From: Brian Adkins <lojicdotcom@...>
Date: 2007-10-26T22:10:01+09:00
Subject: Re: Skip the first invocation e.g. skip_first { foo }

On Oct 25, 11:24 pm, Morton Goldberg <m_goldb...@ameritech.net> wrote:
> On Oct 25, 2007, at 7:30 PM, Brian Adkins wrote:
>
>
>
> > On Oct 25, 7:18 pm, Morton Goldberg <m_goldb...@ameritech.net> wrote:
> >> On Oct 25, 2007, at 6:40 PM, Brian Adkins wrote:
>
> >>> Consider the following code:
>
> >>> first = true
> >>> 3.times do
> >>>   if first
> >>>     first = false
> >>>   else
> >>>     puts 'foo'
> >>>   end
> >>>   ...
> >>> end
>
> >>> I'd like to be able to do the following instead:
>
> >>> 3.times do
> >>>   skip_first { puts 'foo' }
> >>>   ...
> >>> end
>
> >>> However, I'm pretty sure that's impossible - especially when you
> >>> consider running the above code twice would require state to be
> >>> initialized twice, so I expect some initialization outside the
> >>> loop is
> >>> necessary.
>
> >>> So, what's the most elegant way to solve this?
>
> >>> Here are a couple I've come up with minus some implementation
> >>> details.
> >>> They work, but I'm not very pleased with either one. I don't recall
> >>> ever needing 'n' to be other than 1, but it feels strange to not
> >>> generalize it. I also realize it's more typical to want to execute
> >>> code only on the first loop invocation, but I had the opposite need
> >>> when this question arose.
>
> >>> # Use an object for state
> >>> skip_first = SkipN.new(1)
> >>> 3.times do
> >>>   skip_first.run { puts 'hi' }
> >>>   ...
> >>> end
>
> >>> # Use a closure for state
> >>> skip_first = skipn(1)
> >>> 3.times do
> >>>   skip_first.call lambda { puts 'hi' }
> >>>   ...
> >>> end
>
> >>> I experimented with using a binding, but I discovered that a new
> >>> binding is created each time times invokes the block, so apparently
> >>> it's not possible to introduce a variable within the lexical
> >>> scope of
> >>> the block for the duration of the 3.times invocations - or I missed
> >>> something.
>
> >> It's not entirely clear to me what you are trying to accomplish, so
> >> this may way off base.
>
> > What I'm trying to accomplish is a convenient way to execute a block
> > of code on every iteration except the first (or only on the first
> > iteration).
>
> >> For some reason you are ignoring that Integer#times passes an index
> >> into its block,
>
> > Yes, that's because a loop index will not always be available. The
> > times example was just an example. Consider:
>
> > foo.each do |bar|
> >   skip_first { ... }
> >   ...
> > end
>
> Well, I'm pretty simple minded, so I would use a simple class to
> solve this problem. Something like
>
> <code>
> class Skipper
>     def initialize(n)
>        @n = n
>     end
>     def call
>        yield if (@n -= 1) < 0
>     end
> end
>
> skip_1 = Skipper.new(1)
> a = []
> 4.times { |i| skip_1.call { a << i } }
> a # => [1, 2, 3]
>
> skip_2 = Skipper.new(2)
> a = []
> %w[a b c d].each { |w| skip_2.call { a << w } }
> a # => ["c", "d"]
> </code>
>
> But it would appear you've already rejected that kind of solution.
> Why don't you like it?

I think it's the least objectionable at this point :) It's just not as
nice as:

skip_first { foo }