs
def initialize(name, legs)
@name, @legs = name, legs
end
def <=>(o)
return @legs <=> o.legs
end
end
c = MyClass.new('cat', 4)
s = MyClass.new('snake', 0)
p = MyClass.new('parrot', 2)
c < s # -> false
s < c # -> true
p >= s # -> true
p.between?(s, c) # -> true
[p, s, c].sort # -> [snake, parrot, cat]
|
All MyClass must do is define its own semantics for the
operator <=>, and mix-in the Comparable
module. Comparable's methods now become indistinguishable from
MyClass's and your class suddenly sprouts new
functionality. And because the same Comparable module is used
my many classes, your new class will share a consistent and well
understood semantic.
You can define a class method in the class definition, and you can define a class method at the top level?
class Demo def Demo.classMethod end end def Demo.anotherClassMethod end |
There is only one significant difference between the two. In the
class definition you can refer to the class's constants directly, as
the constants are within scope. At the top level, you have to use the
Class::CONST notation.
load and require?
load will load and execute a Ruby program (*.rb).
require loads Ruby programs as well, but will also load binary
Ruby extension modules (shared libraries or DLLs). In addition,
require ensures that a feature is never loaded more than
once.
include and extend?
include mixes a module into a class or another module. Methods
from that the module are called function-style (without a receiver).
extend is used to include a module in an object(instance).
Methods in the module become methods in the object.
self mean?
self is the currently executing receiver--the object to which
a method is applied. A function-style method call implies
self as the receiver.
Say file1 contains:
var1 = 99 |
and some other file loads it in:
require 'file1' puts var1 Produces: prog.rb:2: undefined local variable or method `var1' for #<Object:0x401c1ce0> (NameError) |
You get an error because load and require arrange
for local variables to be stored into a separate, anonymous namespace,
effectively discarding them. This is designed to protect your code
from being polluted.
instance_methods(nil) return?
The method instance_methods returns an array containing the
names of methods that the receiver responds to. This will include the
methods in superclasses and in mixed-in modules.
instance_methods(nil) returns
the name of just those methods which are defined in the object's class.
It depends. In Ruby versions prior to 1.5.2, the random number
generator had (by default) a constant seed, and so would produce the
same series of numbers each time a program was run. If you needed less
deterministic behaviors, you called srand to set up a less
predictable seed.
Newer Rubys (Rubies?) have a different behavior. If rand is called
without a prior call to srand, Ruby will generate its own
random(ish) seed. Successive runs of a program that does not use
srand will generate different sequences of random numbers. To
get the old, predictable, behavior (perhaps for testing), call
srand with a constant seed.
Fixnum, true, nil, and false are
implemented as immediate values. With immediate values, variables hold
the objects themselves, rather than references to them.
Singleton methods cannot be defined for such objects. Two
Fixnums of the same value always represent the same
object instance, so (for example) instance variables for the
Fixnum with the value "one" are shared between all the "ones"
is the system. This makes it impossible to define a singleton method
for just one of these.
nil and false?
First the similarity. nil and false are the only two
values that evaluate to false in a boolean context.
However, they are instances of different classes (NilClass
and FalseClass), and have different behaviors elsewhere.
We recommend that predicate methods (those whose name ends with a
question mark) return true or false. Other methods
that need to indicate failure should return nil.
open("example", "r+").readlines.each_with_index{|l, i|
l[0,0] = (i+1).to_s + ": " }
|
This program does not add line numbers to the file "example". It does read the contents of the file, and for each line read prepend the line number, but the data is never written back. The code below does update the file (although somewhat dangerously, as it takes no backup before starting the update):
io = open("example", "r+")
ary = io.readlines
ary.each_with_index{|l, i| l[0,0] = (i+1).to_s + ": "}
io.rewind
io.print ary
io.close
|
Using the command-line option -i, or built-in variable $-i,
you can read a file and replace it.
The code in the preceding question, which added line numbers to file, is probably best written using this technique:
$ ruby -i -ne 'print "#$.: #$_"' example |
If you want to preserve the original file, use -i.bak to
create a backup.
This code will not work correctly:
open('file', 'w').print "This is a file.\n"
system 'cp file newfile'
|
Because I/O is buffered, file is being copied before
its contents have been written to disk. newfile will probably
be empty. However, when the program terminates, the buffers are
flushed, and file has the expected content.
The problem doesn't arise if you close file before copying:
f = open('file', 'w')
f.print "This is a file.\n"
f.close
system "cp file newfile"
|
As you read from a file, Ruby increments a line number counter in the
global variable $.. This is also available using the
lineno attribute of the File object.
The special constant ARGF is a file-like object that can be
used to read all the input files specified on the command line (or
standard input if there are no files). ARGF is used implicitly by code
such as:
while gets print $_ end |
In this case, $. will be the cumulative number of lines read
across all input files. To get the line number in the current file,
use
ARGF.file.lineno |
You can also get the name of the current file using
ARGF.file.path.
less to display my program's output?
I tried the following, but nothing came out:
f = open '|less', 'w' f.print "abc\n" |
That's because the program ends immediately, and less never
gets a chance to see the stuff you've written to it, never mind to
display it. Use close to wait until less ends.
f = open '|less', 'w' f.print "abc\n" f.close |
File object which is no longer referenced?
A File object which is no longer referenced becomes
eligible for garbage collection. The file will be closed automatically when
the File object is garbage collected.
There are at least four good ways of ensuring that you do close a file:
(1) f = open "file"
begin
f.each {|l| print l}
ensure
f.close
end
(2) File.open("file") { |f|
f.readlines.each { |l| print l }
}
(3) IO.foreach("file") {|l| print l}
(4) IO.readlines("file").each {|l| print l}
|
Dir.glob("*").sort{|a,b| File.mtime(b) <=> File.mtime(a)}
|
Although this works (returning a list in reverse chronological order) it isn't very efficient, as it fetches the files' modification times from the operating system on every comparison.
More efficiency can be bought with some extra complexity:
Dir.glob("*").collect! {|f| [File.mtime(f), f]}.
sort{|a,b| b[0] <=> a[0]}.collect! {|e| e[1]}
|
freq = Hash.new(0)
open("example").read.scan(/\w+/){|w| freq[w] += 1}
freq.keys.sort.each {|k| print k, "-", freq[k], "\n"}
Produces:
and-1
is-3
line-3
one-1
this-3
three-1
two-1
|
false?
Q: An empty string ("") returns true in a conditional
expression! In Perl, it's false.
A: In Ruby, only nil and false are false in
conditional contexts. This is a way of gaining speed--both
nil and false have immediate values, so they can be
tested without having to chase a reference to an object.
You can use empty?, compare the string to "", or compare
length to 0 to find out if a string is empty.
If you want your strings to sort 'AAA', 'BBB', ..., 'ZZZ', 'aaa', 'bbb', then the built-in comparison will work just fine.
If you want to sort ignoring case distinctions, compare downcased versions of the strings in the sort block:
array = %w( z bB Bb BB bb Aa aA AA aa a )
puts array.sort { |a,b| a.downcase <=> b.downcase }
Produces:
a
aa
AA
aA
Aa
bb
BB
bB
Bb
z
|
If you want to sort so that the 'A's and 'a's come together, but 'a' is considered greater than 'A' (so 'Aa' comes after 'AA' but before 'AB'), use:
puts array.sort { |a,b|
(a.downcase <=> b.downcase).nonzero? || a <=> b
}
Produces:
a
AA
Aa
aA
aa
BB
Bb
bB
bb
z
|
"abcd"[0] return?
It returns the character code for ``a'', 97(Fixnum).
You can express a character code as an integer constant by prefixing
the character with a question mark, so ?a is also 97(Fixnum).
If a holds the string to be expanded, you could use one of:
1 while a.sub!(/(^[^\t]*)\t(\t*)/){$1+' '*(8-$1.size%8+8*$2.size)}
#or
1 while a.sub!(/\t(\t*)/){' '*(8-$~.begin(0)%8+8*$1.size)}
#or
a.gsub!(/([^\t]{8})|([^\t]*)\t/n){[$+].pack("A8")}
|
Regexp.quote('\\') escapes a backslash.
It gets trickier if you're using sub and gsub, Say you
write gsub(/\\/, '\\\\'), hoping to replace each backslash
with two. The second argument is converted to '\\' in syntax
analysis. When the substitution occurs, the regular expression engine
converts this to '\', so the net effect is to replace each single
backslash with another single backslash. You need to write
gsub(/\\/, '\\\\\\')!
However, using the fact that \& contains the matched string,
you could also write gsub(/\\/, '\&\&').
If you use the block form of gsub,
i.e. gsub(/\\/){'\\\\'}, the string for substitution is
analyzed only once (during the syntax pass) and the result is what you
intended.
sub and sub!?
In sub, a copy of the receiver is generated, substituted
and returned.
In sub!, the receiver is altered and returned if any match was
found. Otherwise, nil is returned.
Methods like sub! are called
destructive methods
which alter the attribute of the receiver. If there are two similar
methods and one is destructive, the destructive one has a suffix !.
def foo(str)
str = str.sub(/foo/, "baz")
end
obj = "foo"
foo(obj) # -> "baz"
obj # -> "foo"
def foo(str)
str = str.sub!(/foo/, "baz")
end
foo(obj) # -> "baz"
obj # -> "baz"
|
\Z matches just before the last \n if the string ends with a \n, otherwise it matches at the end of a string.
.." and "..."?
.. includes the right hand side in the range, ... does not.
A Proc object generated by Proc.new, proc, or lambda
can be referenced from a variable, so that variable could be said to
be a function pointer. You can also get references to methods
within a particular object instance using Object.method.
thread and fork?
Ruby threads are implemented within the interpreter, while
fork invokes the operating system to create a separately
executing subprocess.
Thread and fork have following characteristics:
fork is slow, thread is not.fork does not share the memory space.thread does not cause thrashing.thread works on DOS.thread gets in a deadlock, the whole process
stops.fork can take advantage of pauses waiting for I/O to
complete, thread does not (at least not without some help).You probably shouldn't mix fork and thread.
Marshal is used to store an object in a file or a string, and later reconstitute it. Objects may be stored using:
Marshal.dump obj [, io ] [, lev] |
io is a writable IO object, lev designates the level
to which objects are dereferred and stored. If lev levels of
dereferring are done and object references still exist, then
dump stores just the reference, not the object referenced. This is
not good, as these referenced objects cannot be subsequently
reconstructed.
If io is omitted, the marshaled objects are returned in a string.
You can load objects back using:
obj = Marshal.load io #or obj = Marshal.load str |
where io is a readable IO object, str is the dumped
string.
Ruby supports a flexible exception handling scheme:
begin statements which may raise exceptions. rescue [exception class names] statements when an exception occurred. rescue [exception class names] statements when an exception occurred. ensure statements that will always run end |
If an exception occurs in the begin clause, the
rescue clause with the matching exception name
is executed. The ensure clause is executed whether an exception
occurred or not. rescue and ensure clauses may be omitted.
If no exception class is designated for rescue clause,
StandardError exception is implied, and exceptions which are
in a is_a? relation to StandardError are captured.
This expression returns the value of the begin clause.
The latest exception is accessed by the global variable $! (and
so its type can be determined using $!.type).
trap?
trap associates code blocks with external events (signals).
trap("PIPE") {raise "SIGPIPE"}
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You can try using irb. The following is paraphrased from Goto
Kentaro (Gotoken), and originally appeared in ruby-talk:444.
irb directory tree.
irb/ directory to the $RUBYLIB
environment variable
irb somewhere in your path.
chmod +x $RUBYLIB/irb/irb.rb
rehash to tell your login shell about the
new command.
irbIf 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 |
There is a gdb-like debugger for Ruby.
ruby -r debug your_program |
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.
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.
Your Tk version may be old, try a newer version.
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.
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.
a ? b : c mean?
It's the same as saying if a then b else c end.
Assuming that the file ends in a linefeed, the following code may give the fastest result.
open("example").read.count("\n") # -> 3
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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.
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
# inject(n) { |n, i| ...}
def inject(n)
each { |i|
n = yield(n, i)
}
n
end
end
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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
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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:
# --------------------------------------------------------------------
# Simple Producer/Consumer
# --------------------------------------------------------------------
# Connect a simple counting task and a printing task together using
# continuations.
#
# Usage: count(limit)
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
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# --------------------------------------------------------------------
# Filtering Out Multiples of a Given Number
# --------------------------------------------------------------------
# Create a filter that is both a consumer and producer. Insert it
# between the counting task and the printing task.
#
# Usage: omit (2, limit)
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
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# --------------------------------------------------------------------
# Prime Number Generator
# --------------------------------------------------------------------
# Create a prime number generator. When a new prime number is
# discovered, dynamically add a new multiple filter to the chain of
# producers and consumers.
#
# Usage: primes (limit)
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
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