From: daz <dooby@...10.karoo.co.uk>
Date: 2005-07-13T22:40:51+09:00
Subject: Re: accessing index inside map


Brian Candler wrote:
> >
> > [1];  obj.enum(:each_with_index).inject(0) {|sum,(a,i)| sum + a*i}
> >
> > [2];  i=-1; obj.inject(0) {|sum, a| sum + a*(i+=1)}
> >
> > Can anyone shade me in on what, if any, advantage [1] has over [2]:
> > -- in terms of:
> >
> >  a) Readability
> >  b) Ease of use and understanding
> >  c) Performance
> >  d) Flexibility
> >  e) Accuracy
> >  f) Memory usage
> >  g) Anything else you care to throw in  ;-)
>
> 1. It retains the enapsulation of "iterating with an index";
>

OK.  That's why I want it provided automatically inside the block.
It's information known to, but not revealed by, the iterator.  We're the
programmers ... they're our blocks ... why can't we discover a fundamental
detail like which iteration we're on ??

> you are reimplementing it in your inner block.

Both examples implement an index.  One as an external variable, the other
as an extra block parameter provided by #each_with_index.
I can't see anything I've reimplemented.  We may be at cross-purposes ?


> (Violation of DRY principle).

Not so.  There's no index, and I need one, so I have to create it.
So do you.  It's no more a violation of DRY than having to
open a file _every_ flippin' time I want to read it.


> 2. It's more general; it works with other iteration methods. Consider
> Hash#each_value and Hash#each_key, or String#each_byte for example.
>

I've considered.  I can't think of any that it wouldn't work with.
Is there anything more general than that ?

>     require 'enumerator'
>     "abc".to_enum(:each_byte).collect    #=> [97, 98, 99]
>
> Of course you could rewrite this too, but then you're reimplementing
> 'collect':
>
>     res = []
>     "abc".each_byte { |x| res << x }
>     res

OK, _that_ would be a reimplementation of 'collect' but I can't respond
with an example of what I'd do because you haven't used an index.
I'm talking index only, albeit within a slightly expanded topic.


However, dawning is in progress, perhaps :-)

#     "abc".to_enum(:each_byte).collect    #=> [97, 98, 99]

I was about to describe the #each_byte block as a "proxy receiver"
for #collect, but I think I'll wait before spreading untruths
because it feels reversed (?)

>
> 3. If you were using the index in more than one place in the inner block, it
> would become more verbose.
>
>     i=0; obj.inject(0) { ... use i ... use i ...; i+=1 }
>

Absurd ;-)
If your block had 30 uses of the index, my version would have 31.

I'll say again - we may be mixing subjects (accessing an index
versus Enumeration and it's probably just me who doesn't know the
difference, yet).

>
> Brian.
>

Thanks BC,

daz