From: Josh Cheek <josh.cheek@...>
Date: 2011-01-18T21:11:15+09:00
Subject: Re: Calling by Reference - Two Questions

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On Mon, Jan 17, 2011 at 6:07 PM, Mike Stephens <rubfor@recitel.net> wrote:

>
> So my first question is why won't Ruby let me do this:
>
> a1 = [1,2]
>
> public
> def myswap5
>  a3 = [0,0]
>  a3[0] = self[1]
>  a3[1] = self[0]
>  self = a3
>  end
>
> a1.myswap5 # gives error message: Can't change the value of self
>
>
You actually can (though I'm not sure what you think is happening when you
say self=a3, so maybe it is not exactly the same):

ary = [1,2]

def ary.swap
  new_contents = [ self[1] , self[0] ]
  replace new_contents
end

ary # => [1, 2]
ary.swap
ary # => [2, 1]


ary replaced its contents with new_contents

My second question is from this:
>
> a1 = [1,2]
>
> def myswap6(a2)
>  a2.reverse!
>  end
>
>  myswap6 a1
>  puts " #{a1}" #21
>
> What magic is reverse! using, and how can I avail myself of it?
>
>
It is written in C, the core being (at least for 1.8):

    p1 = RARRAY(ary)->ptr;
    p2 = p1 + RARRAY(ary)->len - 1;    /* points last item */
    while (p1 < p2) {
        tmp = *p1;
        *p1++ = *p2;
        *p2-- = tmp;
    }


You could replicate it in Ruby like this:

ary = [1,2,3,4]

def ary.my_reverse
  lower , higher = 0 , size-1
  while lower < higher
    self[lower] , self[higher] = self[higher] , self[lower]
    lower  += 1
    higher -= 1
  end
  self
end

ary.my_reverse # => [4, 3, 2, 1]
ary.my_reverse # => [1, 2, 3, 4]
ary << 5
ary.my_reverse # => [5, 4, 3, 2, 1]
ary.my_reverse # => [1, 2, 3, 4, 5]

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