Truth value evaluating of an object

6 Message(s) by 5 Author(s) originally posted in ruby programming

From: Dave River

Date: Saturday, October 27, 2007

I know ruby treat an object

<object-oriented> In {object-oriented programming}, an
	instance of the data structure and behaviour defined by the
	object's {class}.  Each object has its own values for the
	{instance variables} of its class and can respond to the
	{methods} defined by its class.

	For example, an object of the "Point" class might have
	instance variables "x" and "y" and might respond to the "plot"
	method by drawing a dot on the screen at those coordinates.

	(2004-01-26)

as false whenever it is nil or false.
However, I wonder if there are any other ways to change this behavior.

For example, I define a class

1. <programming> The prototype for an {object} in an
	{object-oriented language}; analogous to a {derived type} in a
	{procedural language}.  A class may also be considered to be a
	set of objects which share a common structure and behaviour.
	The structure of a class is determined by the {class
	variables} which represent the {state} of an object of that
	class and the behaviour is given by a set of {methods}
	associated with the class.

	Classes are related in a {class hierarchy}.  One class may be
	a specialisation (a "{subclass}") of another (one of its
	"{superclasses}") or it may be composed of other classes or it
	may use other classes in a {client-server} relationship.  A
	class may be an {abstract class} or a {concrete class}.

	See also {signature}.

	2. <programming> See {type class}.

	3. <networking> One of three types of {Internet addresses}
	distinguished by their most significant bits.

	3. <language> A language developed by the {Andrew Project}.
	It was one of the first attempts to add {object-oriented}
	features to {C}.

	(1995-05-01)

called AreYouOk.
class AreYouOk
def initialize(ok)
@xxxxxxxxxxx = ok
end
end

x = AreYouOk.new(false)
puts "you are ok" if x

Since x isn't nil, ruby prints " you are ok".
However, I want ruby to make the decision based on the @xxxxxxxxxxx instance
variable. Are there any ways to do that?

I know that there is a method

<programming> The name given in {Smalltalk} and other
	{object-oriented languages} to a procedure or routine
	associated with one or more {classes}.  An {object} of a
	certain class knows how to perform actions, e.g. printing
	itself or creating a new instance of itself, rather than the
	function (e.g. printing) knowing how to handle different types
	of object.

	Different classes may define methods with the same name
	(i.e. methods may be {polymorphic}).  The term "method" is used
	both for a named operation, e.g. "PRINT" and also for the code
	which a specific class provides to perform tha
	t operation.

	Most methods operate on objects that are instances of a
	certain class.  Some object-oriented languages call these
	"object methods" to distinguish then from "{class methods}".

	In {Smalltalk}, a method is defined by giving its name,
	documentation, temporary local variables and a sequence of
	expressions separated by "."s.

	(2000-03-22)

called __bool__ in Python. You can define
your __bool__ method in your class. The truth value of an object is
based on the return value of __bool__. Does ruby provide similar
mechanism?
--
Posted via http://www.ruby-forum.com/.

From: Arlen Christian Mart Cuss

Date: Saturday, October 27, 2007

Hi,

wrote in message:

I know that there is a method called __bool__ in Python. You can define
your __bool__ method in your class. The truth value of an object is
based on the return value of __bool__. Does ruby provide similar
mechanism?

I do not think I'm speaking out of school when I say that there's no such
way to do so.

Ruby

<language>

	1. A {relational language} designed by Jones and M. Sheeran in
	1986 for describing and designing circuits (a {hardware
	description language}).  Ruby programs denote {binary
	relations} and programs are built-up inductively from
	primitive relations using a pre-defined set of {relational
	operators}.  Ruby programs also have a geometric
	interpretation as networks of primitive relations connected by
	wires, which is important when layout is considered in circuit
	design.

	Ruby has been continually developed since 1986, and has been
	used to design many different kinds of circuits, including
	{systolic arrays}, {butterfly networks} and arithmetic
	circuits.

	{(ftp://ftp.cs.chalmers.se/pub/misc/ruby/)}.

	E-mail: <graham@cs.chalmers.se>.

	["Ruby - A Language of Relations and Higher-Order Functions",
	M. Sheeran, Proc 3rd Banff Workshop on Hardware Verification,
	Springer 1990].

	(1994-10-27)

	2. One of five pedagogical languages based on {Markov
	algorithms}, used in Higman's report (below).  The other
	languages are {Brilliant}, {Diamond}, {Nonpareil}, and
	{Pearl}.

	["Nonpareil, a Machine Level Machine Independent Language for
	the Study of Semantics", B. Higman, ULICS Intl Report No ICSI
	170, U London (1968)].

	(1994-10-27)

	3. A fully {object oriented} {interpreted} {scripting
	language} by Yukihiro Matsumoto <matz@netlab.co.jp>.

	Similar in scope to {Perl} and {Python}, Ruby has high-level
	{data types}, automatic {memory management}, {dynamic typing},
	a {module} system, {exceptions}, and a rich standard library.
	Other features are {CLU}-style {iterators} for {loop
	abstraction}, {singleton classes}/{methods} and {lexical
	closures}.

	In Ruby, everything is an {object}, including the basic data
	types.  For example, the number 1 is an instance of {class}
	Fixnum.

	Current version (stable): 1.6.7, as of 2002-03-01.

	{Ruby Home (http://www.ruby-lang.org/)}.

	{Ruby Central (http://www.rubycentral.com/)}.

	["Programming Ruby - The Pragmatic Programmer's Guide", David
	Thomas, Andrew Hunt, Yukihiro Matsumoto pub. Addison Wesley
	2000].

	(2002-06-19)

and libraries likely rely on 'if obj' returning false if it's false
or nil - overriding

<programming> Redefining in a {child class} a {method} or
	function member defined in a {parent class}.

	Not to be confused with "{overloading}".

	(1996-12-21)

this behaviour will probably cause some strange
behaviour which may result in your object being inadvertently modified!

Instead, just make an accessor like ok?, so you can test "if x.ok?".

If you need some way to test this across several objects, consider this:

class Object
def ok?
self
end
end
class AreYouOk
def ok?
@xxxxxxxxxxx
end
end

Thus 'if x.ok?' returns the same as 'if x' for any object, but @xxxxxxxxxxx for
yours!

Cheers
Arlen

From: Dave River

Date: Saturday, October 27, 2007

Thus 'if x.ok?' returns the same as 'if x' for any object, but @xxxxxxxxxxx for
yours!Thanks for your explanation!

In fact, I am making a wrapper

<programming> Code which is combined with another piece of
	{code} to determine how that code is executed.  The wrapper
	acts as an interface between its caller and the wrapped code.
	This may be done for compatibility, e.g. if the wrapped code
	is in a different programming language or uses different
	calling conventions, or for security, e.g. to prevent the
	calling program from executing certain functions.  The
	implication is that the wrapped code can only be accessed via
	the wrapper.

	(1998-12-15)

class called Boolean

<mathematics, logic> 1. {Boolean algebra}.

	<programming> 2. (bool) The type of an expression with two
	possible values, "true" and "false".  Also, a variable of
	Boolean type or a function with Boolean arguments or result.
	The most common Boolean functions are {AND}, {OR} and {NOT}.

	(1997-12-01)

which cooperates
with some legacy code

<software> Instructions for a computer in some programming
	language, often {machine language}.  The word "code" is often
	used to distinguish instructions from {data} (e.g. "The code
	is marked 'read-only'") whereas "{software}" is used in
	contrast with "{hardware}" and may consist of more than just
	code.

	(2000-04-08)

in my company because there are some compatibility

<jargon> Different systems (e.g., {programs}, {file formats},
	{protocols}, even {programming languages}) that can work
	together or exchange data are said to be compatible.

	See also {backward compatible}, {forward compatible}.

	(1998-01-15)

problems between different languages.

I write

1. <chat> {Unix}'s simple {talk} command and {protocol}.
	write has been largely superseded by {talk} and then {irc}.

	An enhancement, {RWP}, has been proposed.

	2. <tool> A simple {text editor} for {Windows}.

	(1998-04-28)

some code like the following and the Boolean object hides some
underlying code which solve the compatibility problems.
x = Boolean.new()
if x
do something....
end

If ruby doesn't support

After-sale handholding; something many software vendors
	promise but few deliver.  To hackers, most support people are
	useless - because by the time a hacker calls support he or she
	will usually know the software and the relevant manuals better
	than the support people (sadly, this is *not* a joke or
	exaggeration).  A hacker's idea of "support" is a tte--tte
	or exchange of {electronic mail} with the software's designer.

	[{Jargon File}]

	(1995-02-15)

something like __bool__ in Python, I need to
write some code in the following way.
x = Boolean.new()
if x.evaluate()
do something...
end

But I'd prefer "if x " instead of "if x.evaluate" because it is more
straight forward. So, I'd like to know whether there are any ways to
do so.

Regards,
Dave
--
Posted via http://www.ruby-forum.com/.

From: Phrogz

Date: Saturday, October 27, 2007

wrote in message:

I write some code like the following and the Boolean object hides some
underlying code which solve the compatibility problems.
x = Boolean.new()
if x
do something....
end
If ruby doesn't support something like __bool__ in Python, I need to
write some code in the following way.
x = Boolean.new()
if x.evaluate()
do something...
end
But I'd prefer "if x " instead of "if x.evaluate" because it is more
straight forward. So, I'd like to know whether there are any ways to
do so.

There aren't any ways to do so in Ruby. Sorry. (This question comes
up every few weeks or so.)

From: Robert Klemme

Date: Saturday, October 27, 2007

wrote in message:

Thus 'if x.ok?' returns the same as 'if x' for any object, but @xxxxxxxxxxx for
yours!
Thanks for your explanation!
In fact, I am making a wrapper class called Boolean which cooperates
with some legacy code in my company because there are some compatibility
problems between different languages.
I write some code like the following and the Boolean object hides some
underlying code which solve the compatibility problems.
x = Boolean.new()
if x
do something....
end
If ruby doesn't support something like __bool__ in Python, I need to
write some code in the following way.
x = Boolean.new()
if x.evaluate()
do something...
end
But I'd prefer "if x " instead of "if x.evaluate" because it is more
straight forward. So, I'd like to know whether there are any ways to
do so.

What exactly does Boolean do? Maybe you can get rid of it or do some
other changes so you can directly work with "true" and "false.

Kind regards

robert

From: Morton Goldberg

Date: Saturday, October 27, 2007

wrote in message:

I know ruby treat an object as false whenever it is nil or false.
However, I wonder if there are any other ways to change this behavior.
For example, I define a class called AreYouOk.
class AreYouOk
def initialize(ok)
@xxxxxxxxxxx = ok
end
end
x = AreYouOk.new(false)
puts "you are ok" if x
Since x isn't nil, ruby prints " you are ok".
However, I want ruby to make the decision based on the @xxxxxxxxxxx instance
variable. Are there any ways to do that?
I know that there is a method called __bool__ in Python. You can
define
your __bool__ method in your class. The truth value of an object is
based on the return value of __bool__. Does ruby provide similar
mechanism?

What you are asking about looks to me like a flag

1. <programming> A variable or quantity that can take on one
	of two values; a bit, particularly one that is used to
	indicate one of two outcomes or is used to control which of
	two things is to be done.  "This flag controls whether to
	clear the screen before printing the message."  "The program
	status word contains several flag bits."  See also {hidden
	flag}, {mode bit}.

	2. {command line option}.

	[{Jargon File}]

	(1998-05-02)

class.

<code>
class Flag
def initialize(state=false)
@xxxxxxxxxxx = state
end
def set?
@xxxxxxxxxxx
end
def set
@xxxxxxxxxxx = true
end
def clear
@xxxxxxxxxxx = false
end
end

ok = Flag.new
puts "you are ok" if ok.set?
puts "you aren't ok" unless ok.set?
ok.set
puts "you are ok" if ok.set?
puts "you aren't ok" unless ok.set?

</code>

First the string

<programming> A sequence of {data} values, usually {bytes},
	which usually stand for {characters} (a "character string").
	The {mapping} between values and characters is determined by
	the {character set} which is itself specified implcitly or
	explicitly by the environment in which the string is being
	interpreted.

	The most common character set is {ASCII} but, since the late
	1990s, there has been increased interest in larger character
	sets such as {Unicode} where each character is represented by
	more than eight {bits}.

	Most programming languages consider strings (e.g.
	"124:shabooya:\n", "hello world") basically distinct from
	numbers which are typically stored in fixed-length {binary} or
	{floating-point} representation.

	A {bit string} is a sequence of {bits}.

	(1999-12-21)

"you aren't ok" is printed. After the flag is set,
the string "you are ok" is printed.

Regards, Morton

Next Message: Error in Ruby text comparison?

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