From: Julien Gaugaz <gaugaz@...3s.de>
Date: 2006-12-22T01:07:59+09:00
Subject: Method named ***(other)

Hi!

It's several hours now that i'm trying to find out what's special with 
*** for a method name, without success :(

I'm trying to add an element wise multiplication method to the Matrix 
class. For this I created a collect2 method for Matrix similar to the 
one in Vector as follows:

class Matrix
  def collect2(other)
    result = self.clone
    (0...self.row_size).each {|rowi|
      (0...self.column_size).each {|columni|
        result[rowi,columni] = yield (self[rowi,columni], 
other[rowi,columni])
      }
    }
    result
  end
  def []=(i,j,v)
      @rows[i][j] = v
   end
end

And i would like now to add a method *** to Matrix such that we could 
call matrix1 *** matrix2. Like:
m = Matrix[[1,2,3],[-1,-2,-3], [4,5,6]]
n = Matrix[[1,2,3],[7,8,9], [-1,-2,-3]]
m *** n
=> Matrix[[1, 4, 9], [-7, -16, -27], [-4, -10, -18]]

Now if I define this method as follows:
  def ***(other)
    collect2(other) do |e1,e2|
      if(e1 and e2)
        e1*e2
      end
    end
  end

I get the following error:
SyntaxError: compile error
(irb):550: parse error, unexpected tLPAREN, expecting '\n' or ';'
  def ***(other)
          ^
(irb):557: parse error, unexpected kEND, expecting $
    from (irb):557
    from :0

The same with  def *** other   works (I can define it without error). 
But now a call to m *** n gives me:
SyntaxError: compile error
(irb):33: parse error, unexpected tSTAR
m *** n
     ^
    from (irb):33
    from :0

And m.*** n (notice the period after m) gives:
TypeError: Array can't be coerced into Fixnum
    from (irb):8:in `*'
    from (irb):8:in `**'
    from (irb):24:in `collect2'
    from (irb):23:in `each'
    from (irb):23:in `collect2'
    from (irb):22:in `each'
    from (irb):22:in `collect2'
    from (irb):6:in `**'
    from (irb):35
    from :0

Whereas if I name the method with two stars only, i.e. def **(other), it 
can be defined, and then m ** n gives the expected result.

I therefore have the following questions:
- Why is it possible to define def *** other  but not def ***(other) ?
- What is the difference between m.*** n  and m *** n (with and without 
period after m) ?
- Why do I get an error with m.*** n (with period) but not with m ** n ?

I would greatly appreciate any answer of pointers to some information 
sources (I tried but it is not easy to search for *** in a web search 
engine ;-) )

Thanks a lot in advance for your help and advices!

Julien