From: Eric Mahurin <eric_mahurin@...>
Date: 2005-09-20T11:35:51+09:00
Subject: Re: ruby idiom for python's for/else while/else

--- why the lucky stiff <ruby-talk@whytheluckystiff.net> wrote:

> William Morgan wrote:
> 
> >Excerpts from Gergely Kontra's mail of 19 Sep 2005 (CDT):
> >  
> >
> >>Is there a similar construct to python's:
> >>
> >>    
> >>
> >>>>>for n in range(2, 10):
> >>>>>          
> >>>>>
> >>...     for x in range(2, n):
> >>...         if n % x == 0:
> >>...             print n, 'equals', x, '*', n/x
> >>...             break
> >>...     else:
> >>...         # loop fell through without finding a factor
> >>...         print n, 'is a prime number'
> >>... 
> >>    
> >>
> >
> >I don't think there's a direct idiom. I'd write this in Ruby
> as:
> >
> >(2...10).each do |n|
> >  fac = (2...n).find { |x| n % x == 0 }
> >  if fac
> >    puts "#{n} equals #{fac} * #{n / fac}"
> >  else
> >    puts "#{n} is a prime number"
> >  end
> >end
> >
> >This is much clearer IMO.
> >
> Yeah, this isn't any clearer, but here's one that
> demonstrates the 
> possibilities in tying iterators and conditionals together. 
> It works 
> because `break' causes the iterator to return nil.
> 
>   for n in 2...10
>     puts "#{ n } is a prime number" if
>       for x in 2...n
>         if n % x == 0
>           puts "#{ n } equals #{ x }*#{ n/x }"
>           break
>         end
>       end
>   end
> 
> _why


This is annoying - builtin loops and "loop" returns nil whereas
many of the object method loops (for..in is just the each
method) return the object.  Using a builtin loop, you can do
something like this to get something much closer to python's
while/else (becomes while/or in ruby):

(2...10).each do |n|
  x = 2
  while x<n
    if n % x == 0
      puts "#{ n } equals #{ x }*#{ n/x }"
      break(x)
    end
    x += 1
  end or 
  puts "#{ n } is a prime number"
end

I think that's the closest your going to get to python's
while/else.  You have the use the opposite polarity for the for
loop as why demonstrated.  One of my RCR's is about this
inconsistency.  I think all of the loop methods should return
nil by default (with break(ret) causing it to return something
else).




		
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