Extending Float


#1

Maybe I haven’t wrapped my head around OOP in the way I should have, can
anyone explain to me why I can’t do this?:

class Percentage < Float
def to_s(decimalplaces = 0)
(((self * 10**(decimalplaces+2)).round)/10**decimalplaces).to_s+"%"
end
end

puts Percentage.new(0.5)

I just get the following error:
NoMethodError: undefined method ‘new’ for Percentage:Class

I hope its obvious what I’m trying to do, let me know if there’s a way
to achieve this that I’m missing!
Thanks in advance


#2

On Thu, May 21, 2009 at 5:11 PM, Jp Hastings-spital
removed_email_address@domain.invalid wrote:


I just get the following error:
NoMethodError: undefined method ‘new’ for Percentage:Class

I hope its obvious what I’m trying to do, let me know if there’s a way
to achieve this that I’m missing!
Thanks in advance

Use a delegate class

require ‘delegate’
class Percentage < DelegateClass(Float)
def to_s(decimalplaces = 0)
(((self * 10**(decimalplaces+2)).round)/10**decimalplaces).to_s+"%"
end
end

percentage = Percentage.new(0.5)
percentage.to_s #=> “50%”

Andrew T.
http://ramblingsonrails.com
http://www.linkedin.com/in/andrewtimberlake

“I have never let my schooling interfere with my education” - Mark Twain


#3

Jp Hastings-spital wrote:

puts Percentage.new(0.5)

I just get the following error:
NoMethodError: undefined method ‘new’ for Percentage:Class

That’s because you can’t do Float.new either.


#4

Thanks so much - and for being so quick!


#5

Brian C. wrote:

That’s because you can’t do Float.new either.

So, out of interest, how does a Float get initialized (what function is
called)? Or is that some fancy inbuilt something-or-other?
Logically I’d assume I’d be able to do
my_pc = Percentage = 0.5
or something along those lines. What do you reckon?


#6

On Fri, May 22, 2009 at 7:30 PM, Jp Hastings-spital
removed_email_address@domain.invalid wrote:

Brian C. wrote:

That’s because you can’t do Float.new either.

So, out of interest, how does a Float get initialized (what function is
called)? Or is that some fancy inbuilt something-or-other?
Logically I’d assume I’d be able to do
my_pc = Percentage = 0.5
or something along those lines. What do you reckon?
This is what happenes in numeric.c
rb_cFloat = rb_define_class(“Float”, rb_cNumeric);

rb_undef_alloc_func(rb_cFloat);
rb_undef_method(CLASS_OF(rb_cFloat), "new");

Probably for some good reasons, however you are king

irb(main):001:0> class Float
irb(main):002:1> def self.new x
irb(main):003:2> Float( x )
irb(main):004:2> end
irb(main):005:1> end
=> nil
irb(main):006:0> class W < Float
irb(main):007:1> end
=> nil
irb(main):008:0> x=W::new 42
=> 42.0

How floats get initialized? I dunno, but it is my guess that the
parser generates some code very similar to what is in Kernel#Float.
But for some reason my grep skills elude me to find the code of
Kernel#Float.


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