From: GOTO Kentaro <gotoken@...>
Date: 2001-11-16T15:54:27+09:00
Subject: [ruby-talk:25393] Re: Arrays, iterators, and map/collect

In message "[ruby-talk:25387] Re: Arrays, iterators, and map/collect"
    on Fri, 16 Nov 2001 14:34:03 +0900,
    "Hal E. Fulton" <hal9000@hypermetrics.com> writes:

> ----- Original Message -----
> From: Robert Feldt <feldt@ce.chalmers.se>
> To: ruby-talk ML <ruby-talk@ruby-lang.org>
> Sent: Thursday, November 15, 2001 11:21 PM
> Subject: [ruby-talk:25385] Re: Arrays, iterators, and map/collect

> I guess the other question is: Is map! inefficient about mapping
> an object onto itself? I.e., what if I left out the if?
> 
> In some sense I am bothered that x += y creates a new object.
> I don't see any way around it, though.

When each member is a kind of container, `replace' works.

  strings = %w[ alpha beta gamma ]
  strings.each do |s|
    s.replace("pre.#{s}") if /^a/ =~ s
  end
  p strings  #=> ["pre.alpha", "beta", "gamma"]

> I did toy with the (bad) idea of a ":=" assignment in Ruby, which
> guaranteed to re-use the object on the lefthand side... :)
> Then one could do:
>
>   strings.each {|s| s:="pre."+s }
> 
> with impunity.

Now, the operation is not to the element but the array.  It should not
be a operation to block params, but should be one to the array itself,
IMHO.

However, each_index looks ugly.  So, maybe gsub like interface is
needed.  E.g.:

  class Array
    def change_if(arg)
      if block_given?
        if arg.is_a? Proc
          each_with_index{|e,i| self[i] = yield(e) if arg.call(e)}
        else
          each_with_index{|e,i| self[i] = yield(e) if arg === e}
        end
      end
    end
  end

  strings = %w[ alpha beta gamma ]
  strings.change_if(/^a/) do |s|
    "pre." + s
  end
  p strings

-- Gotoken