From: Peter Vandenabeele <peter@...>
Date: 2011-12-10T21:22:04+09:00
Subject: Re: Ruby module error

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On Sat, Dec 10, 2011 at 12:39 PM, Vadim Pirogov <zzcrash@gmail.com> wrote:

> How in ruby i can write module ?
> so we have:
> a=-4
> b=-9
>
> g = Math.sqrt(Math.Module(a))*(Math.Module(b))
> def g(a, b)
> Math.sqrt(Math.Module(a))*(Math.Module(b))
> end
>
> end when i write "puts g" i have a error. I try write Math.mod but
> nothing.
>  "rb:4: undefined method `Module' for Math:Module (NoMethodError)"
>

For the module Math, you can easily see the documented
instance and class methods with

$ ri Math

The term "Module" is not one of them, so that is why you
get the error:
"undefined method `Module' for Math:Module (NoMethodError)"

You can use the module methods with

Math.sin(0.75)

You can use the instance methods with

include Math
sin(0.75)

Regarding your question how to write a module,
as example, I have written an Average module:

peterv@ASUS:~$ cat average.rb
module Average
  def geometric(*a)
    case a.size
    when 0
      raise "At least one argument required"
    when 1
      a[0]
    when 2
      Math.sqrt(a[0]*a[1])
    else
      (a.inject(1){|acc,e| acc*e})**(1.0/a.size)
    end
  end

  def arithmetic(*a)
    case a.size
    when 0
      raise "At least one argument required"
    when 1
      a[0]
    else
      (a.inject(0){|acc,e| acc+e})/Float(a.size) # force a float
    end
  end
end


and when I 'include' that, I can use the methods as instance
methods (on main).

An alternative would have been to define both methods as
module methods (with "def self.geometric"). This would have
the advantage of avoiding polluting the general name space
with the "include Average" .

peterv@ASUS:~$ irb
001:0> require 'average'
=> true
002:0> include Average
=> Object
003:0> geometric()
RuntimeError: At least one argument required
from ./average.rb:5:in `geometric'
from (irb):3
004:0> geometric(4)
=> 4
005:0> geometric(4,9)
=> 6.0
006:0> geometric(4,9,48)
=> 12.0
007:0> arithmetic()
RuntimeError: At least one argument required
from ./average.rb:18:in `arithmetic'
from (irb):7
008:0> arithmetic(7)
=> 7
009:0> arithmetic(4,7)
=> 5.5
010:0> arithmetic(4,7,6,9)
=> 6.5

In my "Average" module, I have now used the Math.sqrt
module function of the Math module to demonstrate the
proper use of such a module function.

HTH,

Peter

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