From: "Mauricio Fernández" <batsman.geo@...>
Date: 2002-12-05T18:32:43+09:00
Subject: Re: Enumerable#map_with_indices (was Re: Conditional block operations)

On Thu, Dec 05, 2002 at 12:03:29PM +0900, Hal E. Fulton wrote:
> ----- Original Message -----
> From: <dblack@candle.superlink.net>
> To: "ruby-talk ML" <ruby-talk@ruby-lang.org>
> Sent: Wednesday, December 04, 2002 8:47 PM
> Subject: Re: Enumerable#map_with_indices (was Re: Conditional block
> operations)
> 
> 
> > It was more a hint to Matz, though I think mwi is off the table :-)
> 
> I'm not sure it's off the table forever... Matz
> did not seem opposed to it, just unconvinced.
> 
> Here's an exchange from Nov 18 -- Matz is quoted,
> then (I guess) you, then me...
> 
> > > > map_with_index was rejected just because no one proposed its typical
> > > > usage.  YANGI principle applied.
> > >
> > > Hmmmm....  I'll try to think of some examples.  It certainly seems to
> > > me to be at least as useful as each_with_index, potentially (i.e., if
> > > it existed :-)
> >
> > I'd go so far as to say that map_with_index
> > does for map exactly what each_with_index
> > does for each.
> >
> > I do find it useful.
> 
> So I'd say (unless Matz says otherwise): Create a
> rationale for its existence and submit again.
> 
> Here's a little contrived example of how I'd show
> the relationship between ewi and mwi.
> 
> Problem: Take two arrays a and b, known to be the
> same size and contain numbers, and produce an array
> c which is the sum of the corresponding elements.
> 
> Hmm, can't use a.each; must index into the other
> array. Need each_with_index.
> 
>   c = []
>   a.each_with_index {|x,i| c << x + b[i]}	
>
> Now, same problem, but we want to modify a in-place
> rather than creating c.
> 
>   a.map_with_index! {|x,i| x + b[i]}
> 
> And in fact, with mwi (non-bang) the first problem
> also can be solved:
> 
>   c = a.map_with_index {|x,i| x + b[i]}
    c = (0...a.size).map {|i| a[i] + b[i]}

I think that matz wasn't really convinced by this sort of examples, and
IIRC he'd normally give such a solution w/ map and state that YANGNI.

> 
> No doubt David or someone can come up with a more
> compelling example.

At any rate, I'd take mwi for completeness sake. Of course I might not
really need it, but it looks more natural to me than using map and I'd
no longer have NameErrors when trying to call "map_with_index" (cause I
really expected it to be there!)


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