From: Jeff Wood <jeff.darklight@...>
Date: 2005-12-24T04:38:35+09:00
Subject: Re: Why not Python? (No, no, I am not a spy)

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heh, darn, bug there ...

# yes assumptions about positive integer numbers...
# factorial of 0 is undefined ...

# complete recursive:

def factorial( x )
  return 1 if x < 1
  x * factorial( x - 1 )
end

# tail-recursive:

def factorial( x, sum =3D 1 )
  return 1 if x < 1
  factorial( ( x - 1 ), ( sum * x ) )
end

# iterative:

def factorial( x )
  sum =3D 1
  for i in (1..x).to_a
    sum *=3D i
  end
end


On 12/23/05, Jeff Wood <jeff.darklight@gmail.com> wrote:
>
> ah, but to me that isn't completely iterative.... iterative is using a
> loop within a function to make sure it and all functions it calls do not
> call back up the tree...
>
> hence:
>
> # yes assumptions about positive integer numbers...
> # complete recursive:
>
> def factorial( x )
>   return 0 if x < 1
>   x * factorial( x - 1 )
> end
>
> # tail-recursive:
>
> def factorial( x, sum =3D 1 )
>   return 0 if x < 1
>   factorial( ( x - 1 ), ( sum * x ) )
> end
>
> # iterative:
>
> def factorial( x )
>   sum =3D 1
>   for i in (1..x).to_a
>     sum *=3D i
>   end
> end
>
> ... anyways.
>
> j.
>
>
>
> On 12/23/05, Christian Neukirchen <chneukirchen@gmail.com> wrote:
> >
> > Jeff Wood <jeff.darklight@gmail.com> writes:
> >
> > > ... I believe the question was in regards to implementing factorial i=
n
> > a
> > > tail-recursive manner.
> > >
> > > I believe the statement was:
> > >
> > > "I'm a complete newb to lisp, I've been reading Practical Common Lisp
> > & On
> > > Lisp.  I've also been working through "The little schemer" and SICP..=
.
> > I'm
> > > running CMUCL under SLIME & Emacs (v21.4). ( phew ) ... I've written
> > an
> > > iterative & a normally recursive version of factorial ... Now I'm
> > trying to
> > > figure out how to implement it in a tail-recursive fashion ... can
> > anybody
> > > help me write a tail-recursive version ?? I'm not familiar with the
> > call
> > > pattern yet." ...
> >
> > I see, the iterative version at
> >   http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-21.html#%_thm_3.=
9<http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-21.html#%25_thm_3.9>
> > actually is the tail-recursive version. :-)
> >
> > They could have told you that, of course.
> >
> > > or something like that ...
> > >
> > > Jeff Wood
> > --
> > Christian Neukirchen  <chneukirchen@gmail.com>  http://chneukirchen.org
> >
> >
>
>
> --
> "Remember. Understand. Believe. Yield! -> http://ruby-lang.org"
>
> Jeff Wood




--
"Remember. Understand. Believe. Yield! -> http://ruby-lang.org"

Jeff Wood

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