From: "Hal E. Fulton" <hal9000@...>
Date: 2002-12-05T12:03:29+09:00
Subject: Re: Enumerable#map_with_indices (was Re: Conditional block operations)

----- 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]}

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


Hal