From: Alex Young <alex@...>
Date: 2007-12-13T21:51:04+09:00
Subject: Re: suprised by destructive map (not map!)

Boris Schmid wrote:
> Occasionally I run into bits of ruby that are surprising to me (although
> might not be to matz). Here is one of them. Should I file this as a bug
> (I thought .map is supposed to be non-destructive), or just keep in mind
> which classes are passed by value and which are passed by reference.
> 
> Basically: map (and naturally also map!) is destructive when your array
> includes elements which are normally not passed by value but by
> reference.
> 
> ruby 1.8.6 (2007-09-24 patchlevel 111) [x86_64-linux]
> boris:falcon:~:irb
> 
> "unexpected (for me)"
> irb(main):001:0> array = [[1],[2]]
> => [[1], [2]]
> irb(main):002:0> array.map {|i| i << 3}
> => [[1, 3], [2, 3]]
> irb(main):003:0> array
> => [[1, 3], [2, 3]]
> irb(main):004:0>

It's not map that's destructive here, it's the #<< method.  Map iterates 
over the original array, and creates a new one with the results of the 
block - it doesn't copy the original array first.

-- 
Alex