Class similar to Length class

Viskiz

Hello

I'd like to create Area and Volume classes derived from the Float. These classes should be similar to Length class. How conversion from String to Length is made? Length.new() is not implemented, so it seems it doesn't have constructor? How it should be done?

Jim

You can get a Length from a String:

"100cm".to_l

But since you are implementing your own classes, you can write a "new" constructor.


class Area
  def initialize(area)
    @internal_representation_of_an_area = area
  end
end

a = Area.new(12.0)

And Dan will be along shortly to explain why you need to wrap your class in a module.

Dan Rathbun

@viskiz said:

I'd like to create Area and Volume classes derived from the Float. These classes should be similar to Length class.

BUT... Actually ... I'm not sure an Area or Volume class SHOULD ever be implemented.

What we are talking about, are really "properties" of objects, which are best implemented in ruby attributes, with getter and setter methods. And conversion methods to convert to other units.

You can still do if it's the best way (for you.)

A class CAN have attributes, which are instances of some other class.

So class Square, could have an @area attribute, that is an instance of class Area, which has @x and @y attributes, which are instances of class Length, and so on ...

Dan Rathbun

Along with this issue, is the question "Should a Length class have been implemented?"

Yes I understand Sketchup's Length class and why they did it. It was easier (or quicker to implement,) than the "precise" alternative, which is a class for each 'unit of measure.'

Classes Meter, Inch, Feet, Yard, ... etc. are more intuitive to me.

When I see Sketchup's Length class, I keep wanting to find it's unit attribute, to see what it is currently set to. But it does not have one! Because really Sketchup's Length class is an alias for an Inch class.

(I had played a bit defining classes in a General UOM or Units module for global Ruby use. Not finished with it, and it's just an exercise in Ruby programming. I have an interest in metrology as well. There may already be such a module, perhaps in Sketchy Physics?)

Dan Rathbun

@jim said:

And Dan will be along shortly to explain why you need to wrap your class in a module.

The rule is:

IF the class is to be generic (used by,) the entire Ruby Universe, then yes it CAN be defined at the TopLevel, as a Ruby Base class. What is clear, is that you should be VERY certain that your new classname, is not already being used at Ruby's TopLevel, before you define it there.

IF the class is only to be used with a specific application (Sketchup for example,) it should be in the application's namespace (as most of Sketchup's classes are.)

If the class is only meant to be use by YOU, (but possibly in several or many of your plugins,) then it should be within the Developer's namespace.

IF the class is only meant to be used with a specific plugin, then it should be defined within the plugin's namespace (which should defined within the Developer's namespace.)

But still, even with toplevel base classes, it is often nice to group classes together into a 'catagory' module.

I'd think this would best in a Geometry or Geometric module. (Not to be confused with the Sketchup specific Geom module.)

If I were to create a generic Integral class, I would think the best place be within the Math module? (or pehaps within a Calculus submodule of ::Math.)

Viskiz

@jim said:

You can get a Length from a String:

"100cm".to_l

I'm interested how Length class is made? In String class it should look like this:

def to_l
    Length.new(self)
end

but Length.new throws 'undefined method' error. So how to_l works?

P.S. Thanks for reminding me about modules. I've completely forgotten about them

Dan Rathbun

@viskiz said:

In String class it should look like this:

def to_l
>     Length.new(self)
> end

but Length.new throws 'undefined method' error. So how to_l works?

Length is subclass of Float which also does not have a new method.
You create Floats in Ruby using literal expressions:
num = 1.2345

String.to_l is much more complex than you imagine.
It has a string parsing algorithm in it, that separates the numeric characters from the alpha characters, and does conversions from various units to inches.
More like:

def to_l
  text=self.strip
  numpart=text.to_f
  if numlast = text.rindex(/\d/)
    unitpart = text.slice(numlast+1..text.length-1)
  else
    unitpart=nil
  end
  if unitpart
    case unitpart.downcase
    when 'cm'
      return (numpart * 0.3937).to_l
    # other when statements
    # for other units
    end # case
  else
    return numpart.to_l
  end #if
end

Actually the extendend Numeric conversion methods are used, see:
API: Numeric baseclass

Dan Rathbun

@viskiz said:

I'm interested how Length class is made?

It is simply a subclass, which inherits methods from it's superclass.

class Distance < Float
  # add custom methods here
end

P.S.: DO NOT make changes to the Length class and then publish this in a plugin or script !!