From: David Alan Black <dblack@...>
Date: 2001-11-17T08:28:34+09:00
Subject: [ruby-talk:25686] Re: Arrays, iterators, and map/collect

Hi --

On Sat, 17 Nov 2001, Hal E. Fulton wrote:

> From: Robert Feldt <feldt@ce.chalmers.se>
>
> > On Fri, 16 Nov 2001, Hal E. Fulton wrote:
> >
> > > But what if I want to do an operation
> > > conditionally? Is this still a good way to do it,
> > > in a case where the mapped value is
> > > sometimes/always the same as the original?
> > >
> > >   strings.map! { |s| if s[0]==?a then s else "pre." + s end }
> > >
> > I would do it like this but allow myself to shorten it to
> >
> >   strings.map! {|s| s[0]==?a ? s : ("pre." + s)}
> >
> > since its a one-liner.
> >
> > Don't you feel it is natural? Why?
> >
>
> I'm having difficulty expressing myself.
>
> This example is natural enough.
>
> One problem I have is: When I travel through
> an array, I expect to use an iterator.
>
> What if the map! operation is really only a
> side effect? I guess it is possible to do
> other stuff inside the block, but that makes
> it less obvious what is happening.
>
> I guess I would like to iterate through an array
> performing arbitrary operations and have the
> option to change the values of array elements
> if I wished. "replace" is OK I suppose... I haven't
> tried it.
>
> I don't like the idiom of explicitly indexing into
> the array for this case...

If you *really* want to see side-effect programming:

  strings = %w[ alpha beta gamma ]
  strings.find_all {|s| s[0,0] = "pre." if s[0] == ?a}

:-)

I think the map! version is pretty clear, since that's what
map! basically does -- though I have to say, sometimes it
looks/feels better to me to do:

   strings = strings.map { ... }

(though I don't know if there are performance considerations).


David

-- 
David Alan Black
home: dblack@candle.superlink.net
work: blackdav@shu.edu
Web:  http://pirate.shu.edu/~blackdav