mond:/pool/PROG/ruby # irb
irb(main):001:0> require "complex"
=> true
irb(main):002:0> c=Complex.new 1,1
=> Complex(1, 1)
irb(main):003:0> c.arg
=> 0.785398163397448
irb(main):004:0> c.angle
=> 0.785398163397448
irb(main):005:0> class << c
irb(main):006:1> def angle
irb(main):007:2> (arg/Math::PI)*180
irb(main):008:2> end
irb(main):009:1> end
TypeError: can't define singleton method "angle" for Complex
from (irb):6
irb(main):010:0> c.arg
=> 0.785398163397448
irb(main):011:0> c.angle
=> 45.0
irb(main):012:0>
what does TypeError means here?
can someone enlighten me
I have one more related question
I would like to have the angle method
to return the angle in degrees for each
Complex instance, how can I achieve it
the obvious didn't work for me
irb(main):014:0* class << Complex
irb(main):015:1> def angle
irb(main):016:2> (self.arg/Math::PI)*180
irb(main):017:2> end
irb(main):018:1> end
=> nil
irb(main):019:0> d=Complex.new 1,1
=> Complex(1, 1)
irb(main):020:0> d
=> Complex(1, 1)
irb(main):021:0> d.arg
=> 0.785398163397448
irb(main):022:0> d.angle
=> 0.785398163397448
irb(main):023:0>
irb(main):005:0> class << c
irb(main):006:1> def angle
irb(main):007:2> (arg/Math::PI)*180
irb(main):008:2> end
irb(main):009:1> end
Do you mean to add a method to an object?
This would work:
def c.angle
(arg/Math::PI)*180
end
I hope this will help.
···
TypeError: can't define singleton method "angle" for Complex
from (irb):6
from :0
irb(main):010:0> c.arg
=> 0.785398163397448
irb(main):011:0> c.angle
=> 45.0
irb(main):012:0>
what does TypeError means here?
can someone enlighten me
I have one more related question
I would like to have the angle method
to return the angle in degrees for each
Complex instance, how can I achieve it
the obvious didn't work for me
irb(main):014:0* class << Complex
irb(main):015:1> def angle
irb(main):016:2> (self.arg/Math::PI)*180
irb(main):017:2> end
irb(main):018:1> end
=> nil
irb(main):019:0> d=Complex.new 1,1
=> Complex(1, 1)
irb(main):020:0> d
=> Complex(1, 1)
irb(main):021:0> d.arg
=> 0.785398163397448
irb(main):022:0> d.angle
=> 0.785398163397448
irb(main):023:0>
You're using the wrong syntax here. To re-open a class, just use
normal class definition syntax:
class Complex
def angle
arg/Math::PI*180.0
end
end
However, I would advise you to use a name like 'angle_in_degrees'
rather than override a basic function in this way. It will bite you
later when you include other libraries that expect angle in radians.
Regards,
Sean
···
On 11/6/05, Daniel Schüle <uval@rz.uni-karlsruhe.de> wrote:
irb(main):005:0> class << c
irb(main):006:1> def angle
irb(main):007:2> (arg/Math::PI)*180
irb(main):008:2> end
irb(main):009:1> end
TypeError: can't define singleton method "angle" for Complex
from (irb):6
from :0
irb(main):010:0> c.arg
=> 0.785398163397448
irb(main):011:0> c.angle
=> 45.0
I think the error message is misleading, because
class << c
def angle;end
end
should add a singleton method to instance c
or overwrite the behavior of inherited method
as expected the behaviuor of c.angle is changed
it should not "define singleton method for Complex"
You're using the wrong syntax here. To re-open a class, just use
normal class definition syntax:
class Complex
def angle
arg/Math::PI*180.0
end
end
I found out that I also can use
Complex.class_eval do
def angle
arg / Math::PI * 180
end
end
this would "update" the method for all instances of Complex class
class << Complex
def angle; "angle"; end
end
would create singleton_method for Complex class itself
Complex.angle => "angle"
what is the advantage or typical use of having singleton method for a class?
I could imagine that since Complex is a Class it has some inherited methods from it .. and singleton method would belong only to Complex
nevertheless I can't find scenarious where I could use it
but I am a newbee
thanks to all for your replies
Daniel
···
On 11/6/05, Daniel Schüle <uval@rz.uni-karlsruhe.de> wrote:
> irb(main):005:0> class << c
> irb(main):006:1> def angle
> irb(main):007:2> (arg/Math::PI)*180
> irb(main):008:2> end
> irb(main):009:1> end
> TypeError: can't define singleton method "angle" for Complex
> from (irb):6
> from :0
> irb(main):010:0> c.arg
> => 0.785398163397448
> irb(main):011:0> c.angle
> => 45.0
I think the error message is misleading, because
class << c
def angle;end
end
should add a singleton method to instance c
or overwrite the behavior of inherited method
as expected the behaviuor of c.angle is changed
it should not "define singleton method for Complex"
Regards, Daniel
You are correct.
What Nobu said is that you'll see a more helpful message
when it's easier to do so.
At the moment, it seems, the method is defined in your
singleton object then an exception is raised with a
misleading message. IRB traps the error and continues.
require 'complex'
c=Complex.new 1,1
begin
def c.angle
(arg/Math::PI)*180
end
rescue => e
p e
end
#<TypeError: can't define singleton method "angle" for Complex>
p c.arg #.->=> 0.785398163397448
p c.angle #.->=> 45.0
# Complex is unchanged
c2=Complex.new 1,1
p c2.arg #.->=> 0.785398163397448
p c2.angle #.->=> 0.785398163397448
You're using the wrong syntax here. To re-open a class, just use
normal class definition syntax:
class Complex
def angle
arg/Math::PI*180.0
end
end
I found out that I also can use
Complex.class_eval do
def angle
arg / Math::PI * 180
end
end
this would "update" the method for all instances of Complex class
Or:
def Complex.angle
...
class << Complex
def angle; "angle"; end
end
would create singleton_method for Complex class itself
Complex.angle => "angle"
what is the advantage or typical use of having singleton method for a class?
Singleton methods of classes are generally referred to as "class
methods", and they're actually the most frequent use-case for
singleton methods. You'll find a lot of them in the core. Here are
some examples:
Regexp.escape(string)
File.open(filename)
Math.sqrt(num) # Math is actually a Module, not a Class, but same
# idea
Class.new
etc.
David
···
On Mon, 7 Nov 2005, Daniel Schüle wrote:
On 11/6/05, Daniel Schüle <uval@rz.uni-karlsruhe.de> wrote:
Sean O'Halpin wrote:
> > irb(main):014:0* class << Complex
I think Daniel was, correctly, trying to limit his change
to his own object. He didn't want to affect the class.
If you read what Daniel wrote:
I would like to have the angle method
to return the angle in degrees for each
Complex instance, how can I achieve it
the obvious didn't work for me
and his subsequent message that Complex.class_eval does what he wants
I think you will see that all he really wanted to do was to redefine
the instance method for angle which is more usually done by re-opening
class as I showed.
All the singleton stuff is a bit of a red herring as far as Daniel is concerned.
I would think that most people on this list would agree that this:
class Complex
def angle
arg/Math::PI*180.0
end
end
>
> Sean O'Halpin wrote:
> > > irb(main):014:0* class << Complex
>
> I think Daniel was, correctly, trying to limit his change
> to his own object. He didn't want to affect the class.
If you read what Daniel wrote:
>> [[ I have one more related question ]]
>> I would like to have the angle method
>> to return the angle in degrees for each
>> Complex instance, how can I achieve it
>> the obvious didn't work for me
and his subsequent message that Complex.class_eval does what he wants
I think you will see that all he really wanted to do was to redefine
the instance method for angle which is more usually done by re-opening
class as I showed.
Yes, I was defending Daniel's possible intentions which were
ambiguous before his follow-up message.
I see now that in his second (related) question, he *was*
wanting to affect the class and that that was the part you
were responding to.
I would think that most people on this list would agree that this:
> class Complex
> def angle
> arg/Math::PI*180.0
> end
> end
is the usual way to do this.
Now we're focused on instance methods, I think you're on safe ground
Simple misunderstanding -- no aggression intended.
Regards,
Sean
daz
···
On 11/6/05, daz wrote:
> > On 11/6/05, Daniel Schüle wrote:
> I found out that I also can use
> > Complex.class_eval do
> def angle
> arg / Math::PI * 180
> end
> end
> > this would "update" the method for all instances of Complex class
Or:
def Complex.angle
Whoops -- wrong. That's actually a way to define the singleton method
on Complex, as discussed later in your post.
